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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
commit 29e63b8d9f
1 changed files with 2 additions and 0 deletions
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