[PATCH] KVM: arm64: Ensure 'params' is initialised when looking up sys register

[PATCH] KVM: arm64: Ensure 'params' is initialised when looking up sys register

[Will Deacon]

Commit 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce find_reg_by_id()")
introduced 'find_reg_by_id()', which looks up a system register only if
the 'id' index parameter identifies a valid system register. As part of
the patch, existing callers of 'find_reg()' were ported over to the new
interface, but this breaks 'index_to_sys_reg_desc()' in the case that the
initial lookup in the vCPU target table fails because we will then call
into 'find_reg()' for the system register table with an uninitialised
'param' as the key to the lookup.

GCC 10 is bright enough to spot this (amongst a tonne of false positives,
but hey!):

  | arch/arm64/kvm/sys_regs.c: In function ‘index_to_sys_reg_desc.part.0.isra’:
  | arch/arm64/kvm/sys_regs.c:983:33: warning: ‘params.Op2’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  |   983 |   (u32)(x)->CRn, (u32)(x)->CRm, (u32)(x)->Op2);
  | [...]

Revert the hunk of 4b927b94d5df which breaks 'index_to_sys_reg_desc()' so
that the old behaviour of checking the index upfront is restored.

Cc: <stable@vger.kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
Fixes: 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce find_reg_by_id()")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 46822afc57e0..01a515e0171e 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2360,8 +2360,11 @@ static const struct sys_reg_desc *index_to_sys_reg_desc(struct kvm_vcpu *vcpu,
 	if ((id & KVM_REG_ARM_COPROC_MASK) != KVM_REG_ARM64_SYSREG)
 		return NULL;
 
+	if (!index_to_params(id, &params))
+		return NULL;
+
 	table = get_target_table(vcpu->arch.target, true, &num);
-	r = find_reg_by_id(id, &params, table, num);
+	r = find_reg(&params, table, num);
 	if (!r)
 		r = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
 
-- 
2.24.1.735.g03f4e72817-goog

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Re: [PATCH] KVM: arm64: Ensure 'params' is initialised when looking up sys register

[Marc Zyngier]

On 2019-12-12 09:40, Will Deacon wrote:
> Commit 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce 
> find_reg_by_id()")
> introduced 'find_reg_by_id()', which looks up a system register only 
> if
> the 'id' index parameter identifies a valid system register. As part 
> of
> the patch, existing callers of 'find_reg()' were ported over to the 
> new
> interface, but this breaks 'index_to_sys_reg_desc()' in the case that 
> the
> initial lookup in the vCPU target table fails because we will then 
> call
> into 'find_reg()' for the system register table with an uninitialised
> 'param' as the key to the lookup.
>
> GCC 10 is bright enough to spot this (amongst a tonne of false 
> positives,
> but hey!):
>
>   | arch/arm64/kvm/sys_regs.c: In function 
> ‘index_to_sys_reg_desc.part.0.isra’:
>   | arch/arm64/kvm/sys_regs.c:983:33: warning: ‘params.Op2’ may be 
> used uninitialized in this function [-Wmaybe-uninitialized]
>   |   983 |   (u32)(x)->CRn, (u32)(x)->CRm, (u32)(x)->Op2);
>   | [...]
>
> Revert the hunk of 4b927b94d5df which breaks 
> 'index_to_sys_reg_desc()' so
> that the old behaviour of checking the index upfront is restored.

Huhuh... Well spotted GCC 10! And thanks Will for the fix.

>
> Cc: <stable@vger.kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
> Fixes: 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce 
> find_reg_by_id()")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 46822afc57e0..01a515e0171e 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2360,8 +2360,11 @@ static const struct sys_reg_desc
> *index_to_sys_reg_desc(struct kvm_vcpu *vcpu,
>  	if ((id & KVM_REG_ARM_COPROC_MASK) != KVM_REG_ARM64_SYSREG)
>  		return NULL;
>
> +	if (!index_to_params(id, &params))
> +		return NULL;
> +
>  	table = get_target_table(vcpu->arch.target, true, &num);
> -	r = find_reg_by_id(id, &params, table, num);
> +	r = find_reg(&params, table, num);
>  	if (!r)
>  		r = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));

Applied, thanks.

         M.
-- 
Jazz is not dead. It just smells funny...
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