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>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index bddcb2cd945a..2c591fbc75a9 100644
--- 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;
