[PATCH] KVM: x86: Add more protection against undefined behavior in rsvd_bits()

[PATCH] KVM: x86: Add more protection against undefined behavior in rsvd_bits()

[Sean Christopherson]

Add compile-time asserts in rsvd_bits() to guard against KVM passing in
garbage hardcoded values, and cap the upper bound at '63' for dynamic
values to prevent generating a mask that would overflow a u64.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 581925e476d6..261be1d2032b 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -44,8 +44,15 @@
 #define PT32_ROOT_LEVEL 2
 #define PT32E_ROOT_LEVEL 3
 
-static inline u64 rsvd_bits(int s, int e)
+static __always_inline u64 rsvd_bits(int s, int e)
 {
+	BUILD_BUG_ON(__builtin_constant_p(e) && __builtin_constant_p(s) && e < s);
+
+	if (__builtin_constant_p(e))
+		BUILD_BUG_ON(e > 63);
+	else
+		e &= 63;
+
 	if (e < s)
 		return 0;
 
-- 
2.30.0.284.gd98b1dd5eaa7-goog

Re: [PATCH] KVM: x86: Add more protection against undefined behavior in rsvd_bits()

[Paolo Bonzini]

On 13/01/21 21:45, Sean Christopherson wrote:
> Add compile-time asserts in rsvd_bits() to guard against KVM passing in
> garbage hardcoded values, and cap the upper bound at '63' for dynamic
> values to prevent generating a mask that would overflow a u64.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/mmu.h | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 581925e476d6..261be1d2032b 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -44,8 +44,15 @@
>   #define PT32_ROOT_LEVEL 2
>   #define PT32E_ROOT_LEVEL 3
>   
> -static inline u64 rsvd_bits(int s, int e)
> +static __always_inline u64 rsvd_bits(int s, int e)
>   {
> +	BUILD_BUG_ON(__builtin_constant_p(e) && __builtin_constant_p(s) && e < s);
> +
> +	if (__builtin_constant_p(e))
> +		BUILD_BUG_ON(e > 63);
> +	else
> +		e &= 63;
> +
>   	if (e < s)
>   		return 0;
>   
> 

Queued for 5.11, thanks.

Paolo