fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds

Commit 56124d6c ("fsverity: support enabling with tree block size <
PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read
the file's data, instead of direct pagecache accesses.

An unintended consequence of this is that the
'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became
reachable by fuzz tests.  This happens if FS_IOC_ENABLE_VERITY is called
on a fd opened with access mode 3, which means "ioctl access only".

Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds.  But
ioctl-only fds are a weird Linux extension that is rarely used and that
few people even know about.  (The documentation for FS_IOC_ENABLE_VERITY
even specifically says it requires O_RDONLY.)  It's probably not
worthwhile to make the ioctl internally open a new fd just to handle
this case.  Thus, just reject the ioctl on such fds for now.

Fixes: 56124d6c ("fsverity: support enabling with tree block size < PAGE_SIZE")
Reported-by: syzbot+51177e4144d764827c45@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=2281afcbbfa8fdb92f9887479cc0e4180f1c6b28
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230406215106.235829-1-ebiggers@kernel.orgReviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/verity/enable.c

@@ -357,6 +357,13 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 	err = file_permission(filp, MAY_WRITE);
 	if (err)
 		return err;
+	/*
+	 * __kernel_read() is used while building the Merkle tree.  So, we can't
+	 * allow file descriptors that were opened for ioctl access only, using
+	 * the special nonstandard access mode 3.  O_RDONLY only, please!
+	 */
+	if (!(filp->f_mode & FMODE_READ))
+		return -EBADF;
 
 	if (IS_APPEND(inode))
 		return -EPERM;