crypto: user - clean up report structure copying

There have been a pretty ridiculous number of issues with initializing
the report structures that are copied to userspace by NETLINK_CRYPTO.
Commit 4473710df1f8 ("crypto: user - Prepare for CRYPTO_MAX_ALG_NAME
expansion") replaced some strncpy()s with strlcpy()s, thereby
introducing information leaks.  Later two other people tried to replace
other strncpy()s with strlcpy() too, which would have introduced even
more information leaks:

    - https://lore.kernel.org/patchwork/patch/954991/
    - https://patchwork.kernel.org/patch/10434351/

Commit cac5818c25d0 ("crypto: user - Implement a generic crypto
statistics") also uses the buggy strlcpy() approach and therefore leaks
uninitialized memory to userspace.  A fix was proposed, but it was
originally incomplete.

Seeing as how apparently no one can get this right with the current
approach, change all the reporting functions to:

- Start by memsetting the report structure to 0.  This guarantees it's
  always initialized, regardless of what happens later.
- Initialize all strings using strscpy().  This is safe after the
  memset, ensures null termination of long strings, avoids unnecessary
  work, and avoids the -Wstringop-truncation warnings from gcc.
- Use sizeof(var) instead of sizeof(type).  This is more robust against
  copy+paste errors.

For simplicity, also reuse the -EMSGSIZE return value from nla_put().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 3 insertions, 7 deletions

diff --git a/crypto/kpp.c b/crypto/kpp.c
index a90edc27af77..bc2f1006a2f7 100644
--- a/crypto/kpp.c
+++ b/crypto/kpp.c
@@ -30,15 +30,11 @@ static int crypto_kpp_report(struct sk_buff *skb, struct crypto_alg *alg)
 {
 	struct crypto_report_kpp rkpp;
 
-	strncpy(rkpp.type, "kpp", sizeof(rkpp.type));
+	memset(&rkpp, 0, sizeof(rkpp));
 
-	if (nla_put(skb, CRYPTOCFGA_REPORT_KPP,
-		    sizeof(struct crypto_report_kpp), &rkpp))
-		goto nla_put_failure;
-	return 0;
+	strscpy(rkpp.type, "kpp", sizeof(rkpp.type));
 
-nla_put_failure:
-	return -EMSGSIZE;
+	return nla_put(skb, CRYPTOCFGA_REPORT_KPP, sizeof(rkpp), &rkpp);
 }
 #else
 static int crypto_kpp_report(struct sk_buff *skb, struct crypto_alg *alg)