[PATCH] target/i386/cpu: don't assert on out-of-range CPUID 0x1f subleaf

[PATCH] target/i386/cpu: don't assert on out-of-range CPUID 0x1f subleaf

[Ewan Hai-oc]

encode_topo_cpuid1f() asserts count <= CPU_TOPOLOGY_LEVEL_SOCKET, but
count is the subleaf index passed straight from the guest's ECX;
cpu_x86_cpuid() does not clamp it for leaf 0x1f. On a TCG vCPU exposing
0x1f, any CPUID(0x1f) with ECX > 5 aborts QEMU. SeaBIOS already trips
this during POST (its cpuid() helper leaves a stale ECX from the
EFLAGS.ID probe), and guests reach it too,the kernel walks 0x1f
subleaves in a loop and /dev/cpu/*/cpuid forwards arbitrary subleaves.

Per the SDM, CPUID never faults and a subleaf past the last topology
level returns the invalid encoding (EAX=EBX=0, ECX level type 0, EDX the
x2APIC ID), exactly as the legacy 0xB leaf already does. Return that
instead of asserting. KVM is unaffected.

Affected models: SapphireRapids (v4+), GraniteRapids (v3+), SierraForest
(v3+), ClearwaterForest (v3+), DiamondRapids and YongFeng (v3+).

Fixes: 822bce9f58df ("i386/cpu: Decouple CPUID[0x1F] subleaf with specific topology level")
Signed-off-by: Ewan Hai <ewanhai-oc@zhaoxin.com>
---
 target/i386/cpu.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 8929a75c7c..e035e4cf45 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -427,7 +427,14 @@ static void encode_topo_cpuid1f(CPUX86State *env, uint32_t count,
     unsigned long level, base_level, next_level;
     uint32_t num_threads_next_level, offset_next_level;
 
-    assert(count <= CPU_TOPOLOGY_LEVEL_SOCKET);
+    /* Out-of-range subleaf: SDM mandates the invalid encoding, no fault. */
+    if (count > CPU_TOPOLOGY_LEVEL_SOCKET) {
+        *eax = 0;
+        *ebx = 0;
+        *ecx = count & 0xff;
+        *edx = cpu->apic_id;
+        return;
+    }
 
     /*
      * Find the No.(count + 1) topology level in avail_cpu_topo bitmap.

base-commit: cc329c491768b2d91eb0b0984f3baa0bf805776d
-- 
2.34.1

Re: [PATCH] target/i386/cpu: don't assert on out-of-range CPUID 0x1f subleaf

[Ewan Hai-oc]

Gentle ping. Any feedback on this fix?

Ewan

Re: [PATCH] target/i386/cpu: don't assert on out-of-range CPUID 0x1f subleaf

[Zhao Liu]

On Tue, Jun 09, 2026 at 05:00:46PM +0800, Ewan Hai-oc wrote:
> Date: Tue, 9 Jun 2026 17:00:46 +0800
> From: Ewan Hai-oc <ewanhai-oc@zhaoxin.com>
> Subject: [PATCH] target/i386/cpu: don't assert on out-of-range CPUID 0x1f
>  subleaf
> X-Mailer: git-send-email 2.34.1
> 
> encode_topo_cpuid1f() asserts count <= CPU_TOPOLOGY_LEVEL_SOCKET, but
> count is the subleaf index passed straight from the guest's ECX;
> cpu_x86_cpuid() does not clamp it for leaf 0x1f. On a TCG vCPU exposing
> 0x1f, any CPUID(0x1f) with ECX > 5 aborts QEMU. SeaBIOS already trips
> this during POST (its cpuid() helper leaves a stale ECX from the
> EFLAGS.ID probe), and guests reach it too,the kernel walks 0x1f
> subleaves in a loop and /dev/cpu/*/cpuid forwards arbitrary subleaves.
> 
> Per the SDM, CPUID never faults and a subleaf past the last topology
> level returns the invalid encoding (EAX=EBX=0, ECX level type 0, EDX the
> x2APIC ID), exactly as the legacy 0xB leaf already does. Return that
> instead of asserting. KVM is unaffected.
> 
> Affected models: SapphireRapids (v4+), GraniteRapids (v3+), SierraForest
> (v3+), ClearwaterForest (v3+), DiamondRapids and YongFeng (v3+).
> 
> Fixes: 822bce9f58df ("i386/cpu: Decouple CPUID[0x1F] subleaf with specific topology level")
> Signed-off-by: Ewan Hai <ewanhai-oc@zhaoxin.com>
> ---
>  target/i386/cpu.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

(cc stable list)

Yes, good catch!

On a TCG vCPU exposing 0x1f, requesting an invalid subleaf triggers
this abort, e.g.:

cpuid -1 -l 0x1f -s 8

Thanks for the fix,

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>


Regards,
Zhao