Re: [PATCH v2 01/66] KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED_CPUID hits max entries

[Jim Mattson]

On Mon, Mar 2, 2020 at 3:57 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Fix a long-standing bug that causes KVM to return 0 instead of -E2BIG
> when userspace's array is insufficiently sized.
>
> This technically breaks backwards compatibility, e.g. a userspace with a
> hardcoded cpuid->nent could theoretically be broken as it would see an
> error instead of success if cpuid->nent is less than the number of
> entries required to fully enumerate the host CPU.  But, the lowest known
> cpuid->nent hardcoded by a VMM is 100 (lkvm and selftests), and the

I have an existence proof for 98. :-)

> largest realistic limit on Intel and AMD is well under a 100.  E.g.
> Intel's Icelake server with all the bells and whistles tops out at ~60
> entries (variable due to SGX sub-leafs), and AMD's CPUID documentation
> allows for less than 50 (KVM hard caps CPUID 0xD at a single sub-leaf).
>
> 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")
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>