f2fs: Require FMODE_WRITE for atomic write ioctls

From: Jann Horn <[email protected]>

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.

---
 fs/f2fs/file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 62f2521cd955e..2982cc5b37d78 100644
@@ -2103,10 +2103,13 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
 	struct inode *pinode;
 	loff_t isize;
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;
 
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;

@@ -2200,10 +2203,13 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;
 
 	ret = mnt_want_write_file(filp);
 	if (ret)

@@ -2232,10 +2238,13 @@ static int f2fs_ioc_abort_atomic_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;
 
 	ret = mnt_want_write_file(filp);
 	if (ret)

base-commit: aa4cd140bba57b7064b4c7a7141bebd336d32087

2.47.0.rc0.187.ge670bccf7e-goog

I've queued these backports, thanks!

Thanks,
Sasha