Avi Kivity wrote:
Marcelo Tosatti wrote:
Matt T. Yourst notes that kvm_arch_vcpu_ioctl_set_sregs lacks validity
checking for the new cr3 value:
"Userspace callers of KVM_SET_SREGS can pass a bogus value of
cr3 to
the kernel. This will trigger a NULL pointer access in gfn_to_rmap()
when userspace next tries to call KVM_RUN on the affected VCPU and kvm
attempts to activate the new non-existent page table root.
This happens since kvm only validates that cr3 points to a valid guest
physical memory page when code *inside* the guest sets cr3. However, kvm
currently trusts the userspace caller (e.g. QEMU) on the host machine to
always supply a valid page table root, rather than properly validating
it along with the rest of the reloaded guest state."
http://sourceforge.net/tracker/?func=detail&atid=893831&aid=2687641&group_id=180599
Check for a valid cr3 address in kvm_arch_vcpu_ioctl_set_sregs, triple
fault in case of failure.
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Index: kvm/arch/x86/kvm/x86.c
===================================================================
--- kvm.orig/arch/x86/kvm/x86.c
+++ kvm/arch/x86/kvm/x86.c
@@ -3986,7 +3986,13 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct
vcpu->arch.cr2 = sregs->cr2;
mmu_reset_needed |= vcpu->arch.cr3 != sregs->cr3;
- vcpu->arch.cr3 = sregs->cr3;
+
+ down_read(&vcpu->kvm->slots_lock);
+ if (gfn_to_memslot(vcpu->kvm, sregs->cr3 >> PAGE_SHIFT))
+ vcpu->arch.cr3 = sregs->cr3;
+ else
+ set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
+ up_read(&vcpu->kvm->slots_lock);
kvm_set_cr8(vcpu, sregs->cr8);
Isn't this self-defeating? If you drop slots_lock, cr3 may be invalid
again by the time you set cvpu->arch.cr3?
Uh, sorry, of course not. I misread down as up. Bad day for me. Will
apply the patch.
Still, don't we have a problem if userspace drops the memory slot where
cr3 points to?
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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