Re: [PATCH] KVM: x86: Fix the NULL pointer parameter in check_cr_write()

From: Yu Zhang
Date: Wed Sep 20 2017 - 02:58:58 EST

On 9/18/2017 11:56 PM, Jim Mattson wrote:
kvm_cpuid ultimately wants to write all four of the GPRs passed in by
reference. I don't see any advantage to allowing some of these
pointers to be NULL.

Thanks for your comments, Jim & David.

2 reasons I did not choose to change kvm_cpuid():
1> like Jim's comments, kvm_cpuid() will eventually write the *eax - *edx no matter
a cpuid entry is found or not;
2> currently, return value of kvm_cpuid() is either true when an entry is found or false
otherwise. We can change kvm_cpuid() to check the pointers of GPRs against NULL and
return false immediately. Then the false value would have 2 different meanings - entry
not found, or invalid params.

Paolo, any suggestion? :-)


Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>

On Mon, Sep 18, 2017 at 5:19 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
On 18.09.2017 12:45, Yu Zhang wrote:
Routine check_cr_write() will trigger emulator_get_cpuid()->
kvm_cpuid() to get maxphyaddr, and NULL is passed as values
for ebx/ecx/edx. This is problematic because kvm_cpuid() will
dereference these pointers.

Fixes: d1cd3ce90044 ("KVM: MMU: check guest CR3 reserved bits based on its physical address width.")
Reported-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
arch/x86/kvm/emulate.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 16bf665..15f527b 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4102,10 +4102,12 @@ static int check_cr_write(struct x86_emulate_ctxt *ctxt)
ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
if (efer & EFER_LMA) {
u64 maxphyaddr;
- u32 eax = 0x80000008;
+ u32 eax, ebx, ecx, edx;

- if (ctxt->ops->get_cpuid(ctxt, &eax, NULL, NULL,
- NULL, false))
+ eax = 0x80000008;
+ ecx = 0;
+ if (ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx,
+ &edx, false))
maxphyaddr = eax & 0xff;
maxphyaddr = 36;

Not sure if fixing kvm_cpuid() would be better.

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>