Jeongjun Park aha310510@gmail.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@vger.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@googlegroups.com Fixes: 44a30220bc0a ("shmem: recalculate file inode when fstat") Signed-off-by: Jeongjun Park aha310510@gmail.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;
  

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