[PATCH net v2] net: reduce XMIT_RECURSION_LIMIT under KASAN

[Tristan Madani <tristmd@gmail.com>]

From: Tristan Madani <tristan@talencesecurity.com>

Virtual network devices (ipvlan, macvlan, bonding) can enter legitimate
transmit recursion when combined with packet forwarding configurations
such as IPVS NAT.  The existing XMIT_RECURSION_LIMIT (8) in
__dev_queue_xmit() detects and breaks these loops, but the allowed
depth is too high for KASAN-instrumented kernels: each recursion level
consumes significantly more stack due to KASAN inline instrumentation,
and the cumulative usage overflows the kernel stack before the limit
fires.

On x86_64, CONFIG_KASAN doubles THREAD_SIZE from 16KB to 32KB
(KASAN_STACK_ORDER=1), but KASAN per-access checks inflate individual
function frames by roughly 2-3x.  For an ipvlan L3 + IPVS NAT routing
loop, objdump measurements on a non-KASAN kernel show ~1.4KB of stack
consumed per recursion level (across 17 functions from __dev_queue_xmit
through the full IP output path and back).  At KASAN ~2.3x inflation
factor that becomes ~3.3KB per level.  Nine levels -- reached before the
current limit fires -- total ~30KB plus the initial call chain, which
exceeds the 32KB KASAN stack.  The overflow hits the VMAP_STACK guard
page and causes a non-recoverable kernel panic (BUG: stack guard page
was hit).

On non-KASAN kernels the same loop is safely caught by the existing
limit: the "Dead loop on virtual device" message fires and the packet
is dropped without any stack overflow.

Reduce XMIT_RECURSION_LIMIT to 3 when CONFIG_KASAN is enabled.  This
keeps the recursion counter well within the 32KB KASAN stack budget
while preserving the established limit of 8 for production kernels.

The recursion path triggering this is:

  __dev_queue_xmit -> dev_hard_start_xmit -> ipvlan_start_xmit
  -> ipvlan_queue_xmit -> ipvlan_process_outbound -> ip_local_out
  -> nf_hook (IPVS) -> ip_vs_in_hook -> ip_vs_nat_xmit -> ip_output
  -> ip_finish_output2 -> neigh_resolve_output -> __dev_queue_xmit

Tested:
  - KASAN kernel (6.8.12 x86_64): panic before fix, "Dead loop"
    drop after fix.
  - Non-KASAN kernel (6.8.12 x86_64): "Dead loop" drop both before
    and after fix (no behavior change for production kernels).

Fixes: 2ad7bf363841 ("ipvlan: Initial check-in of the IPVLAN driver.")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
v2: Switch from per-driver recursion guard in ipvlan_core.c to reducing
    the global XMIT_RECURSION_LIMIT under CONFIG_KASAN, as suggested
    by Eric Dumazet.

 include/linux/netdevice.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 9981d637f8b54..bdcb61d352afb 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3640,7 +3640,11 @@ struct page_pool_bh {
 };
 DECLARE_PER_CPU(struct page_pool_bh, system_page_pool);
 
+#ifdef CONFIG_KASAN
+#define XMIT_RECURSION_LIMIT	3
+#else
 #define XMIT_RECURSION_LIMIT	8
+#endif
 
 #ifndef CONFIG_PREEMPT_RT
 static inline int dev_recursion_level(void)
-- 
2.47.3