[PATCH 6.6 464/538] f2fs: Require FMODE_WRITE for atomic write ioctls

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6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

commit 4f5a100f87f32cb65d4bb1ad282a08c92f6f591e upstream.

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>
Reviewed-by: Chao Yu <chao@xxxxxxxxxx>
Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/f2fs/file.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2100,6 +2100,9 @@ static int f2fs_ioc_start_atomic_write(s
 	loff_t isize;
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(idmap, inode))
 		return -EACCES;
 
@@ -2206,6 +2209,9 @@ static int f2fs_ioc_commit_atomic_write(
 	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;
 
@@ -2238,6 +2244,9 @@ static int f2fs_ioc_abort_atomic_write(s
 	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;
 






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