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mpls: reload header after pskb_may_pull()

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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: 9f427a0e47 ("net: mpls: Fix multipath selection for LSR use case")
Cc: stable@vger.kernel.org
Signed-off-by: Qing Ming <a0yami@mailbox.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260814095404.7205-1-a0yami@mailbox.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Qing Ming 2026-08-14 17:54:04 +08:00 committed by Paolo Abeni
parent 21040c7f93
commit 29e63b8d9f
1 changed files with 2 additions and 0 deletions
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2
net/mpls/af_mpls.c
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@ -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);