[Patch net] af_key: relax availability checks for skb size calculation

[Patch net] af_key: relax availability checks for skb size calculation

[Cong Wang]

From: Cong Wang <cong.wang@bytedance.com>

xfrm_probe_algs() probes kernel crypto modules and changes the
availability of struct xfrm_algo_desc. But there is a small window
where ealg->available and aalg->available get changed between
count_ah_combs()/count_esp_combs() and dump_ah_combs()/dump_esp_combs(),
in this case we may allocate a smaller skb but later put a larger
amount of data and trigger the panic in skb_put().

Fix this by relaxing the checks when counting the size, that is,
skipping the test of ->available. We may waste some memory for a few
of sizeof(struct sadb_comb), but it is still much better than a panic.

Reported-by: syzbot+b2bf2652983d23734c5c@syzkaller.appspotmail.com
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
---
 net/key/af_key.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/key/af_key.c b/net/key/af_key.c
index c12dbc51ef5f..ef9b4ac03e7b 100644
--- a/net/key/af_key.c
+++ b/net/key/af_key.c
@@ -2902,7 +2902,7 @@ static int count_ah_combs(const struct xfrm_tmpl *t)
 			break;
 		if (!aalg->pfkey_supported)
 			continue;
-		if (aalg_tmpl_set(t, aalg) && aalg->available)
+		if (aalg_tmpl_set(t, aalg))
 			sz += sizeof(struct sadb_comb);
 	}
 	return sz + sizeof(struct sadb_prop);
@@ -2920,7 +2920,7 @@ static int count_esp_combs(const struct xfrm_tmpl *t)
 		if (!ealg->pfkey_supported)
 			continue;
 
-		if (!(ealg_tmpl_set(t, ealg) && ealg->available))
+		if (!(ealg_tmpl_set(t, ealg)))
 			continue;
 
 		for (k = 1; ; k++) {
@@ -2931,7 +2931,7 @@ static int count_esp_combs(const struct xfrm_tmpl *t)
 			if (!aalg->pfkey_supported)
 				continue;
 
-			if (aalg_tmpl_set(t, aalg) && aalg->available)
+			if (aalg_tmpl_set(t, aalg))
 				sz += sizeof(struct sadb_comb);
 		}
 	}
-- 
2.25.1

Re: [Patch net] af_key: relax availability checks for skb size calculation

[Steffen Klassert]

On Sat, Dec 26, 2020 at 04:50:20PM -0800, Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> xfrm_probe_algs() probes kernel crypto modules and changes the
> availability of struct xfrm_algo_desc. But there is a small window
> where ealg->available and aalg->available get changed between
> count_ah_combs()/count_esp_combs() and dump_ah_combs()/dump_esp_combs(),
> in this case we may allocate a smaller skb but later put a larger
> amount of data and trigger the panic in skb_put().
> 
> Fix this by relaxing the checks when counting the size, that is,
> skipping the test of ->available. We may waste some memory for a few
> of sizeof(struct sadb_comb), but it is still much better than a panic.
> 
> Reported-by: syzbot+b2bf2652983d23734c5c@syzkaller.appspotmail.com
> Cc: Steffen Klassert <steffen.klassert@secunet.com>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>

Patch applied, thanks!