Re: [syzbot] [mm?] KCSAN: data-race in generic_fillattr / shmem_unlink (3)

[Dmitry Vyukov]

On Mon, 24 Apr 2023 at 10:10, syzbot
<syzbot+f682b67a78ce05867e78@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    cb0856346a60 Merge tag 'mm-hotfixes-stable-2023-04-19-16-3..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10b9e8bfc80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=fa4baf7c6b35b5d5
> dashboard link: https://syzkaller.appspot.com/bug?extid=f682b67a78ce05867e78
> compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a02dd7789fb2/disk-cb085634.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/a1a1eac454f6/vmlinux-cb085634.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/bb0447014913/bzImage-cb085634.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+f682b67a78ce05867e78@xxxxxxxxxxxxxxxxxxxxxxxxx

I think shmem_unlink() needs to use i_size_write() to update the size.
Writes to i_size are not assumed to be atomic throughout the kernel
code.

> ==================================================================
> BUG: KCSAN: data-race in generic_fillattr / shmem_unlink
>
> read-write to 0xffff888104d167b8 of 8 bytes by task 23887 on cpu 1:
>  shmem_unlink+0x102/0x190 mm/shmem.c:3036
>  vfs_unlink+0x261/0x3e0 fs/namei.c:4250
>  do_unlinkat+0x266/0x4f0 fs/namei.c:4316
>  __do_sys_unlink fs/namei.c:4364 [inline]
>  __se_sys_unlink fs/namei.c:4362 [inline]
>  __x64_sys_unlink+0x30/0x40 fs/namei.c:4362
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> read to 0xffff888104d167b8 of 8 bytes by task 27823 on cpu 0:
>  i_size_read include/linux/fs.h:866 [inline]
>  generic_fillattr+0x13f/0x1e0 fs/stat.c:58
>  shmem_getattr+0x32a/0x3b0 mm/shmem.c:1068
>  vfs_getattr_nosec+0x1e3/0x3c0 fs/stat.c:133
>  vfs_getattr fs/stat.c:170 [inline]
>  vfs_statx+0x156/0x300 fs/stat.c:242
>  vfs_fstatat fs/stat.c:276 [inline]
>  __do_sys_newfstatat fs/stat.c:446 [inline]
>  __se_sys_newfstatat+0x8a/0x2a0 fs/stat.c:440
>  __x64_sys_newfstatat+0x55/0x60 fs/stat.c:440
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> value changed: 0x00000000000000c8 -> 0x00000000000000b4
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 27823 Comm: udevd Not tainted 6.3.0-rc7-syzkaller-00089-gcb0856346a60 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/30/2023
> ==================================================================
>
>
> ---
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