bpf: rework prog_digest into prog_tag

Commit 7bd509e311f4 ("bpf: add prog_digest and expose it via
fdinfo/netlink") was recently discussed, partially due to
admittedly suboptimal name of "prog_digest" in combination
with sha1 hash usage, thus inevitably and rightfully concerns
about its security in terms of collision resistance were
raised with regards to use-cases.

The intended use cases are for debugging resp. introspection
only for providing a stable "tag" over the instruction sequence
that both kernel and user space can calculate independently.
It's not usable at all for making a security relevant decision.
So collisions where two different instruction sequences generate
the same tag can happen, but ideally at a rather low rate. The
"tag" will be dumped in hex and is short enough to introspect
in tracepoints or kallsyms output along with other data such
as stack trace, etc. Thus, this patch performs a rename into
prog_tag and truncates the tag to a short output (64 bits) to
make it obvious it's not collision-free.

Should in future a hash or facility be needed with a security
relevant focus, then we can think about requirements, constraints,
etc that would fit to that situation. For now, rework the exposed
parts for the current use cases as long as nothing has been
released yet. Tested on x86_64 and s390x.

Fixes: 7bd509e311f4 ("bpf: add prog_digest and expose it via fdinfo/netlink")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 6 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 1eb4f1303756..503d4211988a 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -146,10 +146,11 @@ void __bpf_prog_free(struct bpf_prog *fp)
 	vfree(fp);
 }
 
-int bpf_prog_calc_digest(struct bpf_prog *fp)
+int bpf_prog_calc_tag(struct bpf_prog *fp)
 {
 	const u32 bits_offset = SHA_MESSAGE_BYTES - sizeof(__be64);
-	u32 raw_size = bpf_prog_digest_scratch_size(fp);
+	u32 raw_size = bpf_prog_tag_scratch_size(fp);
+	u32 digest[SHA_DIGEST_WORDS];
 	u32 ws[SHA_WORKSPACE_WORDS];
 	u32 i, bsize, psize, blocks;
 	struct bpf_insn *dst;
@@ -162,7 +163,7 @@ int bpf_prog_calc_digest(struct bpf_prog *fp)
 	if (!raw)
 		return -ENOMEM;
 
-	sha_init(fp->digest);
+	sha_init(digest);
 	memset(ws, 0, sizeof(ws));
 
 	/* We need to take out the map fd for the digest calculation
@@ -204,13 +205,14 @@ int bpf_prog_calc_digest(struct bpf_prog *fp)
 	*bits = cpu_to_be64((psize - 1) << 3);
 
 	while (blocks--) {
-		sha_transform(fp->digest, todo, ws);
+		sha_transform(digest, todo, ws);
 		todo += SHA_MESSAGE_BYTES;
 	}
 
-	result = (__force __be32 *)fp->digest;
+	result = (__force __be32 *)digest;
 	for (i = 0; i < SHA_DIGEST_WORDS; i++)
-		result[i] = cpu_to_be32(fp->digest[i]);
+		result[i] = cpu_to_be32(digest[i]);
+	memcpy(fp->tag, result, sizeof(fp->tag));
 
 	vfree(raw);
 	return 0;