6.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Qing Ming <redacted>
commit 29e63b8d9fc150cc191b1c6eb7e16e1247e1b650 upstream.
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: <redacted>
Signed-off-by: Qing Ming <redacted>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260814095404.7205-1-a0yami@mailbox.org (local)
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/mpls/af_mpls.c | 2 ++
1 file changed, 2 insertions(+)
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -197,6 +197,7 @@ static u32 mpls_multipath_hash(struct mp
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),@@ -207,6 +208,7 @@ static u32 mpls_multipath_hash(struct mp
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);