linux-kernel - Re: [PATCH RESEND] mm: shmem: fix data-race in shmem

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Message-ID: <CAO9qdTG9wxBSct-rrgG_DevaLZgy4AFirKu+3uu=HOPD1-PRBg@mail.gmail.com>
Date: Wed, 16 Oct 2024 23:12:43 +0900
From: Jeongjun Park <aha310510@...il.com>
To: akpm@...ux-foundation.org, hughd@...gle.com
Cc: yuzhao@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	syzbot <syzkaller@...glegroups.com>, stable@...r.kernel.org
Subject: Re: [PATCH RESEND] mm: shmem: fix data-race in shmem_getattr()

Jeongjun Park <aha310510@...il.com> wrote:
>
> I got the following KCSAN report during syzbot testing:
>
> ==================================================================
> BUG: KCSAN: data-race in generic_fillattr / inode_set_ctime_current
>
> write to 0xffff888102eb3260 of 4 bytes by task 6565 on cpu 1:
>  inode_set_ctime_to_ts include/linux/fs.h:1638 [inline]
>  inode_set_ctime_current+0x169/0x1d0 fs/inode.c:2626
>  shmem_mknod+0x117/0x180 mm/shmem.c:3443
>  shmem_create+0x34/0x40 mm/shmem.c:3497
>  lookup_open fs/namei.c:3578 [inline]
>  open_last_lookups fs/namei.c:3647 [inline]
>  path_openat+0xdbc/0x1f00 fs/namei.c:3883
>  do_filp_open+0xf7/0x200 fs/namei.c:3913
>  do_sys_openat2+0xab/0x120 fs/open.c:1416
>  do_sys_open fs/open.c:1431 [inline]
>  __do_sys_openat fs/open.c:1447 [inline]
>  __se_sys_openat fs/open.c:1442 [inline]
>  __x64_sys_openat+0xf3/0x120 fs/open.c:1442
>  x64_sys_call+0x1025/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:258
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> read to 0xffff888102eb3260 of 4 bytes by task 3498 on cpu 0:
>  inode_get_ctime_nsec include/linux/fs.h:1623 [inline]
>  inode_get_ctime include/linux/fs.h:1629 [inline]
>  generic_fillattr+0x1dd/0x2f0 fs/stat.c:62
>  shmem_getattr+0x17b/0x200 mm/shmem.c:1157
>  vfs_getattr_nosec fs/stat.c:166 [inline]
>  vfs_getattr+0x19b/0x1e0 fs/stat.c:207
>  vfs_statx_path fs/stat.c:251 [inline]
>  vfs_statx+0x134/0x2f0 fs/stat.c:315
>  vfs_fstatat+0xec/0x110 fs/stat.c:341
>  __do_sys_newfstatat fs/stat.c:505 [inline]
>  __se_sys_newfstatat+0x58/0x260 fs/stat.c:499
>  __x64_sys_newfstatat+0x55/0x70 fs/stat.c:499
>  x64_sys_call+0x141f/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:263
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> value changed: 0x2755ae53 -> 0x27ee44d3
>
> Since there is no special protection when shmem_getattr() calls
> generic_fillattr(), data-race occurs by functions such as shmem_unlink()
> or shmem_mknod(). This can cause unexpected results, so commenting it out
> is not enough.
>
> Therefore, when calling generic_fillattr() from shmem_getattr(), it is
> appropriate to protect the inode using inode_lock_shared() and
> inode_unlock_shared() to prevent data-race.
>

Cc: stable@...r.kernel.org

I think this patch should be applied from next rc version and also stable
version. When calling generic_fillattr(), if you don't hold read lock,
data-race will occur in inode member variables, which can cause unexpected
behavior. This problem is also present in several stable versions, so I think
it should be fixed as soon as possible.

Regards,

Jeongjun Park

> Reported-by: syzbot <syzkaller@...glegroups.com>
> Fixes: 44a30220bc0a ("shmem: recalculate file inode when fstat")
> Signed-off-by: Jeongjun Park <aha310510@...il.com>
> ---
>  mm/shmem.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5a77acf6ac6a..9beeb47c3743 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1154,7 +1154,9 @@ static int shmem_getattr(struct mnt_idmap *idmap,
>         stat->attributes_mask |= (STATX_ATTR_APPEND |
>                         STATX_ATTR_IMMUTABLE |
>                         STATX_ATTR_NODUMP);
> +       inode_lock_shared(inode);
>         generic_fillattr(idmap, request_mask, inode, stat);
> +       inode_unlock_shared(inode);
>
>         if (shmem_is_huge(inode, 0, false, NULL, 0))
>                 stat->blksize = HPAGE_PMD_SIZE;
> --