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    Subject[PATCH v2 01/66] KVM: x86: Return -E2BIG when KVM_GET_SUPPORTED_CPUID hits max entries
    Date
    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
    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@redhat.com>
    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@redhat.com>
    Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    ---
    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;
    --
    2.24.1
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