Re: [syzbot] [mm?] KCSAN: data-race in generic_fillattr / shmem_mknod (2)

[Dmitry Vyukov]

On Mon, 24 Apr 2023 at 09:58, syzbot
<syzbot+702361cf7e3d95758761@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    457391b03803 Linux 6.3
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13226cf0280000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8c81c9a3d360ebcf
> dashboard link: https://syzkaller.appspot.com/bug?extid=702361cf7e3d95758761
> 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/6438dcb1c42d/disk-457391b0.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/20596c249e47/vmlinux-457391b0.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/cb715366a3e5/bzImage-457391b0.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+702361cf7e3d95758761@xxxxxxxxxxxxxxxxxxxxxxxxx

I think shmem_mknod() 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_mknod
>
> read-write to 0xffff8881049a9408 of 8 bytes by task 3383 on cpu 0:
>  shmem_mknod+0xdd/0x180 mm/shmem.c:2942
>  shmem_create+0x34/0x40 mm/shmem.c:2994
>  lookup_open fs/namei.c:3416 [inline]
>  open_last_lookups fs/namei.c:3484 [inline]
>  path_openat+0xd96/0x1d00 fs/namei.c:3712
>  do_filp_open+0xf6/0x200 fs/namei.c:3742
>  do_sys_openat2+0xb5/0x2a0 fs/open.c:1348
>  do_sys_open fs/open.c:1364 [inline]
>  __do_sys_openat fs/open.c:1380 [inline]
>  __se_sys_openat fs/open.c:1375 [inline]
>  __x64_sys_openat+0xf3/0x120 fs/open.c:1375
>  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 0xffff8881049a9408 of 8 bytes by task 4291 on cpu 1:
>  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: 0x0000000000001644 -> 0x0000000000001658
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 4291 Comm: udevd Not tainted 6.3.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/14/2023
> ==================================================================
>
>
> ---
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>
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