Re: [PATCH] fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds

Christian Brauner
2023-04-11T08:56:21Z

On Thu, Apr 06, 2023 at 02:51:06PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> Commit 56124d6c87fd ("fsverity: support enabling with tree block size <
> PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read
> the file's data, instead of direct pagecache accesses.
>
> An unintended consequence of this is that the
> 'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became
> reachable by fuzz tests. This happens if FS_IOC_ENABLE_VERITY is called
> on a fd opened with access mode 3, which means "ioctl access only".
>
> Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds. But
> ioctl-only fds are a weird Linux extension that is rarely used and that
> few people even know about. (The documentation for FS_IOC_ENABLE_VERITY
> even specifically says it requires O_RDONLY.) It's probably not
> worthwhile to make the ioctl internally open a new fd just to handle
> this case. Thus, just reject the ioctl on such fds for now.
>
> Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE")
> Reported-by: syzbot+51177e4144d764827c45@syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?id=2281afcbbfa8fdb92f9887479cc0e4180f1c6b28
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---

Looks good to me,
Reviewed-by: Christian Brauner <brauner@kernel.org>