[PATCH v2] virtio-net: validate RSS indirections_len in post_load

[Junjie Cao <junjie.cao@intel.com>]

virtio_net_handle_rss() enforces that indirections_len is a non-zero
power of two no larger than VIRTIO_NET_RSS_MAX_TABLE_LEN, but
virtio_net_rss_post_load() applies none of these checks to values
restored from the migration stream.

A corrupted save file or crafted migration stream can set
indirections_len to 0.  Even if it also clears redirect,
virtio_load() calls set_features_nocheck() after the device vmstate
(including the RSS subsection and its post_load) has already been
loaded, re-deriving redirect from the negotiated guest features.
When VIRTIO_NET_F_RSS was negotiated, redirect is set back to true
regardless of the migration stream value.  The receive path then
computes

    hash & (indirections_len - 1)   /* wraps to 0xFFFFFFFF via int promotion */

and uses the result to index into indirections_table, which was not
allocated by the VMState loader when the element count is zero (see
vmstate_handle_alloc()), resulting in a NULL pointer dereference that
crashes QEMU:

  #0  virtio_net_process_rss    ../hw/net/virtio-net.c:1901
  #1  virtio_net_receive_rcu    ../hw/net/virtio-net.c:1921
  #2  virtio_net_do_receive     ../hw/net/virtio-net.c:2061
  #3  nc_sendv_compat           ../net/net.c:823
  #4  qemu_deliver_packet_iov   ../net/net.c:870

The RSS subsection is only loaded when rss_data.enabled is true (via
virtio_net_rss_needed()), and the command path always produces
indirections_len in {1, 2, 4, …, 128}, so an unconditional check
cannot reject a legitimate migration stream.

Factor the validation into virtio_net_rss_indirections_len_valid()
and call it from both virtio_net_handle_rss() and
virtio_net_rss_post_load().

Fixes: e41b711485e5 ("virtio-net: add migration support for RSS and hash report")
Cc: qemu-stable@nongnu.org
Signed-off-by: Junjie Cao <junjie.cao@intel.com>
---
v1 -> v2:
  - Drop the redundant == 0 check; is_power_of_2() already handles
    it. [MST]
  - Factor the validation into virtio_net_rss_indirections_len_valid()
    and use it from both virtio_net_handle_rss() and
    virtio_net_rss_post_load(), replacing the two separate checks in
    the command path. [MST]

v1: https://lore.kernel.org/qemu-devel/20260323131531.1976-1-junjie.cao@intel.com/

 hw/net/virtio-net.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 2a5d642a64..f0e3beb290 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1374,6 +1374,11 @@ static void virtio_net_unload_ebpf(VirtIONet *n)
     ebpf_rss_unload(&n->ebpf_rss);
 }
 
+static bool virtio_net_rss_indirections_len_valid(uint16_t len)
+{
+    return is_power_of_2(len) && len <= VIRTIO_NET_RSS_MAX_TABLE_LEN;
+}
+
 static uint16_t virtio_net_handle_rss(VirtIONet *n,
                                       struct iovec *iov,
                                       unsigned int iov_cnt,
@@ -1411,14 +1416,9 @@ static uint16_t virtio_net_handle_rss(VirtIONet *n,
     if (!do_rss) {
         n->rss_data.indirections_len = 0;
     }
-    if (n->rss_data.indirections_len >= VIRTIO_NET_RSS_MAX_TABLE_LEN) {
-        err_msg = "Too large indirection table";
-        err_value = n->rss_data.indirections_len;
-        goto error;
-    }
     n->rss_data.indirections_len++;
-    if (!is_power_of_2(n->rss_data.indirections_len)) {
-        err_msg = "Invalid size of indirection table";
+    if (!virtio_net_rss_indirections_len_valid(n->rss_data.indirections_len)) {
+        err_msg = "Invalid indirection table length";
         err_value = n->rss_data.indirections_len;
         goto error;
     }
@@ -3427,6 +3427,13 @@ static int virtio_net_rss_post_load(void *opaque, int version_id)
         n->rss_data.supported_hash_types = VIRTIO_NET_RSS_SUPPORTED_HASHES;
     }
 
+    if (!virtio_net_rss_indirections_len_valid(n->rss_data.indirections_len)) {
+        error_report("virtio-net: saved image has invalid RSS "
+                     "indirections_len: %u",
+                     n->rss_data.indirections_len);
+        return -EINVAL;
+    }
+
     return 0;
 }
 
-- 
2.43.0