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7f1de03e31
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_GENERIC 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. Eight levels -- the current
limit -- consume ~26KB plus the initial call chain (~8KB), 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 4 when CONFIG_KASAN is enabled.
The deepest legitimate transmit recursion observed in the kernel
selftests is 5 levels of __dev_queue_xmit nesting, in VXLAN symmetric
routing topologies with VRF (vxlan_symmetric, vxlan_asymmetric):
__dev_queue_xmit(vrf) depth 1
__dev_queue_xmit(vlan-svi) depth 2
__dev_queue_xmit(bridge) depth 3
__dev_queue_xmit(vxlan) depth 4
__dev_queue_xmit(veth) depth 5
Since the recursion check fires when the counter exceeds the limit
(strictly greater than), a limit of 4 permits 5 levels of nesting
while blocking the 6th. At ~3.3KB per level, five levels consume
~16.5KB; combined with the ~8KB initial call chain, total usage is
~24.5KB -- well within the 32KB KASAN stack with ~7.5KB of margin.
A limit of 3 (v2/v3 of this patch) allows only 4 levels, which broke
the VXLAN symmetric selftests: the 5th __dev_queue_xmit call was
incorrectly dropped, as reported by Jakub Kicinski and the kernel test
robot.
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 (at recursion level 4 instead of 8).
- Non-KASAN kernel (6.8.12 x86_64): "Dead loop" drop both before
and after fix (no behavior change for production kernels).
- Measured max __dev_queue_xmit nesting depth via bpftrace in a
VXLAN symmetric cross-VLAN topology (VRF + VLAN + bridge + VXLAN +
veth underlay): 5 levels, confirming limit=4 is sufficient.
Cc: stable+noautosel@kernel.org # KASAN is non-production
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Link: https://patch.msgid.link/20260902123040.2172805-1-tristmd@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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