fsverity: Explicitly require FMODE_READ to retrieve hashes

The comment in fsverity_ioctl_read_metadata(), which justifies not
checking for FMODE_READ, doesn't consider that the nonstandard access
mode 3 (ioctl-only) exists.

Opening an ioctl-only fd generally requires read (and write) permission.
However, there are gaps with certain LSMs: an ioctl-only fd requests
read (and write) permission via security_inode_permission, but not
security_file_open.  With certain LSMs relying on the latter, such as
Landlock and AppArmor, an ioctl-only fd can be opened without the LSM
granting read (and write) permission.

Since FS_IOC_READ_VERITY_METADATA returns hashes of file contents
blocks, it should require read permission.  This also applies to
FS_IOC_MEASURE_VERITY, which returns a hash for the whole file.

Meanwhile, ioctl-only fds are a rarely-used feature, not known to be
used with either ioctl.

Therefore, just disallow these ioctls on ioctl-only file descriptors,
similar to what was already done with FS_IOC_ENABLE_VERITY.

More specifically, explicitly check for FMODE_READ.  This also covers
write-only fds to non-verity files which otherwise would have failed
with ENODATA.  This change in error code for write-only fds should be
fine too, as these ioctls cannot be usefully used with write-only fds
anyway, but it's worth noting as another technicality.

Fixes: 4dd893d832 ("fs-verity: implement FS_IOC_MEASURE_VERITY ioctl")
Fixes: e17fe6579d ("fs-verity: add FS_IOC_READ_VERITY_METADATA ioctl")
Cc: stable@vger.kernel.org
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
Link: https://patch.msgid.link/20261022211522.1489943-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Documentation/filesystems/fsverity.rst

@@ -170,6 +170,7 @@ fatal signal), no changes are made to the file.
 FS_IOC_ENABLE_VERITY can fail with the following errors:
 
 - ``EACCES``: the process does not have write access to the file
+- ``EBADF``: the file descriptor is not open for reading
 - ``EBADMSG``: the builtin signature is malformed
 - ``EBUSY``: this ioctl is already running on the file
 - ``EEXIST``: the file already has verity enabled
@@ -232,6 +233,7 @@ regardless of the size of the file.
 
 FS_IOC_MEASURE_VERITY can fail with the following errors:
 
+- ``EBADF``: the file descriptor is not open for reading
 - ``EFAULT``: the caller provided inaccessible memory
 - ``ENODATA``: the file is not a verity file
 - ``ENOTTY``: this type of filesystem does not implement fs-verity
@@ -310,6 +312,7 @@ blocks from disk without actually verifying the path to the root node.
 
 FS_IOC_READ_VERITY_METADATA can fail with the following errors:
 
+- ``EBADF``: the file descriptor is not open for reading
 - ``EFAULT``: the caller provided inaccessible memory
 - ``EINTR``: the ioctl was interrupted before any data was read
 - ``EINVAL``: reserved fields were set, or ``offset + length``

fs/verity/measure.c

@@ -31,6 +31,16 @@ int fsverity_ioctl_measure(struct file *filp, void __user *_uarg)
 	const struct fsverity_hash_alg *hash_alg;
 	struct fsverity_digest arg;
 
+	/*
+	 * This ioctl returns a hash of the file contents, so it should require
+	 * read permission.  Disallow it on ioctl-only (i.e., nonstandard access
+	 * mode 3) fds, which can in some cases be acquired without passing all
+	 * LSM read-permission checks.  Also disallow write-only fds, which can
+	 * refer only to non-verity files which would fail with ENODATA anyway.
+	 */
+	if (!(filp->f_mode & FMODE_READ))
+		return -EBADF;
+
 	vi = fsverity_get_info(inode);
 	if (!vi)
 		return -ENODATA; /* not a verity file */

fs/verity/read_metadata.c

@@ -166,13 +166,19 @@ int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg)
 	int length;
 	void __user *buf;
 
+	/*
+	 * This ioctl returns file contents block hashes, so it should require
+	 * read permission.  Disallow it on ioctl-only (i.e., nonstandard access
+	 * mode 3) fds, which can in some cases be acquired without passing all
+	 * LSM read-permission checks.  Also disallow write-only fds, which can
+	 * refer only to non-verity files which would fail with ENODATA anyway.
+	 */
+	if (!(filp->f_mode & FMODE_READ))
+		return -EBADF;
+
 	vi = fsverity_get_info(inode);
 	if (!vi)
 		return -ENODATA; /* not a verity file */
-	/*
-	 * Note that we don't have to explicitly check that the file is open for
-	 * reading, since verity files can only be opened for reading.
-	 */
 
 	if (copy_from_user(&arg, uarg, sizeof(arg)))
 		return -EFAULT;