[PATCH net] mpls: reload header after pskb_may_pull()

mpls_select_multipath() calls mpls_multipath_hash() to choose a nexthop
when an MPLS route has multiple nexthops.  While walking the MPLS label
stack, the hash routine caches hdr for the current label.  After finding
the bottom-of-stack label, it calls pskb_may_pull() before reading the
inner IP header.

If an skb is constructed with the inner IP header in nonlinear data and
insufficient tailroom in the linear head, pskb_may_pull() calls
pskb_expand_head() to replace the skb head and free the old one.  This
leaves hdr pointing to freed memory.  The IPv6 path can invalidate hdr
again when it performs a second pull for the larger header.

The issue was found through static analysis.  A reproducer sending a legal
Geneve packet through a bareudp/MPLS multipath setup triggered the same
KASAN report in 2 of 2 unpatched runs:

  BUG: KASAN: slab-use-after-free in mpls_select_multipath
  Read of size 1 at addr ffff88800ecc6e20 by task ksoftirqd/1/23

  Call Trace:
   mpls_select_multipath
   mpls_forward
   __netif_receive_skb_list_core
   netif_receive_skb_list_internal
   napi_complete_done
   gro_cell_poll
   __napi_poll
   net_rx_action

  Freed by task 23:
   kfree
   pskb_expand_head
   __pskb_pull_tail
   mpls_select_multipath

Reload hdr from the current skb head after each successful pull before
deriving the inner IPv4 or IPv6 header pointer.

Fixes: 9f427a0e474a ("net: mpls: Fix multipath selection for LSR use case")
Cc: stable@vger.kernel.org
Signed-off-by: Qing Ming <redacted>
---
 net/mpls/af_mpls.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 961be5054a03..17b78dcbf8ab 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c

@@ -221,6 +221,7 @@ static u32 mpls_multipath_hash(struct mpls_route *rt, struct sk_buff *skb)
 		if (pskb_may_pull(skb, mpls_hdr_len + sizeof(struct iphdr))) {
 			const struct iphdr *v4hdr;
 
+			hdr = mpls_hdr(skb) + label_index;
 			v4hdr = (const struct iphdr *)(hdr + 1);
 			if (v4hdr->version == 4) {
 				hash = jhash_3words(ntohl(v4hdr->saddr),

@@ -231,6 +232,7 @@ static u32 mpls_multipath_hash(struct mpls_route *rt, struct sk_buff *skb)
 						 sizeof(struct ipv6hdr))) {
 				const struct ipv6hdr *v6hdr;
 
+				hdr = mpls_hdr(skb) + label_index;
 				v6hdr = (const struct ipv6hdr *)(hdr + 1);
 				hash = __ipv6_addr_jhash(&v6hdr->saddr, hash);
 				hash = __ipv6_addr_jhash(&v6hdr->daddr, hash);

base-commit: 24ef02f934eeb48830cff6b739abc3c62b1d107b
-- 
2.53.0