[PATCH net] net: xps: re-validate the queue index after switching to the subordinate device

__netif_set_xps_queue() reassigns dev to the subordinate device
(netdev_get_tx_queue(dev, index)->sb_dev) but keeps using index, which
was only validated against the ORIGINAL device's queue count.  A
subordinate with fewer TX queues than the parent's index then overflows
sb_dev->_tx[] / the XPS map:

  BUG: KASAN: slab-out-of-bounds in __netif_set_xps_queue+0x178c/0x1ee8
  Write of size 4 at addr ffff000005d3fae0 by task A-26-F1-S2/162
   __netif_set_xps_queue+0x178c/0x1ee8
   netif_set_xps_queue+0x38/0x58
   xps_cpus_store+0x1dc/0x2b8

The store is netdev_queue_numa_node_write() on sub->_tx[index].  The
top-of-function WARN_ON_ONCE(index >= dev->num_tx_queues) only checks the
parent and is a debug assertion.  Re-check index against the device that
is actually indexed once dev has been rebound.

Note this is a different defect from the "reject an out of range traffic
class" guard (4da3b7b8b50f), which bounds tc, not index.

Fixes: ffcfe25bb50f ("net: Add support for subordinate device traffic classes")
Assisted-by: LLM
Signed-off-by: Dudu Lu <redacted>
---
Tested base
-----------
Detected on Linux 7.3-rc4 (commit 93f51579e7df), arm64 QEMU guest.
Re-checked against mainline f47d223627f8 (v7.3-341): the affected code
is unchanged between the two, so the bug is still present there.
Checked against linux-next next-20261021 (commit 9a482184d910): the
affected code is unchanged there as well, so the bug is still present in
linux-next too.

Reproducer status
-----------------
A reproducer was written and executed on an arm64 QEMU guest (KASAN); it
fires at the claimed sink, and still fires on a kernel built with 4da3b7b8b50f
applied (identity established by disassembling both kernels: same store, same
target offset #0x160).  The guest has no SR-IOV NIC, so the reproducer stands
in for that hardware with a small module that binds an sb_dev the same way
netdev_bind_sb_channel_queue() does for i40e/ixgbe.  The reproducer source is
not posted here per Documentation/process/security-bugs.rst; it is available on
request.

Triggering conditions
---------------------
CAP_NET_ADMIN to set the XPS mask.  An sb_dev is bound in-tree by
netdev_bind_sb_channel_queue(), which the i40e and ixgbe SR-IOV paths call
when a VF's channel is bound to the PF.  The QEMU guest used for the
reproducer has no SR-IOV NIC, so the sb_dev binding was set up through a
small module; the defect itself is reached in-tree on that hardware.

Notes
-----
Built and checkpatch-clean: this patch was applied on top of 7.3-rc4
(commit 93f51579e7df) and a full arm64 kernel (defconfig) was built with
it -- no new warnings or errors.  scripts/checkpatch.pl reports 0 errors,
0 warnings.

 net/core/dev.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index e76762e29360..d5ce25144806 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -2900,6 +2900,14 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 		/* If queue belongs to subordinate dev use its map */
 		dev = netdev_get_tx_queue(dev, index)->sb_dev ? : dev;
 
+		/* dev may now be the subordinate device, whose queue
+		 * count can be smaller than the parent's.  index was only
+		 * validated against the parent above, so re-check it before
+		 * it is used to index the (possibly different) dev->_tx[].
+		 */
+		if (index >= dev->real_num_tx_queues)
+			return -EINVAL;
+
 		tc = netdev_txq_to_tc(dev, index);
 		if (tc < 0 || tc >= num_tc)
 			return -EINVAL;