Re: [PATCH net-next 2/3] net, neigh: Use NLA_POLICY_MASK helper for NDA_FLAGS_EXT attribute

On 10/14/21 5:13 AM, David Ahern wrote:

On 10/13/21 7:21 AM, Daniel Borkmann wrote:

quoted

Instead of open-coding a check for invalid bits in NTF_EXT_MASK, we can just
use the NLA_POLICY_MASK() helper instead, and simplify NDA_FLAGS_EXT sanity
check this way.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
  net/core/neighbour.c | 6 +-----
  1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 4fc601f9cd06..922b9ed0fe76 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c

@@ -1834,7 +1834,7 @@ const struct nla_policy nda_policy[NDA_MAX+1] = {
  	[NDA_MASTER]		= { .type = NLA_U32 },
  	[NDA_PROTOCOL]		= { .type = NLA_U8 },
  	[NDA_NH_ID]		= { .type = NLA_U32 },
-	[NDA_FLAGS_EXT]		= { .type = NLA_U32 },
+	[NDA_FLAGS_EXT]		= NLA_POLICY_MASK(NLA_U32, NTF_EXT_MASK),
  	[NDA_FDB_EXT_ATTRS]	= { .type = NLA_NESTED },
  };
  

@@ -1936,10 +1936,6 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
  	if (tb[NDA_FLAGS_EXT]) {
  		u32 ext = nla_get_u32(tb[NDA_FLAGS_EXT]);
  
-		if (ext & ~NTF_EXT_MASK) {
-			NL_SET_ERR_MSG(extack, "Invalid extended flags");
-			goto out;
-		}
  		BUILD_BUG_ON(sizeof(neigh->flags) * BITS_PER_BYTE <
  			     (sizeof(ndm->ndm_flags) * BITS_PER_BYTE +
  			      hweight32(NTF_EXT_MASK)));

I get that NLA_POLICY_MASK wants to standardize the logic, but the
generic extack message "reserved bit set" is less useful than the one here.

If the expectation/recommendation is that NLA_POLICY_MASK() should be used, then
it would probably make sense for NLA_POLICY_MASK() itself to improve. For example,
NLA_POLICY_MASK() could perhaps take an optional error string which it should
return via extack rather than the standard "reserved bit set" one or such.. on
the other hand, I see that NL_SET_ERR_MSG_ATTR() already points out the affected
attribute via setting extack->bad_attr, so it be sufficient to figure out that it's
about reserved bits inside NDA_FLAGS_EXT given this is propagated back to user
space via NLMSGERR_ATTR_OFFS.

Thanks,
Daniel