[PATCH] LoadPin: Annotate struct dm_verity_loadpin_trusted_root_digest with __counted_by

[Kees Cook]

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct dm_verity_loadpin_trusted_root_digest.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Alasdair Kergon <agk@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: dm-devel@xxxxxxxxxx
Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
Cc: linux-security-module@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/dm-verity-loadpin.h | 2 +-
 security/loadpin/loadpin.c        | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/dm-verity-loadpin.h b/include/linux/dm-verity-loadpin.h
index 552b817ab102..3ac6dbaeaa37 100644
--- a/include/linux/dm-verity-loadpin.h
+++ b/include/linux/dm-verity-loadpin.h
@@ -12,7 +12,7 @@ extern struct list_head dm_verity_loadpin_trusted_root_digests;
 struct dm_verity_loadpin_trusted_root_digest {
 	struct list_head node;
 	unsigned int len;
-	u8 data[];
+	u8 data[] __counted_by(len);
 };
 
 #if IS_ENABLED(CONFIG_SECURITY_LOADPIN_VERITY)
diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index ebae964f7cc9..a9d40456a064 100644
--- a/security/loadpin/loadpin.c
+++ b/security/loadpin/loadpin.c
@@ -336,6 +336,7 @@ static int read_trusted_verity_root_digests(unsigned int fd)
 			rc = -ENOMEM;
 			goto err;
 		}
+		trd->len = len;
 
 		if (hex2bin(trd->data, d, len)) {
 			kfree(trd);
@@ -343,8 +344,6 @@ static int read_trusted_verity_root_digests(unsigned int fd)
 			goto err;
 		}
 
-		trd->len = len;
-
 		list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
 	}
 
-- 
2.34.1