Re: [f2fs-dev] [PATCH] f2fs: Require FMODE_WRITE for atomic write ioctls

[Eric Biggers]

On Tue, Aug 06, 2024 at 04:07:16PM +0200, Jann Horn via Linux-f2fs-devel wrote:
> The F2FS ioctls for starting and committing atomic writes check for
> inode_owner_or_capable(), but this does not give LSMs like SELinux or
> Landlock an opportunity to deny the write access - if the caller's FSUID
> matches the inode's UID, inode_owner_or_capable() immediately returns true.
> 
> There are scenarios where LSMs want to deny a process the ability to write
> particular files, even files that the FSUID of the process owns; but this
> can currently partially be bypassed using atomic write ioctls in two ways:
> 
>  - F2FS_IOC_START_ATOMIC_REPLACE + F2FS_IOC_COMMIT_ATOMIC_WRITE can
>    truncate an inode to size 0
>  - F2FS_IOC_START_ATOMIC_WRITE + F2FS_IOC_ABORT_ATOMIC_WRITE can revert
>    changes another process concurrently made to a file
> 
> Fix it by requiring FMODE_WRITE for these operations, just like for
> F2FS_IOC_MOVE_RANGE. Since any legitimate caller should only be using these
> ioctls when intending to write into the file, that seems unlikely to break
> anything.
> 
> Fixes: 88b88a667971 ("f2fs: support atomic writes")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  fs/f2fs/file.c | 9 +++++++++
>  1 file changed, 9 insertions(+)

Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>

- Eric