fs-verity: factor out fsverity_get_descriptor()

The FS_IOC_READ_VERITY_METADATA ioctl will need to return the fs-verity
descriptor (and signature) to userspace.

There are a few ways we could implement this:

- Save a copy of the descriptor (and signature) in the fsverity_info
struct that hangs off of the in-memory inode. However, this would
waste memory since most of the time it wouldn't be needed.

- Regenerate the descriptor from the merkle_tree_params in the
fsverity_info. However, this wouldn't work for the signature, nor for
the salt which the merkle_tree_params only contains indirectly as part
of the 'hashstate'. It would also be error-prone.

- Just get them from the filesystem again. The disadvantage is that in
general we can't trust that they haven't been maliciously changed
since the file has opened. However, the use cases for
FS_IOC_READ_VERITY_METADATA don't require that it verifies the chain
of trust. So this is okay as long as we do some basic validation.

In preparation for implementing the third option, factor out a helper
function fsverity_get_descriptor() which gets the descriptor (and
appended signature) from the filesystem and does some basic validation.

As part of this, start checking the sig_size field for overflow.
Currently fsverity_verify_signature() does this. But the new ioctl will
need this too, so do it earlier.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Victor Hsieh <[e-mail]>
Reviewed-by: Jaegeuk Kim <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>

fs/verity/fsverity_private.h

···
 				     const u8 *salt, size_t salt_size);
 
 struct fsverity_info *fsverity_create_info(const struct inode *inode,
-					   void *desc, size_t desc_size);
+					   struct fsverity_descriptor *desc,
+					   size_t desc_size);
 
 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi);
 
 void fsverity_free_info(struct fsverity_info *vi);
+
+int fsverity_get_descriptor(struct inode *inode,
+			    struct fsverity_descriptor **desc_ret,
+			    size_t *desc_size_ret);
 
 int __init fsverity_init_info_cache(void);
 void __init fsverity_exit_info_cache(void);

fs/verity/open.c

···
 }
 
 /*
- * Validate the given fsverity_descriptor and create a new fsverity_info from
- * it.  The signature (if present) is also checked.
+ * Create a new fsverity_info from the given fsverity_descriptor (with optional
+ * appended signature), and check the signature if present.  The
+ * fsverity_descriptor must have already undergone basic validation.
  */
 struct fsverity_info *fsverity_create_info(const struct inode *inode,
-					   void *_desc, size_t desc_size)
+					   struct fsverity_descriptor *desc,
+					   size_t desc_size)
 {
-	struct fsverity_descriptor *desc = _desc;
 	struct fsverity_info *vi;
 	int err;
-
-	if (desc_size < sizeof(*desc)) {
-		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
-			     desc_size);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (desc->version != 1) {
-		fsverity_err(inode, "Unrecognized descriptor version: %u",
-			     desc->version);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
-		fsverity_err(inode, "Reserved bits set in descriptor");
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (desc->salt_size > sizeof(desc->salt)) {
-		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (le64_to_cpu(desc->data_size) != inode->i_size) {
-		fsverity_err(inode,
-			     "Wrong data_size: %llu (desc) != %lld (inode)",
-			     le64_to_cpu(desc->data_size), inode->i_size);
-		return ERR_PTR(-EINVAL);
-	}
 
 	vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
 	if (!vi)
···
 	kmem_cache_free(fsverity_info_cachep, vi);
 }
 
-/* Ensure the inode has an ->i_verity_info */
-static int ensure_verity_info(struct inode *inode)
+static bool validate_fsverity_descriptor(struct inode *inode,
+					 const struct fsverity_descriptor *desc,
+					 size_t desc_size)
 {
-	struct fsverity_info *vi = fsverity_get_info(inode);
-	struct fsverity_descriptor *desc;
-	int res;
+	if (desc_size < sizeof(*desc)) {
+		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
+			     desc_size);
+		return false;
+	}
 
-	if (vi)
-		return 0;
+	if (desc->version != 1) {
+		fsverity_err(inode, "Unrecognized descriptor version: %u",
+			     desc->version);
+		return false;
+	}
+
+	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
+		fsverity_err(inode, "Reserved bits set in descriptor");
+		return false;
+	}
+
+	if (desc->salt_size > sizeof(desc->salt)) {
+		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
+		return false;
+	}
+
+	if (le64_to_cpu(desc->data_size) != inode->i_size) {
+		fsverity_err(inode,
+			     "Wrong data_size: %llu (desc) != %lld (inode)",
+			     le64_to_cpu(desc->data_size), inode->i_size);
+		return false;
+	}
+
+	if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
+		fsverity_err(inode, "Signature overflows verity descriptor");
+		return false;
+	}
+
+	return true;
+}
+
+/*
+ * Read the inode's fsverity_descriptor (with optional appended signature) from
+ * the filesystem, and do basic validation of it.
+ */
+int fsverity_get_descriptor(struct inode *inode,
+			    struct fsverity_descriptor **desc_ret,
+			    size_t *desc_size_ret)
+{
+	int res;
+	struct fsverity_descriptor *desc;
 
 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
 	if (res < 0) {
···
 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
 	if (res < 0) {
 		fsverity_err(inode, "Error %d reading verity descriptor", res);
-		goto out_free_desc;
+		kfree(desc);
+		return res;
 	}
 
-	vi = fsverity_create_info(inode, desc, res);
+	if (!validate_fsverity_descriptor(inode, desc, res)) {
+		kfree(desc);
+		return -EINVAL;
+	}
+
+	*desc_ret = desc;
+	*desc_size_ret = res;
+	return 0;
+}
+
+/* Ensure the inode has an ->i_verity_info */
+static int ensure_verity_info(struct inode *inode)
+{
+	struct fsverity_info *vi = fsverity_get_info(inode);
+	struct fsverity_descriptor *desc;
+	size_t desc_size;
+	int err;
+
+	if (vi)
+		return 0;
+
+	err = fsverity_get_descriptor(inode, &desc, &desc_size);
+	if (err)
+		return err;
+
+	vi = fsverity_create_info(inode, desc, desc_size);
 	if (IS_ERR(vi)) {
-		res = PTR_ERR(vi);
+		err = PTR_ERR(vi);
 		goto out_free_desc;
 	}
 
 	fsverity_set_info(inode, vi);
-	res = 0;
+	err = 0;
 out_free_desc:
 	kfree(desc);
-	return res;
+	return err;
 }
 
 /**