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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 次要用于 vpc 等场景，智能网卡的代码随着
性能的加强导致复杂度始终在回升，驱动的 bug 始终是内核 bug 中的大头，在遇到相似问题时，内核开发者因为对驱动代码不相熟，排查会比拟吃力, 自身波及的背景常识有：dma_pool,dma_page,net_device,mlx5_core_dev 设施，设施卸载，uaf 问题等, 另外，这个 bug 目测在最新的 linux 基线也没有解决, 本文独自拿进去列举是因为 uaf 问题绝对比拟独特。
上面列一下咱们是怎么排查并解决这个问题的。
一、故障景象
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------------- 运行的工夫很短
LOAD AVERAGE: 0.25, 0.23, 0.11
TASKS: 2027
RELEASE: 3.10.0-1062.18.1.el7.x86_64
MEMORY: 127.6 GB
PANIC: "BUG: unable to handle kernel NULL pointer dereference at (null)"
PID: 23283
COMMAND: "spider-agent"
TASK: ffff9d1fbb090000 [THREAD_INFO: ffff9d1f9a0d8000]
CPU: 0
STATE: TASK_RUNNING (PANIC)
crash> bt
PID: 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: 异样地址
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]// 波及的模块
#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，是某过程读取文件触发了一个内核态的空指针援用。
二、故障景象剖析
从堆栈信息看：
1、过后过程关上 fd 编号为 13 的文件，这个从 rdi 的值能够看出。
2、speed_show 和 __ethtool_get_link_ksettings 示意在读取网卡的速率值
上面看下关上的文件是哪个，
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--- 留神对应关系 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--- 留神对应关系 0000:5e:00.1 对应 p3p2
....留神下面 pci 编号与 网卡名称的对应关系，前面会用到。
关上文件读取 speed 自身应该是一个很常见的流程，
上面从 exception RIP: dma_pool_alloc+427 进一步剖析为什么触发了 NULL pointer dereference
开展具体的堆栈如下：
#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常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，从下面的堆栈看，的确是 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: 以后满足条件
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: 示意正在援用
333 offset = page->offset;// 从上次用完的中央开始应用
334 page->offset = *(int *)(page->vaddr + offset);//caq: 出问题的行号
...
}从下面的代码看，page->vaddr 为 NULL,offset 也为 0，才会援用 NULL，page 有两个起源，
第一种是从 pool 中的 page_list 中取，
第二种是从 pool_alloc_page 长期申请，当然申请之后会挂入到 pool 中的 page_list,
上面查看一下这个 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 的确不为空，而且满足
if (page->offset < pool->allocation) 的条件，所以第一个 page 应该是 ffff9d0fa45f4c80
也就是从第一种状况取出的：
crash> dma_page ffff9d0fa45f4c80
struct dma_page {
page_list = {
next = 0xffff9d0fa45f4d00,
prev = 0xffff9d0fa45f4c80
},
vaddr = 0x0, //caq: 这个异样，援用这个将导致 crash
dma = 0,
in_use = 1, //caq: 这个标记为在应用，合乎 page->in_use++;
offset = 0
}
问题剖析到这里，因为 dma_pool 中的 page，申请之后，vaddr 都会初始化，
个别在 pool_alloc_page 中进行初始化，怎么可能会 NULL 呢？
而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: 留神这个长度 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因为以前用过相似的 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，
那间接查看一下 dma_page 的失常大小：
crash> p sizeof(struct dma_page)
$3 = 40按情理长度才 40 字节，就算申请 slab 的话，也应该扩大为 64 字节才对，怎么可能像下面那个 dma_page 一样是 128 字节呢？为了解开这个纳闷，找一个失常的其余节点比照一下：
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
而后依据代码：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--- 依据偏移计算
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: 失常 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-- 失常长度 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 相关驱动代码比拟相熟。
相比于异样的 dma_page, 失常的 dma_page 是一个 64 字节的 slab，所以很显著，
要么这个是一个踩内存问题，要么是一个 uaf(used after free)问题。
个别问题查到这，怎么疾速判断是哪一种类型呢？因为这两种问题，波及到内存错乱，个别都比拟难查，这时候须要跳进去，咱们先看一下其余运行过程的状况，找到了一个过程如下：
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
为什么会关注这个过程，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 中开释资源，所以直觉上，感觉和这个卸载有很大关系。上面剖析一下，reboot 流程外面操作到哪了。
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 }
}从下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、每个设施的 shutdown 流程从 device_shutdown 看是串行的。
从 reboot 的堆栈看，卸载一个 mlx 设施的流程蕴含如下：
pci_device_shutdown–>shutdown–>mlx5_unload_one–>mlx5_detach_device
-->mlx5_cmd_cleanup-->dma_pool_destroy
mlx5_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 去开释
}
kfree(pool);//caq：开释 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: 开释后会将 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 设施，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]要留神，reboot 正在开释的 mlx5_core_dev 是 ffff9d0fa74800c0，这个设施对应的 net_device 是：
p3p1, 而 23283 过程正在拜访的 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咱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
咱们发现 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，以后 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, 曾经卸载完
[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 设施还没 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 设施还没 unload
kobj.name = 0xffff9d1fbe82aae0 "0000:3b:00.1",
因为 p3p2 与 p3p1 均不在 device.list 中，而依据 pci_device_shutdown 的串行卸载流程，以后正在卸载的是 p3p1，所以很确定的是 23283 过程拜访的是卸载后的 cmd_pool, 依据后面形容的卸载流程：
pci_device_shutdown–>shutdown–>mlx5_unload_one–>mlx5_cmd_cleanup–>dma_pool_destroy
此时的 pool 曾经被开释了，pool 中的 dma_page 均有效的。
而后尝试 google 对应的 bug，查看到一个跟以后景象极为类似，redhat 遇到了相似的问题：https://access.redhat.com/sol…
然而，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 <parav@mellanox.com>
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，比方如下的并发：
咱们用一个简略的图来示意一下并发解决：
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--- 持锁胜利后
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--- 拜访 cmd.pool, 触发 crash
所以如果要真正解决这个问题，还须要 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？如果思考影响范畴，不想动公共流程，则应该
在 mlx5e_get_link_ksettings 中判断一下 __LINK_STATE_PRESENT。
这个就留给喜爱跟社区打交道的同学去欠缺吧。
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: 减少一下清理 __LINK_STATE_PRESENT 位
rtnl_unlock();
.......三、故障复现
1、竞态问题，能够制作相似上图 cpu1 与 cpu2 的竞争场景。
四、故障躲避或解决
可能的解决方案是：
1、不要依照红帽 https://access.redhat.com/sol… 那样降级。
2、独自打补丁。
作者简介
Anqing
目前在 OPPO 混合云负责 linux 内核及容器，虚拟机等虚拟化方面的工作
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