This is a note to let you know that I've just added the patch titled
KVM: SEV: only access GHCB fields once
to the 6.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-sev-only-access-ghcb-fields-once.patch
and it can be found in the queue-6.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 7588dbcebcbf0193ab5b76987396d0254270b04a Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date: Fri, 4 Aug 2023 12:56:36 -0400
Subject: KVM: SEV: only access GHCB fields once
From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
commit 7588dbcebcbf0193ab5b76987396d0254270b04a upstream.
A KVM guest using SEV-ES or SEV-SNP with multiple vCPUs can trigger
a double fetch race condition vulnerability and invoke the VMGEXIT
handler recursively.
sev_handle_vmgexit() maps the GHCB page using kvm_vcpu_map() and then
fetches the exit code using ghcb_get_sw_exit_code(). Soon after,
sev_es_validate_vmgexit() fetches the exit code again. Since the GHCB
page is shared with the guest, the guest is able to quickly swap the
values with another vCPU and hence bypass the validation. One vmexit code
that can be rejected by sev_es_validate_vmgexit() is SVM_EXIT_VMGEXIT;
if sev_handle_vmgexit() observes it in the second fetch, the call
to svm_invoke_exit_handler() will invoke sev_handle_vmgexit() again
recursively.
To avoid the race, always fetch the GHCB data from the places where
sev_es_sync_from_ghcb stores it.
Exploiting recursions on linux kernel has been proven feasible
in the past, but the impact is mitigated by stack guard pages
(CONFIG_VMAP_STACK). Still, if an attacker manages to call the handler
multiple times, they can theoretically trigger a stack overflow and
cause a denial-of-service, or potentially guest-to-host escape in kernel
configurations without stack guard pages.
Note that winning the race reliably in every iteration is very tricky
due to the very tight window of the fetches; depending on the compiler
settings, they are often consecutive because of optimization and inlining.
Tested by booting an SEV-ES RHEL9 guest.
Fixes: CVE-2023-4155
Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Andy Nguyen <theflow@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/svm/sev.c | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2442,9 +2442,15 @@ static void sev_es_sync_from_ghcb(struct
memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
}
+static u64 kvm_ghcb_get_sw_exit_code(struct vmcb_control_area *control)
+{
+ return (((u64)control->exit_code_hi) << 32) | control->exit_code;
+}
+
static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
{
- struct kvm_vcpu *vcpu;
+ struct vmcb_control_area *control = &svm->vmcb->control;
+ struct kvm_vcpu *vcpu = &svm->vcpu;
struct ghcb *ghcb;
u64 exit_code;
u64 reason;
@@ -2455,7 +2461,7 @@ static int sev_es_validate_vmgexit(struc
* Retrieve the exit code now even though it may not be marked valid
* as it could help with debugging.
*/
- exit_code = ghcb_get_sw_exit_code(ghcb);
+ exit_code = kvm_ghcb_get_sw_exit_code(control);
/* Only GHCB Usage code 0 is supported */
if (ghcb->ghcb_usage) {
@@ -2470,7 +2476,7 @@ static int sev_es_validate_vmgexit(struc
!kvm_ghcb_sw_exit_info_2_is_valid(svm))
goto vmgexit_err;
- switch (ghcb_get_sw_exit_code(ghcb)) {
+ switch (exit_code) {
case SVM_EXIT_READ_DR7:
break;
case SVM_EXIT_WRITE_DR7:
@@ -2487,18 +2493,18 @@ static int sev_es_validate_vmgexit(struc
if (!kvm_ghcb_rax_is_valid(svm) ||
!kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
- if (ghcb_get_rax(ghcb) == 0xd)
+ if (vcpu->arch.regs[VCPU_REGS_RAX] == 0xd)
if (!kvm_ghcb_xcr0_is_valid(svm))
goto vmgexit_err;
break;
case SVM_EXIT_INVD:
break;
case SVM_EXIT_IOIO:
- if (ghcb_get_sw_exit_info_1(ghcb) & SVM_IOIO_STR_MASK) {
+ if (control->exit_info_1 & SVM_IOIO_STR_MASK) {
if (!kvm_ghcb_sw_scratch_is_valid(svm))
goto vmgexit_err;
} else {
- if (!(ghcb_get_sw_exit_info_1(ghcb) & SVM_IOIO_TYPE_MASK))
+ if (!(control->exit_info_1 & SVM_IOIO_TYPE_MASK))
if (!kvm_ghcb_rax_is_valid(svm))
goto vmgexit_err;
}
@@ -2506,7 +2512,7 @@ static int sev_es_validate_vmgexit(struc
case SVM_EXIT_MSR:
if (!kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
- if (ghcb_get_sw_exit_info_1(ghcb)) {
+ if (control->exit_info_1) {
if (!kvm_ghcb_rax_is_valid(svm) ||
!kvm_ghcb_rdx_is_valid(svm))
goto vmgexit_err;
@@ -2550,8 +2556,6 @@ static int sev_es_validate_vmgexit(struc
return 0;
vmgexit_err:
- vcpu = &svm->vcpu;
-
if (reason == GHCB_ERR_INVALID_USAGE) {
vcpu_unimpl(vcpu, "vmgexit: ghcb usage %#x is not valid\n",
ghcb->ghcb_usage);
@@ -2849,8 +2853,6 @@ int sev_handle_vmgexit(struct kvm_vcpu *
trace_kvm_vmgexit_enter(vcpu->vcpu_id, ghcb);
- exit_code = ghcb_get_sw_exit_code(ghcb);
-
sev_es_sync_from_ghcb(svm);
ret = sev_es_validate_vmgexit(svm);
if (ret)
@@ -2859,6 +2861,7 @@ int sev_handle_vmgexit(struct kvm_vcpu *
ghcb_set_sw_exit_info_1(ghcb, 0);
ghcb_set_sw_exit_info_2(ghcb, 0);
+ exit_code = kvm_ghcb_get_sw_exit_code(control);
switch (exit_code) {
case SVM_VMGEXIT_MMIO_READ:
ret = setup_vmgexit_scratch(svm, true, control->exit_info_2);
Patches currently in stable-queue which might be from pbonzini@xxxxxxxxxx are
queue-6.4/kvm-sev-snapshot-the-ghcb-before-accessing-it.patch
queue-6.4/kvm-sev-only-access-ghcb-fields-once.patch
