Several AMT receive and transmit paths cache a pointer into the skb head
and then call a helper that can reallocate that head before the cached
pointer is used again, so the later access reads or writes freed memory.

v3 addressed only the source-address reads in a subset of the handlers
and described amt_membership_query_handler() and
amt_multicast_data_handler() as unaffected.  As the review pointed out,
that was incomplete: those handlers keep stale eth_hdr() and AMT-header
pointers across later pulls, the record loops in the IGMPv3 and MLDv2
report handlers read the record count and the group record across the
*_mc_may_pull() calls, and amt_update_handler() and amt_dev_xmit() read
the destination address after further pulls.

Patch 1 walks every AMT path and, for each pointer used after a
reallocating call, either snapshots the value before the first pull or
re-derives the pointer after the last one.  This uses the re-derive
approach rather than the per-value snapshot of v3, because the write
sites cannot be expressed as a snapshot and re-derivation is already the
idiom used elsewhere in the file.

Patch 2 is a smaller, separable hardening change: the three handlers
that rewrite the ethernet header do so in place without making the head
private, which corrupts a cloned skb (for example one held by a packet
tap).  It adds skb_cow_head() before the rewrite, split out so the
use-after-free fix is not held up by discussion of the clone case.

Both patches build cleanly (x86_64, CONFIG_AMT=m, W=1) and are
checkpatch --strict clean.

Changes since v3:
 - Rework from the per-value source-address snapshot to re-deriving the
   header pointers after the last reallocating pull, and cover every
   affected handler (amt_dev_xmit, amt_multicast_data_handler,
   amt_membership_query_handler, the IGMPv3 and MLDv2 record loops, and
   the remaining reads in amt_update_handler), not just the
   source-address reads.
 - Correct the v3 commit-message claim that the query and multicast-data
   handlers were unaffected.
 - Add patch 2 (skb_cow_head() before the L2 rewrite).
 - Drop the v2 Acked-by from Taehee Yoo: this series is materially larger
   than what was acked.

v3: https://lore.kernel.org/all/20260626111917.802243-1-michael.bommarito@gmail.com/ (local)
v2: https://lore.kernel.org/all/20260617123443.3586930-1-michael.bommarito@gmail.com/ (local)

Michael Bommarito (2):
  amt: re-read skb header pointers after every pull
  amt: make the head writable before rewriting the L2 header

 drivers/net/amt.c | 117 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 96 insertions(+), 21 deletions(-)


base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
-- 
2.53.0