一例智能网卡(mellanox)的网卡故障分析

1. 背景

这个是在centos 7.6.1810的环境上复现的，智能网卡是目前很多云服务器上的网卡标配，在OPPO主要用于vpc等场景，智能网卡的代码随着功能的增强导致复杂度一直在上升，驱动的bug一直是内核bug中的大头，在遇到类似问题时，内核开发者由于对驱动代码不熟悉，排查会比较费劲,本身涉及的背景知识有：dma_pool,dma_page,net_device,mlx5_core_dev设备，设备卸载，uaf问题等,另外，这个bug目测在最新的linux基线也没有解决,本文单独拿出来列举是因为uaf问题相对比较独特。下面列一下我们是怎么排查并解决这个问题的。

2. 故障现象

OPPO云内核团队接到连通性告警报障，发现机器复位：

UPTIME: 00:04:16-------------运行的时间很短LOAD AVERAGE: 0.25, 0.23, 0.11TASKS: 2027RELEASE: 3.10.0-1062.18.1.el7.x86_64MEMORY: 127.6 GBPANIC: "BUG: unable to handle kernel NULL pointer dereference at           (null)"PID: 23283COMMAND: "spider-agent"TASK: ffff9d1fbb090000  [THREAD_INFO: ffff9d1f9a0d8000]CPU: 0STATE: TASK_RUNNING (PANIC)crash> btPID: 23283  TASK: ffff9d1fbb090000  CPU: 0   COMMAND: "spider-agent" #0 [ffff9d1f9a0db650] machine_kexec at ffffffffb6665b34 #1 [ffff9d1f9a0db6b0] __crash_kexec at ffffffffb6722592 #2 [ffff9d1f9a0db780] crash_kexec at ffffffffb6722680 #3 [ffff9d1f9a0db798] oops_end at ffffffffb6d85798 #4 [ffff9d1f9a0db7c0] no_context at ffffffffb6675bb4 #5 [ffff9d1f9a0db810] __bad_area_nosemaphore at ffffffffb6675e82 #6 [ffff9d1f9a0db860] bad_area_nosemaphore at ffffffffb6675fa4 #7 [ffff9d1f9a0db870] __do_page_fault at ffffffffb6d88750 #8 [ffff9d1f9a0db8e0] do_page_fault at ffffffffb6d88975 #9 [ffff9d1f9a0db910] page_fault at ffffffffb6d84778    [exception RIP: dma_pool_alloc+427]//caq:异常地址    RIP: ffffffffb680efab  RSP: ffff9d1f9a0db9c8  RFLAGS: 00010046    RAX: 0000000000000246  RBX: ffff9d0fa45f4c80  RCX: 0000000000001000    RDX: 0000000000000000  RSI: 0000000000000246  RDI: ffff9d0fa45f4c10    RBP: ffff9d1f9a0dba20   R8: 000000000001f080   R9: ffff9d00ffc07c00    R10: ffffffffc03e10c4  R11: ffffffffb67dd6fd  R12: 00000000000080d0    R13: ffff9d0fa45f4c10  R14: ffff9d0fa45f4c00  R15: 0000000000000000    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018#10 [ffff9d1f9a0dba28] mlx5_alloc_cmd_msg at ffffffffc03e10e3 [mlx5_core]//涉及的模块#11 [ffff9d1f9a0dba78] cmd_exec at ffffffffc03e3c92 [mlx5_core]#12 [ffff9d1f9a0dbb18] mlx5_cmd_exec at ffffffffc03e442b [mlx5_core]#13 [ffff9d1f9a0dbb48] mlx5_core_access_reg at ffffffffc03ee354 [mlx5_core]#14 [ffff9d1f9a0dbba0] mlx5_query_port_ptys at ffffffffc03ee411 [mlx5_core]#15 [ffff9d1f9a0dbc10] mlx5e_get_link_ksettings at ffffffffc0413035 [mlx5_core]#16 [ffff9d1f9a0dbce8] __ethtool_get_link_ksettings at ffffffffb6c56d06#17 [ffff9d1f9a0dbd48] speed_show at ffffffffb6c705b8#18 [ffff9d1f9a0dbdd8] dev_attr_show at ffffffffb6ab1643#19 [ffff9d1f9a0dbdf8] sysfs_kf_seq_show at ffffffffb68d709f#20 [ffff9d1f9a0dbe18] kernfs_seq_show at ffffffffb68d57d6#21 [ffff9d1f9a0dbe28] seq_read at ffffffffb6872a30#22 [ffff9d1f9a0dbe98] kernfs_fop_read at ffffffffb68d6125#23 [ffff9d1f9a0dbed8] vfs_read at ffffffffb684a8ff#24 [ffff9d1f9a0dbf08] sys_read at ffffffffb684b7bf#25 [ffff9d1f9a0dbf50] system_call_fastpath at ffffffffb6d8dede    RIP: 00000000004a5030  RSP: 000000c001099378  RFLAGS: 00000212    RAX: 0000000000000000  RBX: 000000c000040000  RCX: ffffffffffffffff    RDX: 000000000000000a  RSI: 000000c00109976e  RDI: 000000000000000d---read的文件fd编号    RBP: 000000c001099640   R8: 0000000000000000   R9: 0000000000000000    R10: 0000000000000000  R11: 0000000000000206  R12: 000000000000000c    R13: 0000000000000032  R14: 0000000000f710c4  R15: 0000000000000000    ORIG_RAX: 0000000000000000  CS: 0033  SS: 002b                

从堆栈看，是某进程读取文件触发了一个内核态的空指针引用。

3. 故障现象分析

从堆栈信息看：1. 当时进程打开fd编号为13的文件，这个从rdi的值可以看出。2. speed_show 和 __ethtool_get_link_ksettings 表示在读取网卡的速率值下面看下打开的文件是哪个

crash> files 23283
PID: 23283  TASK: ffff9d1fbb090000  CPU: 0   COMMAND: "spider-agent"
ROOT: /rootfs    CWD: /rootfs/home/service/app/spider
 FD       FILE            DENTRY           INODE       TYPE PATH
....
  9 ffff9d0f5709b200 ffff9d1facc80a80 ffff9d1069a194d0 REG  /rootfs/sys/devices/pci0000:3a/0000:3a:00.0/0000:3b:00.0/net/p1p1/speed---这个还在
 10 ffff9d0f4a45a400 ffff9d0f9982e240 ffff9d0fb7b873a0 REG  /rootfs/sys/devices/pci0000:5d/0000:5d:00.0/0000:5e:00.0/net/p3p1/speed---注意对应关系  0000:5e:00.0 对应p3p1
 11 ffff9d0f57098f00 ffff9d1facc80240 ffff9d1069a1b530 REG  /rootfs/sys/devices/pci0000:3a/0000:3a:00.0/0000:3b:00.1/net/p1p2/speed---这个还在
 13 ffff9d0f4a458a00 ffff9d0f9982e0c0 ffff9d0fb7b875f0 REG  /rootfs/sys/devices/pci0000:5d/0000:5d:00.0/0000:5e:00.1/net/p3p2/speed---注意对应关系 0000:5e:00.1 对应p3p2
....

注意上面 pci编号与 网卡名称的对应关系，后面会用到。打开文件读取speed本身应该是一个很常见的流程。下面从 exception RIP: dma_pool_alloc+427 进一步分析为什么触发了NULL pointer dereference展开具体的堆栈如下：

#9 [ffff9d1f9a0db910] page_fault at ffffffffb6d84778
    [exception RIP: dma_pool_alloc+427]
    RIP: ffffffffb680efab  RSP: ffff9d1f9a0db9c8  RFLAGS: 00010046
    RAX: 0000000000000246  RBX: ffff9d0fa45f4c80  RCX: 0000000000001000
    RDX: 0000000000000000  RSI: 0000000000000246  RDI: ffff9d0fa45f4c10
    RBP: ffff9d1f9a0dba20   R8: 000000000001f080   R9: ffff9d00ffc07c00
    R10: ffffffffc03e10c4  R11: ffffffffb67dd6fd  R12: 00000000000080d0
    R13: ffff9d0fa45f4c10  R14: ffff9d0fa45f4c00  R15: 0000000000000000
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
    ffff9d1f9a0db918: 0000000000000000 ffff9d0fa45f4c00 
    ffff9d1f9a0db928: ffff9d0fa45f4c10 00000000000080d0 
    ffff9d1f9a0db938: ffff9d1f9a0dba20 ffff9d0fa45f4c80 
    ffff9d1f9a0db948: ffffffffb67dd6fd ffffffffc03e10c4 
    ffff9d1f9a0db958: ffff9d00ffc07c00 000000000001f080 
    ffff9d1f9a0db968: 0000000000000246 0000000000001000 
    ffff9d1f9a0db978: 0000000000000000 0000000000000246 
    ffff9d1f9a0db988: ffff9d0fa45f4c10 ffffffffffffffff 
    ffff9d1f9a0db998: ffffffffb680efab 0000000000000010 
    ffff9d1f9a0db9a8: 0000000000010046 ffff9d1f9a0db9c8 
    ffff9d1f9a0db9b8: 0000000000000018 ffffffffb680ee45 
    ffff9d1f9a0db9c8: ffff9d0faf9fec40 0000000000000000 
    ffff9d1f9a0db9d8: ffff9d0faf9fec48 ffffffffb682669c 
    ffff9d1f9a0db9e8: ffff9d00ffc07c00 00000000618746c1 
    ffff9d1f9a0db9f8: 0000000000000000 0000000000000000 
    ffff9d1f9a0dba08: ffff9d0faf9fec40 0000000000000000 
    ffff9d1f9a0dba18: ffff9d0fa3c800c0 ffff9d1f9a0dba70 
    ffff9d1f9a0dba28: ffffffffc03e10e3 
#10 [ffff9d1f9a0dba28] mlx5_alloc_cmd_msg at ffffffffc03e10e3 [mlx5_core]
    ffff9d1f9a0dba30: ffff9d0f4eebee00 0000000000000001 
    ffff9d1f9a0dba40: 000000d0000080d0 0000000000000050 
    ffff9d1f9a0dba50: ffff9d0fa3c800c0 0000000000000005 --r12是rdi ,ffff9d0fa3c800c0
    ffff9d1f9a0dba60: ffff9d0fa3c803e0 ffff9d1f9d87ccc0 
    ffff9d1f9a0dba70: ffff9d1f9a0dbb10 ffffffffc03e3c92 
#11 [ffff9d1f9a0dba78] cmd_exec at ffffffffc03e3c92 [mlx5_core]

从堆栈中取出对应的 mlx5_core_dev 为 ffff9d0fa3c800c0

crash> mlx5_core_dev.cmd ffff9d0fa3c800c0 -xo
struct mlx5_core_dev {
  [ffff9d0fa3c80138] struct mlx5_cmd cmd;
}
crash> mlx5_cmd.pool ffff9d0fa3c80138
  pool = 0xffff9d0fa45f4c00------这个就是dma_pool，写驱动代码的同学会经常遇到

出问题的代码行号为：

crash> dis -l dma_pool_alloc+427 -B 5
/usr/src/debug/kernel-3.10.0-1062.18.1.el7/linux-3.10.0-1062.18.1.el7.x86_64/mm/dmapool.c: 334
0xffffffffb680efab <dma_pool_alloc+427>:        mov    (%r15),%ecx
而对应的r15，从上面的堆栈看，确实是null。
    305 void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
    306                      dma_addr_t *handle)
    307 {
...
    315         spin_lock_irqsave(&pool->lock, flags);
    316         list_for_each_entry(page, &pool->page_list, page_list) {
    317                 if (page->offset < pool->allocation)---//caq:当前满足条件
    318                         goto ready;//caq:跳转到ready
    319         }
    320 
    321         /* pool_alloc_page() might sleep, so temporarily drop &pool->lock */
    322         spin_unlock_irqrestore(&pool->lock, flags);
    323 
    324         page = pool_alloc_page(pool, mem_flags & (~__GFP_ZERO));
    325         if (!page)
    326                 return NULL;
    327 
    328         spin_lock_irqsave(&pool->lock, flags);
    329 
    330         list_add(&page->page_list, &pool->page_list);
    331  ready:
    332         page->in_use++;//caq:表示正在引用
    333         offset = page->offset;//从上次用完的地方开始使用
    334         page->offset = *(int *)(page->vaddr + offset);//caq:出问题的行号
...
    }

从上面的代码看，page->vaddr为NULL,offset也为0，才会引用NULL，page有两个来源：第一种是从pool中的page_list中取。第二种是从pool_alloc_page临时申请，当然申请之后会挂入到pool中的page_list。下面查看一下这个page_list：

crash> dma_pool ffff9d0fa45f4c00 -x
struct dma_pool {
  page_list = {
    next = 0xffff9d0fa45f4c80, 
    prev = 0xffff9d0fa45f4c00
  }, 
  lock = {
    {
      rlock = {
        raw_lock = {
          val = {
            counter = 0x1
          }
        }
      }
    }
  }, 
  size = 0x400, 
  dev = 0xffff9d1fbddec098, 
  allocation = 0x1000, 
  boundary = 0x1000, 
  name = "mlx5_cmd\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000", 
  pools = {
    next = 0xdead000000000100, 
    prev = 0xdead000000000200
  }
}

crash> list dma_pool.page_list -H 0xffff9d0fa45f4c00 -s dma_page.offset,vaddr
ffff9d0fa45f4c80
  offset = 0
  vaddr = 0x0
ffff9d0fa45f4d00
  offset = 0
  vaddr = 0x0

从 dma_pool_alloc 函数的代码逻辑看，pool->page_list确实不为空，而且满足if (page->offset < pool->allocation) 的条件，所以第一个page应该是 ffff9d0fa45f4c80也就是从第一种情况取出的：

crash> dma_page ffff9d0fa45f4c80
struct dma_page {
  page_list = {
    next = 0xffff9d0fa45f4d00, 
    prev = 0xffff9d0fa45f4c80
  }, 
  vaddr = 0x0, //caq:这个异常，引用这个将导致crash
  dma = 0, 
  in_use = 1, //caq:这个标记为在使用，符合page->in_use++;
  offset = 0
}

问题分析到这里，因为dma_pool中的page，申请之后，vaddr都会初始化，一般在pool_alloc_page 中进行初始化，怎么可能会NULL呢？然后查看一下这个地址：

crash> kmem ffff9d0fa45f4c80-------这个是dma_pool中的page
CACHE            NAME                 OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE
ffff9d00ffc07900 kmalloc-128//caq:注意这个长度  128       8963     14976    234     8k
  SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
  ffffe299c0917d00  ffff9d0fa45f4000     0     64         29    35
  FREE / [ALLOCATED]
   ffff9d0fa45f4c80  

      PAGE         PHYSICAL      MAPPING       INDEX CNT FLAGS
ffffe299c0917d00 10245f4000                0 ffff9d0fa45f4c00  1 2fffff00004080 slab,head

由于以前用过类似的dma函数，印象中dma_page没有这么大，再看看第二个dma_page如下：

crash> kmem ffff9d0fa45f4d00
CACHE            NAME                 OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE
ffff9d00ffc07900 kmalloc-128              128       8963     14976    234     8k
  SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
  ffffe299c0917d00  ffff9d0fa45f4000     0     64         29    35
  FREE / [ALLOCATED]
   ffff9d0fa45f4d00  

      PAGE         PHYSICAL      MAPPING       INDEX CNT FLAGS
ffffe299c0917d00 10245f4000                0 ffff9d0fa45f4c00  1 2fffff00004080 slab,head

crash> dma_page ffff9d0fa45f4d00
struct dma_page {
  page_list = {
    next = 0xffff9d0fa45f5000, 
    prev = 0xffff9d0fa45f4d00
  }, 
  vaddr = 0x0, -----------caq：也是null
  dma = 0, 
  in_use = 0, 
  offset = 0
}

crash> list dma_pool.page_list -H 0xffff9d0fa45f4c00 -s dma_page.offset,vaddr
ffff9d0fa45f4c80
  offset = 0
  vaddr = 0x0
ffff9d0fa45f4d00
  offset = 0
  vaddr = 0x0
ffff9d0fa45f5000
  offset = 0
  vaddr = 0x0
.........

看来不仅是第一个dma_page有问题，所有在pool中的dma_page单元都一样。那直接查看一下dma_page的正常大小：

crash> p sizeof(struct dma_page)
$3 = 40

按道理长度才40字节，就算申请slab的话，也应该扩展为64字节才对，怎么可能像上面那个dma_page。一样是128字节呢？为了解开这个疑惑，找一个正常的其他节点对比一下：

crash> net
   NET_DEVICE     NAME   IP ADDRESS(ES)
ffff8f9e800be000  lo     127.0.0.1
ffff8f9e62640000  p1p1   
ffff8f9e626c0000  p1p2   
ffff8f9e627c0000  p3p1   -----//caq:以这个为例
ffff8f9e62100000  p3p2   

然后根据代码：通过net_device查看mlx5e_priv：

static int mlx5e_get_link_ksettings(struct net_device *netdev,
                    struct ethtool_link_ksettings *link_ksettings)
{
...
    struct mlx5e_priv *priv    = netdev_priv(netdev);
...
}

static inline void *netdev_priv(const struct net_device *dev)
{
  return (char *)dev + ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
}

crash> px sizeof(struct net_device)
$2 = 0x8c0

crash> mlx5e_priv.mdev ffff8f9e627c08c0---根据偏移计算
  mdev = 0xffff8f9e67c400c0

crash> mlx5_core_dev.cmd 0xffff8f9e67c400c0 -xo
struct mlx5_core_dev {
  [ffff8f9e67c40138] struct mlx5_cmd cmd;
}

crash> mlx5_cmd.pool ffff8f9e67c40138
  pool = 0xffff8f9e7bf48f80

crash> dma_pool 0xffff8f9e7bf48f80
struct dma_pool {
  page_list = {
    next = 0xffff8f9e79c60880, //caq:其中的一个dma_page
    prev = 0xffff8fae6e4db800
  }, 
.......
  size = 1024, 
  dev = 0xffff8f9e800b3098, 
  allocation = 4096, 
  boundary = 4096, 
  name = "mlx5_cmd\000\217\364{\236\217\377\377\300\217\364{\236\217\377\377\200\234>\250\217\217\377\377", 
  pools = {
    next = 0xffff8f9e800b3290, 
    prev = 0xffff8f9e800b3290
  }
}
crash> dma_page 0xffff8f9e79c60880     //caq:查看这个dma_page
struct dma_page {
  page_list = {
    next = 0xffff8f9e79c60840, -------其中的一个dma_page
    prev = 0xffff8f9e7bf48f80
  }, 
  vaddr = 0xffff8f9e6fc9b000, //caq:正常vaddr不可能会NULL的
  dma = 69521223680, 
  in_use = 0, 
  offset = 0
}

crash> kmem 0xffff8f9e79c60880
CACHE            NAME             OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE
ffff8f8fbfc07b00 kmalloc-64--正常长度    64     667921    745024  11641  4k
  SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
  ffffde5140e71800  ffff8f9e79c60000     0     64         64     0
  FREE / [ALLOCATED]
  [ffff8f9e79c60880]

      PAGE         PHYSICAL      MAPPING       INDEX CNT FLAGS
ffffde5140e71800 1039c60000                0        0  1 2fffff00000080 slab

以上操作要求对net_device和mlx5相关驱动代码比较熟悉。相比于异常的dma_page,正常的dma_page是一个64字节的slab，所以很明显，要么这个是一个踩内存问题，要么是一个uaf(used after free )问题。一般问题查到这，怎么快速判断是哪一种类型呢？因为这两种问题，涉及到内存紊乱，一般都比较难查，这时候需要跳出来，我们先看一下其他运行进程的情况，找到了一个进程如下：

crash> bt 48263
PID: 48263  TASK: ffff9d0f4ee0a0e0  CPU: 56  COMMAND: "reboot"
 #0 [ffff9d0f95d7f958] __schedule at ffffffffb6d80d4a
 #1 [ffff9d0f95d7f9e8] schedule at ffffffffb6d811f9
 #2 [ffff9d0f95d7f9f8] schedule_timeout at ffffffffb6d7ec48
 #3 [ffff9d0f95d7faa8] wait_for_completion_timeout at ffffffffb6d81ae5
 #4 [ffff9d0f95d7fb08] cmd_exec at ffffffffc03e41c9 [mlx5_core]
 #5 [ffff9d0f95d7fba8] mlx5_cmd_exec at ffffffffc03e442b [mlx5_core]
 #6 [ffff9d0f95d7fbd8] mlx5_core_destroy_mkey at ffffffffc03f085d [mlx5_core]
 #7 [ffff9d0f95d7fc40] mlx5_mr_cache_cleanup at ffffffffc0c60aab [mlx5_ib]
 #8 [ffff9d0f95d7fca8] mlx5_ib_stage_pre_ib_reg_umr_cleanup at ffffffffc0c45d32 [mlx5_ib]
 #9 [ffff9d0f95d7fcc0] __mlx5_ib_remove at ffffffffc0c4f450 [mlx5_ib]
#10 [ffff9d0f95d7fce8] mlx5_ib_remove at ffffffffc0c4f4aa [mlx5_ib]
#11 [ffff9d0f95d7fd00] mlx5_detach_device at ffffffffc03fe231 [mlx5_core]
#12 [ffff9d0f95d7fd30] mlx5_unload_one at ffffffffc03dee90 [mlx5_core]
#13 [ffff9d0f95d7fd60] shutdown at ffffffffc03def80 [mlx5_core]
#14 [ffff9d0f95d7fd80] pci_device_shutdown at ffffffffb69d1cda
#15 [ffff9d0f95d7fda8] device_shutdown at ffffffffb6ab3beb
#16 [ffff9d0f95d7fdd8] kernel_restart_prepare at ffffffffb66b7916
#17 [ffff9d0f95d7fde8] kernel_restart at ffffffffb66b7932
#18 [ffff9d0f95d7fe00] SYSC_reboot at ffffffffb66b7ba9
#19 [ffff9d0f95d7ff40] sys_reboot at ffffffffb66b7c4e
#20 [ffff9d0f95d7ff50] system_call_fastpath at ffffffffb6d8dede
    RIP: 00007fc9be7a5226  RSP: 00007ffd9a19e448  RFLAGS: 00010246
    RAX: 00000000000000a9  RBX: 0000000000000004  RCX: 0000000000000000
    RDX: 0000000001234567  RSI: 0000000028121969  RDI: fffffffffee1dead
    RBP: 0000000000000002   R8: 00005575d529558c   R9: 0000000000000000
    R10: 00007fc9bea767b8  R11: 0000000000000206  R12: 0000000000000000
    R13: 00007ffd9a19e690  R14: 0000000000000000  R15: 0000000000000000
    ORIG_RAX: 00000000000000a9  CS: 0033  SS: 002b

为什么会关注这个进程，因为这么多年以来，因为卸载模块引发的uaf问题排查不低于20次了，有时候是reboot，有时候是unload，有时候是在work中释放资源，所以直觉上，觉得和这个卸载有很大关系。下面分析一下，reboot流程里面操作到哪了。

2141 void device_shutdown(void)
   2142 {
   2143         struct device *dev, *parent;
   2144 
   2145         spin_lock(&devices_kset->list_lock);
   2146         /*
   2147          * Walk the devices list backward, shutting down each in turn.
   2148          * Beware that device unplug events may also start pulling
   2149          * devices offline, even as the system is shutting down.
   2150          */
   2151         while (!list_empty(&devices_kset->list)) {
   2152                 dev = list_entry(devices_kset->list.prev, struct device,
   2153                                 kobj.entry);
........
   2178                 if (dev->device_rh && dev->device_rh->class_shutdown_pre) {
   2179                         if (initcall_debug)
   2180                                 dev_info(dev, "shutdown_pre\n");
   2181                         dev->device_rh->class_shutdown_pre(dev);
   2182                 }
   2183                 if (dev->bus && dev->bus->shutdown) {
   2184                         if (initcall_debug)
   2185                                 dev_info(dev, "shutdown\n");
   2186                         dev->bus->shutdown(dev);
   2187                 } else if (dev->driver && dev->driver->shutdown) {
   2188                         if (initcall_debug)
   2189                                 dev_info(dev, "shutdown\n");
   2190                         dev->driver->shutdown(dev);
   2191                 }
   }

从上面代码看出以下两点：1.每个device 的 kobj.entry 成员串接在 devices_kset->list 中。2. 每个设备的shutdown流程从 device_shutdown 看是串行的。从reboot 的堆栈看，卸载一个 mlx设备的流程包含如下：pci_device_shutdown-->shutdown-->mlx5_unload_one-->mlx5_detach_device              -->mlx5_cmd_cleanup-->dma_pool_destroymlx5_detach_device的流程分支为：

void dma_pool_destroy(struct dma_pool *pool)
{
.......
        while (!list_empty(&pool->page_list)) {//caq:将pool中的dma_page一一删除
                struct dma_page *page;
                page = list_entry(pool->page_list.next,
                                  struct dma_page, page_list);
                if (is_page_busy(page)) {
.......
                        list_del(&page->page_list);
                        kfree(page);
                } else
                        pool_free_page(pool, page);//每个dma_page去释放
        }

        kfree(pool);//caq：释放pool
.......        
}

static void pool_free_page(struct dma_pool *pool, struct dma_page *page)
{
        dma_addr_t dma = page->dma;

#ifdef  DMAPOOL_DEBUG
        memset(page->vaddr, POOL_POISON_FREED, pool->allocation);
#endif
        dma_free_coherent(pool->dev, pool->allocation, page->vaddr, dma);
        list_del(&page->page_list);//caq:释放后会将page_list成员毒化
        kfree(page);
}

从reboot的堆栈中，查看对应的信息

 #4 [ffff9d0f95d7fb08] cmd_exec at ffffffffc03e41c9 [mlx5_core]
    ffff9d0f95d7fb10: ffffffffb735b580 ffff9d0f904caf18 
    ffff9d0f95d7fb20: ffff9d00ff801da8 ffff9d0f23121200 
    ffff9d0f95d7fb30: ffff9d0f23121740 ffff9d0fa7480138 
    ffff9d0f95d7fb40: 0000000000000000 0000001002020000 
    ffff9d0f95d7fb50: 0000000000000000 ffff9d0f95d7fbe8 
    ffff9d0f95d7fb60: ffff9d0f00000000 0000000000000000 
    ffff9d0f95d7fb70: 00000000756415e3 ffff9d0fa74800c0 ----mlx5_core_dev设备，对应的是 p3p1，
    ffff9d0f95d7fb80: ffff9d0f95d7fbf8 ffff9d0f95d7fbe8 
    ffff9d0f95d7fb90: 0000000000000246 ffff9d0f8f3a20b8 
    ffff9d0f95d7fba0: ffff9d0f95d7fbd0 ffffffffc03e442b 
 #5 [ffff9d0f95d7fba8] mlx5_cmd_exec at ffffffffc03e442b [mlx5_core]
    ffff9d0f95d7fbb0: 0000000000000000 ffff9d0fa74800c0 
    ffff9d0f95d7fbc0: ffff9d0f8f3a20b8 ffff9d0fa74bea00 
    ffff9d0f95d7fbd0: ffff9d0f95d7fc38 ffffffffc03f085d 
 #6 [ffff9d0f95d7fbd8] mlx5_core_destroy_mkey at ffffffffc03f085d [mlx5_core]

要注意，reboot正在释放的 mlx5_core_dev 是 ffff9d0fa74800c0，这个设备对应的net_device是：p3p1,而 23283 进程正在访问的 mlx5_core_dev 是 ffff9d0fa3c800c0 ，对应的是 p3p2。

crash> net
   NET_DEVICE     NAME   IP ADDRESS(ES)
ffff9d0fc003e000  lo     127.0.0.1
ffff9d1fad200000  p1p1   
ffff9d0fa0700000  p1p2   
ffff9d0fa00c0000  p3p1  对应的 mlx5_core_dev 是 ffff9d0fa74800c0
ffff9d0fa0200000  p3p2  对应的 mlx5_core_dev 是 ffff9d0fa3c800c0

我们看下目前还残留在 devices_kset 中的device：

crash> p devices_kset
devices_kset = $4 = (struct kset *) 0xffff9d1fbf4e70c0
crash> p devices_kset.list
$5 = {
  next = 0xffffffffb72f2a38, 
  prev = 0xffff9d0fbe0ea130
}

crash> list -H -o 0x18 0xffffffffb72f2a38 -s device.kobj.name >device.list

我们发现p3p1 与 p3p2均不在 device.list中，

[root@it202-seg-k8s-prod001-node-10-27-96-220 127.0.0.1-2020-12-07-10:58:06]# grep 0000:5e:00.0 device.list //caq:未找到 这个是 p3p1，当前reboot流程正在卸载。
[root@it202-seg-k8s-prod001-node-10-27-96-220 127.0.0.1-2020-12-07-10:58:06]# grep 0000:5e:00.1 device.list //caq:未找到，这个是 p3p2,已经卸载完
[root@it202-seg-k8s-prod001-node-10-27-96-220 127.0.0.1-2020-12-07-10:58:06]# grep 0000:3b:00.0  device.list //caq:这个mlx5设备还没unload
  kobj.name = 0xffff9d1fbe82aa70 "0000:3b:00.0",
[root@it202-seg-k8s-prod001-node-10-27-96-220 127.0.0.1-2020-12-07-10:58:06]# grep 0000:3b:00.1 device.list //caq:这个mlx5设备还没unload
  kobj.name = 0xffff9d1fbe82aae0 "0000:3b:00.1",

由于 p3p2 与 p3p1均不在 device.list中，而根据 pci_device_shutdown的串行卸载流程，当前正在卸载的是 p3p1，所以很确定的是 23283 进程访问的是卸载后的cmd_pool,根据前面描述的卸载流程 ：pci_device_shutdown-->shutdown-->mlx5_unload_one-->mlx5_cmd_cleanup-->dma_pool_destroy此时的pool已经被释放了，pool中的dma_page均无效的。然后尝试google对应的bug，查看到一个跟当前现象极为相似，redhat遇到了类似的问题：https://access.redhat.com/solutions/5132931但是，红帽在这个链接中认为解决了uaf的问题，合入的补丁却是：

commit 4cca96a8d9da0ed8217cfdf2aec0c3c8b88e8911
Author: Parav Pandit <[e-mail]>
Date:   Thu Dec 12 13:30:21 2019 +0200

diff --git a/drivers/infiniband/hw/mlx5/main.c b/drivers/infiniband/hw/mlx5/main.c
index 997cbfe..05b557d 100644
--- a/drivers/infiniband/hw/mlx5/main.c
+++ b/drivers/infiniband/hw/mlx5/main.c
@@ -6725,6 +6725,8 @@ void __mlx5_ib_remove(struct mlx5_ib_dev *dev,
                      const struct mlx5_ib_profile *profile,
                      int stage)
{
+       dev->ib_active = false;
+
        /* Number of stages to cleanup */
        while (stage) {
                stage--;

敲黑板，三遍：这个合入是不能解决对应的bug的，比如如下的并发：我们用一个简单的图来表示一下并发处理：

  CPU1                                                            CPU2
                                                                   dev_attr_show
    pci_device_shutdown                                            speed_show
      shutdown                          
        mlx5_unload_one
          mlx5_detach_device
            mlx5_detach_interface
              mlx5e_detach
               mlx5e_detach_netdev
                 mlx5e_nic_disable
                   rtnl_lock
                     mlx5e_close_locked 
                     clear_bit(MLX5E_STATE_OPENED, &priv->state);---只清理了这个bit
                   rtnl_unlock                   
                                                  rtnl_trylock---持锁成功后
                                                  netif_running 只是判断net_device.state的最低位
                                                    __ethtool_get_link_ksettings
                                                    mlx5e_get_link_ksettings
                                                      mlx5_query_port_ptys()
                                                      mlx5_core_access_reg()
                                                      mlx5_cmd_exec
                                                      cmd_exec
                                                      mlx5_alloc_cmd_msg
          mlx5_cmd_cleanup---清理dma_pool                                                       
                                                      dma_pool_alloc---访问cmd.pool,触发crash

所以如果要真正解决这个问题，还需要 netif_device_detach 中清理 __LINK_STATE_START的bit位，或者在 speed_show 中判断一下 __LINK_STATE_PRESENT 位？如果考虑影响范围，不想动公共流程，则应该在 mlx5e_get_link_ksettings 中 判断一下 __LINK_STATE_PRESENT。这个就留给喜欢跟社区打交道的同学去完善吧。

static void mlx5e_nic_disable(struct mlx5e_priv *priv)
{
.......
  rtnl_lock();
  if (netif_running(priv->netdev))
    mlx5e_close(priv->netdev);
  netif_device_detach(priv->netdev);
  //caq:增加一下清理 __LINK_STATE_PRESENT位 
  rtnl_unlock();
.......

4. 故障复现

1. 竞态问题，可以制造类似上图cpu1 与cpu2 的竞争场景。

5. 故障规避或解决

可能的解决方案是：1. 不要按照红帽https://access.redhat.com/solutions/5132931那样升级。2. 单独打补丁。

作者简介

Anqing  OPPO高级后端工程师

目前负责linux内核及容器，虚拟机等虚拟化方面的工作

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