[PATCH bpf v2 2/2] fsverity: Fix silent truncation in bpf_get_fsverity_digest()

[Eric Biggers]

bpf_get_fsverity_digest() silently truncates the digest if the provided
buffer is too small.  This is a footgun, and it doesn't match the
semantics of the equivalent UAPI (FS_IOC_MEASURE_VERITY).

Change it to return -EOVERFLOW instead, matching FS_IOC_MEASURE_VERITY.

Fixes: 67814c00de31 ("bpf, fsverity: Add kfunc bpf_get_fsverity_digest")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 fs/verity/measure.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/verity/measure.c b/fs/verity/measure.c
index f8b3526af004..68dfccb69772 100644
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -144,14 +144,15 @@ __bpf_kfunc int bpf_get_fsverity_digest(struct file *file, const struct bpf_dynp
 
 	hash_alg = vi->tree_params.hash_alg;
 
+	out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
+	if (out_digest_sz < hash_alg->digest_size)
+		return -EOVERFLOW;
+
 	arg->digest_algorithm = hash_alg - fsverity_hash_algs;
 	arg->digest_size = hash_alg->digest_size;
 
-	out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
-
 	/* copy digest */
-	memcpy(arg->digest, vi->file_digest,
-	       min(hash_alg->digest_size, out_digest_sz));
+	memcpy(arg->digest, vi->file_digest, hash_alg->digest_size);
 
 	/* fill the extra buffer with zeros */
 	if (out_digest_sz > hash_alg->digest_size)
-- 
2.55.0