[PATCH] fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions

[Eric Biggers <ebiggers@kernel.org>]

The BPF verifier and the dynptr abstraction ensure that the memory space
referenced by a dynptr remains valid.  They do not, however, provide any
guarantee that the contents of the memory are stable.  kfuncs are
expected to remain memory-safe even if concurrent modifications occur.

bpf_get_fsverity_digest() didn't follow that: it could crash if
arg->digest_size was concurrently modified.

Fix that by using the known-good value hash_alg->digest_size instead.

Also correctly handle sizes over INT_MAX, which previously caused an
integer overflow and crash.  __bpf_dynptr_size() returns a u64.

Fixes: 67814c00de31 ("bpf, fsverity: Add kfunc bpf_get_fsverity_digest")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 fs/verity/measure.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/verity/measure.c b/fs/verity/measure.c
index cfe2d5e535f9..f8b3526af004 100644
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -122,11 +122,11 @@ __bpf_kfunc int bpf_get_fsverity_digest(struct file *file, const struct bpf_dynp
 {
 	const struct bpf_dynptr_kern *digest_ptr = (struct bpf_dynptr_kern *)digest_p;
 	const struct inode *inode = file_inode(file);
-	u32 dynptr_sz = __bpf_dynptr_size(digest_ptr);
+	u64 dynptr_sz = __bpf_dynptr_size(digest_ptr);
 	struct fsverity_digest *arg;
 	const struct fsverity_info *vi;
 	const struct fsverity_hash_alg *hash_alg;
-	int out_digest_sz;
+	u64 out_digest_sz;
 
 	if (dynptr_sz < sizeof(struct fsverity_digest))
 		return -EINVAL;
@@ -150,11 +150,13 @@ __bpf_kfunc int bpf_get_fsverity_digest(struct file *file, const struct bpf_dynp
 	out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
 
 	/* copy digest */
-	memcpy(arg->digest, vi->file_digest,  min_t(int, hash_alg->digest_size, out_digest_sz));
+	memcpy(arg->digest, vi->file_digest,
+	       min(hash_alg->digest_size, out_digest_sz));
 
 	/* fill the extra buffer with zeros */
 	if (out_digest_sz > hash_alg->digest_size)
-		memset(arg->digest + arg->digest_size, 0, out_digest_sz - hash_alg->digest_size);
+		memset(arg->digest + hash_alg->digest_size, 0,
+		       out_digest_sz - hash_alg->digest_size);
 
 	return 0;
 }

base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
-- 
2.55.0.508.g3f0d502094-goog