[PATCH 0/2] KVM: x86: Fix UBSAN bool warnings in module parameters

[PATCH 0/2] KVM: x86: Fix UBSAN bool warnings in module parameters

[Gal Pressman]

Several KVM module parameters use int to support a special -1 (auto)
value, but rely on param_get_bool() for the sysfs getter.
When userspace reads these parameters before the auto value is resolved,
param_get_bool() interprets the int as a bool, triggering UBSAN "load of
value 255 is not a valid value for type '_Bool'" warnings.

Fix both instances by implementing getter functions that handle the -1
case before falling through to param_get_bool().

Gal Pressman (2):
  KVM: SVM: Fix UBSAN warning when reading avic parameter
  KVM: x86/mmu: Fix UBSAN warning when reading nx_huge_pages parameter

 arch/x86/kvm/mmu/mmu.c  |  5 +++++
 arch/x86/kvm/svm/avic.c | 13 ++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

-- 
2.52.0

[PATCH 1/2] KVM: SVM: Fix UBSAN warning when reading avic parameter

[Gal Pressman]

The avic parameter is stored as an int to support the special value -1
(AVIC_AUTO_MODE), but the cited commit changed it from bool to int while
keeping param_get_bool() as the getter function.
This causes UBSAN to report "load of value 255 is not a valid value for
type '_Bool'" when the parameter is read via sysfs.

The issue happens in two scenarios:

1. During module load: There's a time window between when module
   parameters are registered, and when avic_hardware_setup() runs to
   resolve the value, where the value is -1.

2. On non-AMD systems: On non-AMD hardware, the kvm_is_svm_supported()
   check returns early. The avic_hardware_setup() function never runs,
   so avic remains -1.

Fix that by implementing a getter function that properly reads and
converts the -1 value into an 'auto' string.

Triggered by sos report:
  UBSAN: invalid-load in kernel/params.c:323:33
  load of value 255 is not a valid value for type '_Bool'
  CPU: 0 UID: 0 PID: 4667 Comm: sos Not tainted 6.19.0-rc5net_mlx5_1e86836 #1 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x69/0xa0
   ubsan_epilogue+0x5/0x2b
   __ubsan_handle_load_invalid_value.cold+0x47/0x4c
   ? lock_acquire+0x219/0x2c0
   param_get_bool.cold+0xf/0x14
   param_attr_show+0x51/0x80
   module_attr_show+0x19/0x30
   sysfs_kf_seq_show+0xac/0xf0
   seq_read_iter+0x100/0x410
   copy_splice_read+0x1b4/0x360
   splice_direct_to_actor+0xbd/0x270
   ? wait_for_space+0xb0/0xb0
   do_splice_direct+0x72/0xb0
   ? propagate_umount+0x870/0x870
   do_sendfile+0x3a3/0x470
   __x64_sys_sendfile64+0x5e/0xe0
   do_syscall_64+0x70/0x8c0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53

Fixes: ca2967de5a5b ("KVM: SVM: Enable AVIC by default for Zen4+ if x2AVIC is support")
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
---
 arch/x86/kvm/svm/avic.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 6b77b2033208..48de0f475ca5 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -19,6 +19,7 @@
 #include <linux/amd-iommu.h>
 #include <linux/kvm_host.h>
 #include <linux/kvm_irqfd.h>
+#include <linux/sysfs.h>
 
 #include <asm/irq_remapping.h>
 #include <asm/msr.h>
@@ -76,10 +77,20 @@ static int avic_param_set(const char *val, const struct kernel_param *kp)
 	return param_set_bint(val, kp);
 }
 
+static int avic_param_get(char *buffer, const struct kernel_param *kp)
+{
+	int val = *(int *)kp->arg;
+
+	if (val == AVIC_AUTO_MODE)
+		return sysfs_emit(buffer, "auto\n");
+
+	return param_get_bool(buffer, kp);
+}
+
 static const struct kernel_param_ops avic_ops = {
 	.flags = KERNEL_PARAM_OPS_FL_NOARG,
 	.set = avic_param_set,
-	.get = param_get_bool,
+	.get = avic_param_get,
 };
 
 /*
-- 
2.52.0

Re: [PATCH 1/2] KVM: SVM: Fix UBSAN warning when reading avic parameter

[Naveen N Rao]

On Tue, Feb 10, 2026 at 08:46:20AM +0200, Gal Pressman wrote:
>  
> +static int avic_param_get(char *buffer, const struct kernel_param *kp)
> +{
> +	int val = *(int *)kp->arg;
> +
> +	if (val == AVIC_AUTO_MODE)
> +		return sysfs_emit(buffer, "auto\n");

My preference would be to return 'N' here, so that this continues to 
return a boolean value when read.


- Naveen

Re: [PATCH 1/2] KVM: SVM: Fix UBSAN warning when reading avic parameter

[Gal Pressman]

On 23/02/2026 18:38, Naveen N Rao wrote:
> On Tue, Feb 10, 2026 at 08:46:20AM +0200, Gal Pressman wrote:
>>  
>> +static int avic_param_get(char *buffer, const struct kernel_param *kp)
>> +{
>> +	int val = *(int *)kp->arg;
>> +
>> +	if (val == AVIC_AUTO_MODE)
>> +		return sysfs_emit(buffer, "auto\n");
> 
> My preference would be to return 'N' here, so that this continues to 
> return a boolean value when read.

I guess the boolean conversion used to return 'Y' before this patch, why
is 'N' better?

Re: [PATCH 1/2] KVM: SVM: Fix UBSAN warning when reading avic parameter

[Naveen N Rao]

On Tue, Feb 24, 2026 at 11:18:01AM +0200, Gal Pressman wrote:
> On 23/02/2026 18:38, Naveen N Rao wrote:
> > On Tue, Feb 10, 2026 at 08:46:20AM +0200, Gal Pressman wrote:
> >>  
> >> +static int avic_param_get(char *buffer, const struct kernel_param *kp)
> >> +{
> >> +	int val = *(int *)kp->arg;
> >> +
> >> +	if (val == AVIC_AUTO_MODE)
> >> +		return sysfs_emit(buffer, "auto\n");
> > 
> > My preference would be to return 'N' here, so that this continues to 
> > return a boolean value when read.
> 
> I guess the boolean conversion used to return 'Y' before this patch, why
> is 'N' better?

Because that would reflect the state of AVIC more accurately:
- in the case of kvm-amd module still being loaded, AVIC has not yet 
  been enabled, and
- in the case of a non-AMD system, AVIC is not enabled and is not 
  active.

Also, note that AVIC used to default to 'N' until recently.


- Naveen

Re: [PATCH 1/2] KVM: SVM: Fix UBSAN warning when reading avic parameter

[Gal Pressman]

On 25/02/2026 8:27, Naveen N Rao wrote:
> On Tue, Feb 24, 2026 at 11:18:01AM +0200, Gal Pressman wrote:
>> On 23/02/2026 18:38, Naveen N Rao wrote:
>>> On Tue, Feb 10, 2026 at 08:46:20AM +0200, Gal Pressman wrote:
>>>>  
>>>> +static int avic_param_get(char *buffer, const struct kernel_param *kp)
>>>> +{
>>>> +	int val = *(int *)kp->arg;
>>>> +
>>>> +	if (val == AVIC_AUTO_MODE)
>>>> +		return sysfs_emit(buffer, "auto\n");
>>>
>>> My preference would be to return 'N' here, so that this continues to 
>>> return a boolean value when read.
>>
>> I guess the boolean conversion used to return 'Y' before this patch, why
>> is 'N' better?
> 
> Because that would reflect the state of AVIC more accurately:
> - in the case of kvm-amd module still being loaded, AVIC has not yet 
>   been enabled, and
> - in the case of a non-AMD system, AVIC is not enabled and is not 
>   active.
> 
> Also, note that AVIC used to default to 'N' until recently.

Will change in v2, thanks for the review.

[PATCH 2/2] KVM: x86/mmu: Fix UBSAN warning when reading nx_huge_pages parameter

[Gal Pressman]

The nx_huge_pages parameter is stored as an int (initialized to -1 to
indicate auto mode), but get_nx_huge_pages() calls param_get_bool()
which expects a bool pointer.
This causes UBSAN to report "load of value 255 is not a valid value for
type '_Bool'" when the parameter is read via sysfs during a narrow time
window.

The issue occurs during module load: the module parameter is registered
and its sysfs file becomes readable before the kvm_mmu_x86_module_init()
function runs:

1. Module load begins, static variable initialized to -1
2. mod_sysfs_setup() creates /sys/module/kvm/parameters/nx_huge_pages
3. (Parameter readable, value = -1)
4. do_init_module() runs kvm_x86_init()
5. kvm_mmu_x86_module_init() resolves -1 to bool

If userspace (e.g., sos report) reads the parameter during step 3,
param_get_bool() dereferences the int as a bool, triggering the UBSAN
warning.

Fix that by properly reading and converting the -1 value into an 'auto'
string.

Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
---
 arch/x86/kvm/mmu/mmu.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 02c450686b4a..3644d1db8be1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7488,9 +7488,14 @@ static void kvm_wake_nx_recovery_thread(struct kvm *kvm)
 
 static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp)
 {
+	int val = *(int *)kp->arg;
+
 	if (nx_hugepage_mitigation_hard_disabled)
 		return sysfs_emit(buffer, "never\n");
 
+	if (val == -1)
+		return sysfs_emit(buffer, "auto\n");
+
 	return param_get_bool(buffer, kp);
 }
 
-- 
2.52.0