[PATCH net] ixgbevf: fix use-after-free in VEPA multicast source pruning

[Michael Bommarito <michael.bommarito@gmail.com>]

ixgbevf_clean_rx_irq() prunes frames whose source MAC matches the VF's
own address (VEPA multicast workaround) by freeing the skb and
continuing to the next descriptor:

    dev_kfree_skb_irq(skb);
    continue;

The skb pointer is declared outside the while loop and persists across
iterations.  Because the continue skips the "skb = NULL" reset at the
bottom of the loop, the next iteration enters the "else if (skb)" path
and calls ixgbevf_add_rx_frag() on the freed skb, dereferencing
skb_shinfo(skb)->nr_frags — a use-after-free in NAPI softirq context.

The sibling driver iavf already handles this correctly by nulling the
pointer before continuing.  Apply the same pattern here.

I do not have ixgbevf hardware; the bug was found by static analysis
(scan_drop_continue_loops.py + semgrep drop_continue_in_loop, multi-tool
corroboration with the highest score in the scan).  The UAF was confirmed
under KASAN by loading a test module that reproduces the exact code
pattern (alloc skb, kfree_skb, then read skb_shinfo(skb)->nr_frags):

  BUG: KASAN: slab-use-after-free in ixgbevf_uaf_test_init+0x100/0x1000
  Read of size 8 at addr 000000006163ae78 by task insmod/30
  freed 208-byte region [000000006163adc0, 000000006163ae90)

QEMU emulates igb (82576) but not ixgbe (82599), and the igbvf VF
driver does not include the VEPA source pruning path, so a full
end-to-end reproduction with emulated hardware was not possible.

Fixes: bad17234ba70 ("ixgbevf: Change receive model to use double buffered page based receives")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
index 42f89a179a3f..4ba3be961ab6 100644
--- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
+++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
@@ -1221,6 +1221,7 @@ static int ixgbevf_clean_rx_irq(struct ixgbevf_q_vector *q_vector,
 		    ether_addr_equal(rx_ring->netdev->dev_addr,
 				     eth_hdr(skb)->h_source)) {
 			dev_kfree_skb_irq(skb);
+			skb = NULL;
 			continue;
 		}
 
-- 
2.53.0