f2fs: Require FMODE_WRITE for atomic write ioctls

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@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index bddcb2cd945aa03898f08997561d16e3f1144240..2c591fbc75a9e5d0a4ca8a19a736a82b43edbdc5 100644 (file)
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2131,6 +2131,9 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
        loff_t isize;
        int ret;
 
+       if (!(filp->f_mode & FMODE_WRITE))
+               return -EBADF;
+
        if (!inode_owner_or_capable(idmap, inode))
                return -EACCES;
 
@@ -2239,6 +2242,9 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
        struct mnt_idmap *idmap = file_mnt_idmap(filp);
        int ret;
 
+       if (!(filp->f_mode & FMODE_WRITE))
+               return -EBADF;
+
        if (!inode_owner_or_capable(idmap, inode))
                return -EACCES;
 
@@ -2271,6 +2277,9 @@ static int f2fs_ioc_abort_atomic_write(struct file *filp)
        struct mnt_idmap *idmap = file_mnt_idmap(filp);
        int ret;
 
+       if (!(filp->f_mode & FMODE_WRITE))
+               return -EBADF;
+
        if (!inode_owner_or_capable(idmap, inode))
                return -EACCES;