[v2] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

[PATCH v2] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

Posted by Jiakai Xu 2 months, 1 week ago

The make_xfence_request() function uses a shift operation to check if a
vCPU is in the hart mask:

  if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))

However, when the difference between vcpu_id and hbase
is >= BITS_PER_LONG, the shift operation causes undefined behavior.

This was detected by UBSAN:
  UBSAN: shift-out-of-bounds in arch/riscv/kvm/tlb.c:343:23
  shift exponent 256 is too large for 64-bit type 'long unsigned int'

Fix this by adding a bounds check before the shift operation.

This bug was found by fuzzing the KVM RISC-V interface.

Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
---
V1 -> V2:
- Dropped 'idx' variable and compared vcpu_id against hbase directly,
  as suggested by Andrew Jones.
---
 arch/riscv/kvm/tlb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
index ff1aeac4eb8eb..439c20c2775ab 100644
--- a/arch/riscv/kvm/tlb.c
+++ b/arch/riscv/kvm/tlb.c
@@ -338,7 +338,8 @@ static void make_xfence_request(struct kvm *kvm,
 	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (hbase != -1UL) {
-			if (vcpu->vcpu_id < hbase)
+			if (vcpu->vcpu_id < hbase ||
+				vcpu->vcpu_id >= hbase + BITS_PER_LONG)
 				continue;
 			if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
 				continue;
-- 
2.34.1

Re: [PATCH v2] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

Posted by Anup Patel 2 months, 1 week ago

On Sat, Apr 4, 2026 at 4:50 AM Jiakai Xu <xujiakai2025@iscas.ac.cn> wrote:
>
> The make_xfence_request() function uses a shift operation to check if a
> vCPU is in the hart mask:
>
>   if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
>
> However, when the difference between vcpu_id and hbase
> is >= BITS_PER_LONG, the shift operation causes undefined behavior.
>
> This was detected by UBSAN:
>   UBSAN: shift-out-of-bounds in arch/riscv/kvm/tlb.c:343:23
>   shift exponent 256 is too large for 64-bit type 'long unsigned int'
>
> Fix this by adding a bounds check before the shift operation.
>
> This bug was found by fuzzing the KVM RISC-V interface.
>
> Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
> Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
> Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>

Queued this patch for Linux-7.1

Thanks,
Anup

> ---
> V1 -> V2:
> - Dropped 'idx' variable and compared vcpu_id against hbase directly,
>   as suggested by Andrew Jones.
> ---
>  arch/riscv/kvm/tlb.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
> index ff1aeac4eb8eb..439c20c2775ab 100644
> --- a/arch/riscv/kvm/tlb.c
> +++ b/arch/riscv/kvm/tlb.c
> @@ -338,7 +338,8 @@ static void make_xfence_request(struct kvm *kvm,
>         bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
>         kvm_for_each_vcpu(i, vcpu, kvm) {
>                 if (hbase != -1UL) {
> -                       if (vcpu->vcpu_id < hbase)
> +                       if (vcpu->vcpu_id < hbase ||
> +                               vcpu->vcpu_id >= hbase + BITS_PER_LONG)
>                                 continue;
>                         if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
>                                 continue;
> --
> 2.34.1
>

Re: [PATCH v2] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

Posted by Andrew Jones 2 months, 1 week ago

On Fri, Apr 03, 2026 at 11:20:11PM +0000, Jiakai Xu wrote:
> The make_xfence_request() function uses a shift operation to check if a
> vCPU is in the hart mask:
> 
>   if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
> 
> However, when the difference between vcpu_id and hbase
> is >= BITS_PER_LONG, the shift operation causes undefined behavior.
> 
> This was detected by UBSAN:
>   UBSAN: shift-out-of-bounds in arch/riscv/kvm/tlb.c:343:23
>   shift exponent 256 is too large for 64-bit type 'long unsigned int'
> 
> Fix this by adding a bounds check before the shift operation.
> 
> This bug was found by fuzzing the KVM RISC-V interface.
> 
> Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
> Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
> Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
> ---
> V1 -> V2:
> - Dropped 'idx' variable and compared vcpu_id against hbase directly,
>   as suggested by Andrew Jones.
> ---
>  arch/riscv/kvm/tlb.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
> index ff1aeac4eb8eb..439c20c2775ab 100644
> --- a/arch/riscv/kvm/tlb.c
> +++ b/arch/riscv/kvm/tlb.c
> @@ -338,7 +338,8 @@ static void make_xfence_request(struct kvm *kvm,
>  	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		if (hbase != -1UL) {
> -			if (vcpu->vcpu_id < hbase)
> +			if (vcpu->vcpu_id < hbase ||
> +				vcpu->vcpu_id >= hbase + BITS_PER_LONG)
>  				continue;
>  			if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
>  				continue;
> -- 
> 2.34.1
>

Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>