[PATCH v2] bpf: Add length check for SK_DIAG_BPF_STORAGE_REQ_MAP_FD parsing

The nla_for_each_nested parsing in function bpf_sk_storage_diag_alloc
does not check the length of the nested attribute. This can lead to an
out-of-attribute read and allow a malformed nlattr (e.g., length 0) to
be viewed as a 4 byte integer.

This patch adds an additional check when the nlattr is getting counted.
This makes sure the latter nla_get_u32 can access the attributes with
the correct length.

Fixes: 1ed4d92458a9 ("bpf: INET_DIAG support in bpf_sk_storage")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Lin Ma <redacted>
---
V1 -> V2: moves the check to the counting loop as Jakub suggested,
          alters the commit message accordingly.

 net/core/bpf_sk_storage.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index d4172534dfa8..cca7594be92e 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c

@@ -496,8 +496,11 @@ bpf_sk_storage_diag_alloc(const struct nlattr *nla_stgs)
 		return ERR_PTR(-EPERM);
 
 	nla_for_each_nested(nla, nla_stgs, rem) {
-		if (nla_type(nla) == SK_DIAG_BPF_STORAGE_REQ_MAP_FD)
+		if (nla_type(nla) == SK_DIAG_BPF_STORAGE_REQ_MAP_FD) {
+			if (nla_len(nla) != sizeof(u32))
+				return ERR_PTR(-EINVAL);
 			nr_maps++;
+		}
 	}
 
 	diag = kzalloc(struct_size(diag, maps, nr_maps), GFP_KERNEL);

-- 
2.17.1