Re: [PATCH 01/61] KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED_CPUID hits max entries

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> Fix a long-standing bug that causes KVM to return 0 instead of -E2BIG
> when userspace's array is insufficiently sized.
>
> Note, while the Fixes: tag is accurate with respect to the immediate
> bug, it's likely that similar bugs in KVM_GET_SUPPORTED_CPUID existed
> prior to the refactoring, e.g. Qemu contains a workaround for the broken
> KVM_GET_SUPPORTED_CPUID behavior that predates the buggy commit by over
> two years.  The Qemu workaround is also likely the main reason the bug
> has gone unreported for so long.
>
> Qemu hack:
>   commit 76ae317f7c16aec6b469604b1764094870a75470
>   Author: Mark McLoughlin <markmc@xxxxxxxxxx>
>   Date:   Tue May 19 18:55:21 2009 +0100
>
>     kvm: work around supported cpuid ioctl() brokenness
>
>     KVM_GET_SUPPORTED_CPUID has been known to fail to return -E2BIG
>     when it runs out of entries. Detect this by always trying again
>     with a bigger table if the ioctl() fills the table.
>
> Fixes: 831bf664e9c1f ("KVM: Refactor and simplify kvm_dev_ioctl_get_supported_cpuid")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b1c469446b07..47ce04762c20 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -908,9 +908,14 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>  			goto out_free;
>  
>  		limit = cpuid_entries[nent - 1].eax;
> -		for (func = ent->func + 1; func <= limit && nent < cpuid->nent && r == 0; ++func)
> +		for (func = ent->func + 1; func <= limit && r == 0; ++func) {
> +			if (nent >= cpuid->nent) {
> +				r = -E2BIG;
> +				goto out_free;
> +			}
>  			r = do_cpuid_func(&cpuid_entries[nent], func,
>  				          &nent, cpuid->nent, type);
> +		}
>  
>  		if (r)
>  			goto out_free;

Is fixing a bug a valid reason for breaking buggy userspace? :-)
Personally, I think so. In particular, here the change is both the
return value and the fact that we don't do copy_to_user() anymore so I
think it's possible to meet a userspace which is going to get broken by
the change.

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly