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Subject[PATCH 3.16 024/233] KVM: x86: Fix load damaged SSEx MXCSR register
3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wanpeng Li <wanpeng.li@hotmail.com>

commit a575813bfe4bc15aba511a5e91e61d242bff8b9d upstream.

Reported by syzkaller:

BUG: unable to handle kernel paging request at ffffffffc07f6a2e
IP: report_bug+0x94/0x120
PGD 348e12067
P4D 348e12067
PUD 348e14067
PMD 3cbd84067
PTE 80000003f7e87161

Oops: 0003 [#1] SMP
CPU: 2 PID: 7091 Comm: kvm_load_guest_ Tainted: G OE 4.11.0+ #8
task: ffff92fdfb525400 task.stack: ffffbda6c3d04000
RIP: 0010:report_bug+0x94/0x120
RSP: 0018:ffffbda6c3d07b20 EFLAGS: 00010202
do_trap+0x156/0x170
do_error_trap+0xa3/0x170
? kvm_load_guest_fpu.part.175+0x12a/0x170 [kvm]
? mark_held_locks+0x79/0xa0
? retint_kernel+0x10/0x10
? trace_hardirqs_off_thunk+0x1a/0x1c
do_invalid_op+0x20/0x30
invalid_op+0x1e/0x30
RIP: 0010:kvm_load_guest_fpu.part.175+0x12a/0x170 [kvm]
? kvm_load_guest_fpu.part.175+0x1c/0x170 [kvm]
kvm_arch_vcpu_ioctl_run+0xed6/0x1b70 [kvm]
kvm_vcpu_ioctl+0x384/0x780 [kvm]
? kvm_vcpu_ioctl+0x384/0x780 [kvm]
? sched_clock+0x13/0x20
? __do_page_fault+0x2a0/0x550
do_vfs_ioctl+0xa4/0x700
? up_read+0x1f/0x40
? __do_page_fault+0x2a0/0x550
SyS_ioctl+0x79/0x90
entry_SYSCALL_64_fastpath+0x23/0xc2

SDM mentioned that "The MXCSR has several reserved bits, and attempting to write
a 1 to any of these bits will cause a general-protection exception(#GP) to be
generated". The syzkaller forks' testcase overrides xsave area w/ random values
and steps on the reserved bits of MXCSR register. The damaged MXCSR register
values of guest will be restored to SSEx MXCSR register before vmentry. This
patch fixes it by catching userspace override MXCSR register reserved bits w/
random values and bails out immediately.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kernel/i387.c | 1 +
arch/x86/kvm/x86.c | 9 +++++++--
2 files changed, 8 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -115,6 +115,7 @@ void unlazy_fpu(struct task_struct *tsk)
EXPORT_SYMBOL(unlazy_fpu);

unsigned int mxcsr_feature_mask __read_mostly = 0xffffffffu;
+EXPORT_SYMBOL_GPL(mxcsr_feature_mask);
unsigned int xstate_size;
EXPORT_SYMBOL_GPL(xstate_size);
static struct i387_fxsave_struct fx_scratch;
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3110,11 +3110,14 @@ static void kvm_vcpu_ioctl_x86_get_xsave
}
}

+#define XSAVE_MXCSR_OFFSET 24
+
static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
struct kvm_xsave *guest_xsave)
{
u64 xstate_bv =
*(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)];
+ u32 mxcsr = *(u32 *)&guest_xsave->region[XSAVE_MXCSR_OFFSET / sizeof(u32)];

if (cpu_has_xsave) {
/*
@@ -3122,12 +3125,14 @@ static int kvm_vcpu_ioctl_x86_set_xsave(
* CPUID leaf 0xD, index 0, EDX:EAX. This is for compatibility
* with old userspace.
*/
- if (xstate_bv & ~kvm_supported_xcr0())
+ if (xstate_bv & ~kvm_supported_xcr0() ||
+ mxcsr & ~mxcsr_feature_mask)
return -EINVAL;
memcpy(&vcpu->arch.guest_fpu.state->xsave,
guest_xsave->region, vcpu->arch.guest_xstate_size);
} else {
- if (xstate_bv & ~XSTATE_FPSSE)
+ if (xstate_bv & ~XSTATE_FPSSE ||
+ mxcsr & ~mxcsr_feature_mask)
return -EINVAL;
memcpy(&vcpu->arch.guest_fpu.state->fxsave,
guest_xsave->region, sizeof(struct i387_fxsave_struct));
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