2017-12-07 08:30:15

by Wanpeng Li

Subject: [PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

From: Wanpeng Li <[email protected]>

*** Guest State ***
CR0: actual=0x0000000000000030, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
CR4: actual=0x0000000000002050, shadow=0x0000000000000000, gh_mask=ffffffffffffe871
CR3 = 0x00000000fffbc000
RSP = 0x0000000000000000 RIP = 0x0000000000000000
RFLAGS=0x00000000 DR7 = 0x0000000000000400
^^^^^^^^^^

The failed vmentry is triggered by the following testcase when ept=Y:

#include <unistd.h>
#include <sys/syscall.h>
#include <string.h>
#include <stdint.h>
#include <linux/kvm.h>
#include <fcntl.h>
#include <sys/ioctl.h>

long r[5];
int main()
{
r[2] = open("/dev/kvm", O_RDONLY);
r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
struct kvm_regs regs = {
.rflags = 0,
};
ioctl(r[4], KVM_SET_REGS, &regs);
ioctl(r[4], KVM_RUN, 0);
}

X86 RFLAGS bit 1 is fixed set, userspace can simply clearing bit 1
of RFLAGS with KVM_SET_REGS ioctl which results in vmentry fails.
This patch fixes it by oring X86_EFLAGS_FIXED during ioctl.

Suggested-by: Jim Mattson <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Jim Mattson <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
---
v2 -> v3:
* move Oring X86_EFLAGS_FIXED to kvm_arch_vcpu_ioctl_set_regs
v1 -> v2:
* Oring X86_EFLAGS_FIXED

arch/x86/kvm/x86.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ebe0a3a..0c5d55c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7405,7 +7405,7 @@ int kvm_arch_vcpu_ioctl_set_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
#endif

kvm_rip_write(vcpu, regs->rip);
- kvm_set_rflags(vcpu, regs->rflags);
+ kvm_set_rflags(vcpu, regs->rflags | X86_EFLAGS_FIXED);

vcpu->arch.exception.pending = false;

--
2.7.4

2017-12-07 09:24:41

by Quan Xu

Subject: Re: [PATCH v3] KVM: X86: Fix load RFLAGS w/o the fixed bit

On 2017/12/07 16:30, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> *** Guest State ***
> CR0: actual=0x0000000000000030, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
> CR4: actual=0x0000000000002050, shadow=0x0000000000000000, gh_mask=ffffffffffffe871
> CR3 = 0x00000000fffbc000
> RSP = 0x0000000000000000 RIP = 0x0000000000000000
> RFLAGS=0x00000000 DR7 = 0x0000000000000400
> ^^^^^^^^^^
>
> The failed vmentry is triggered by the following testcase when ept=Y:
>
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <string.h>
> #include <stdint.h>
> #include <linux/kvm.h>
> #include <fcntl.h>
> #include <sys/ioctl.h>
>
> long r[5];
> int main()
> {
> r[2] = open("/dev/kvm", O_RDONLY);
> r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
> r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
> struct kvm_regs regs = {
> .rflags = 0,
> };
> ioctl(r[4], KVM_SET_REGS, &regs);
> ioctl(r[4], KVM_RUN, 0);
> }
>
> X86 RFLAGS bit 1 is fixed set, userspace can simply clearing bit 1
> of RFLAGS with KVM_SET_REGS ioctl which results in vmentry fails.
> This patch fixes it by oring X86_EFLAGS_FIXED during ioctl.
>
> Suggested-by: Jim Mattson <[email protected]>
> Reviewed-by: David Hildenbrand <[email protected]>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Radim Krčmář <[email protected]>
> Cc: Jim Mattson <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
  Reviewed-by: Quan Xu <[email protected]>