[PATCH 1/3] KVM: SEV: snapshot the GHCB before accessing it

From: Paolo Bonzini
Date: Fri Aug 04 2023 - 13:35:06 EST

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Validation of the GHCB is susceptible to time-of-check/time-of-use vulnerabilities.
To avoid them, we would like to always snapshot the fields that are read in
sev_es_validate_vmgexit(), and not use the GHCB anymore after it returns.

This means:

- invoking sev_es_sync_from_ghcb() before any GHCB access, including before
sev_es_validate_vmgexit()

- snapshotting all fields including the valid bitmap and the sw_scratch field,
which are currently not caching anywhere.

The valid bitmap is the first thing to be copied out of the GHCB; then,
further accesses will use the copy in svm->sev_es.

Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/svm/sev.c | 69 +++++++++++++++++++++---------------------
arch/x86/kvm/svm/svm.h | 26 ++++++++++++++++
2 files changed, 61 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 07756b7348ae..e898f0b2b0ba 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2417,15 +2417,18 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
*/
memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));

- vcpu->arch.regs[VCPU_REGS_RAX] = ghcb_get_rax_if_valid(ghcb);
- vcpu->arch.regs[VCPU_REGS_RBX] = ghcb_get_rbx_if_valid(ghcb);
- vcpu->arch.regs[VCPU_REGS_RCX] = ghcb_get_rcx_if_valid(ghcb);
- vcpu->arch.regs[VCPU_REGS_RDX] = ghcb_get_rdx_if_valid(ghcb);
- vcpu->arch.regs[VCPU_REGS_RSI] = ghcb_get_rsi_if_valid(ghcb);
+ BUILD_BUG_ON(sizeof(svm->sev_es.valid_bitmap) != sizeof(ghcb->save.valid_bitmap));
+ memcpy(&svm->sev_es.valid_bitmap, &ghcb->save.valid_bitmap, sizeof(ghcb->save.valid_bitmap));

- svm->vmcb->save.cpl = ghcb_get_cpl_if_valid(ghcb);
+ vcpu->arch.regs[VCPU_REGS_RAX] = kvm_ghcb_get_rax_if_valid(svm, ghcb);
+ vcpu->arch.regs[VCPU_REGS_RBX] = kvm_ghcb_get_rbx_if_valid(svm, ghcb);
+ vcpu->arch.regs[VCPU_REGS_RCX] = kvm_ghcb_get_rcx_if_valid(svm, ghcb);
+ vcpu->arch.regs[VCPU_REGS_RDX] = kvm_ghcb_get_rdx_if_valid(svm, ghcb);
+ vcpu->arch.regs[VCPU_REGS_RSI] = kvm_ghcb_get_rsi_if_valid(svm, ghcb);

- if (ghcb_xcr0_is_valid(ghcb)) {
+ svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm, ghcb);
+
+ if (kvm_ghcb_xcr0_is_valid(svm)) {
vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
kvm_update_cpuid_runtime(vcpu);
}
@@ -2436,6 +2439,7 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
control->exit_code_hi = upper_32_bits(exit_code);
control->exit_info_1 = ghcb_get_sw_exit_info_1(ghcb);
control->exit_info_2 = ghcb_get_sw_exit_info_2(ghcb);
+ svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm, ghcb);

/* Clear the valid entries fields */
memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
@@ -2464,56 +2468,56 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)

reason = GHCB_ERR_MISSING_INPUT;

- if (!ghcb_sw_exit_code_is_valid(ghcb) ||
- !ghcb_sw_exit_info_1_is_valid(ghcb) ||
- !ghcb_sw_exit_info_2_is_valid(ghcb))
+ if (!kvm_ghcb_sw_exit_code_is_valid(svm) ||
+ !kvm_ghcb_sw_exit_info_1_is_valid(svm) ||
+ !kvm_ghcb_sw_exit_info_2_is_valid(svm))
goto vmgexit_err;

switch (ghcb_get_sw_exit_code(ghcb)) {
case SVM_EXIT_READ_DR7:
break;
case SVM_EXIT_WRITE_DR7:
- if (!ghcb_rax_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm))
goto vmgexit_err;
break;
case SVM_EXIT_RDTSC:
break;
case SVM_EXIT_RDPMC:
- if (!ghcb_rcx_is_valid(ghcb))
+ if (!kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
break;
case SVM_EXIT_CPUID:
- if (!ghcb_rax_is_valid(ghcb) ||
- !ghcb_rcx_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm) ||
+ !kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
if (ghcb_get_rax(ghcb) == 0xd)
- if (!ghcb_xcr0_is_valid(ghcb))
+ 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 (!ghcb_sw_scratch_is_valid(ghcb))
+ 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 (!ghcb_rax_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm))
goto vmgexit_err;
}
break;
case SVM_EXIT_MSR:
- if (!ghcb_rcx_is_valid(ghcb))
+ if (!kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
if (ghcb_get_sw_exit_info_1(ghcb)) {
- if (!ghcb_rax_is_valid(ghcb) ||
- !ghcb_rdx_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm) ||
+ !kvm_ghcb_rdx_is_valid(svm))
goto vmgexit_err;
}
break;
case SVM_EXIT_VMMCALL:
- if (!ghcb_rax_is_valid(ghcb) ||
- !ghcb_cpl_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm) ||
+ !kvm_ghcb_cpl_is_valid(svm))
goto vmgexit_err;
break;
case SVM_EXIT_RDTSCP:
@@ -2521,19 +2525,19 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
case SVM_EXIT_WBINVD:
break;
case SVM_EXIT_MONITOR:
- if (!ghcb_rax_is_valid(ghcb) ||
- !ghcb_rcx_is_valid(ghcb) ||
- !ghcb_rdx_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm) ||
+ !kvm_ghcb_rcx_is_valid(svm) ||
+ !kvm_ghcb_rdx_is_valid(svm))
goto vmgexit_err;
break;
case SVM_EXIT_MWAIT:
- if (!ghcb_rax_is_valid(ghcb) ||
- !ghcb_rcx_is_valid(ghcb))
+ if (!kvm_ghcb_rax_is_valid(svm) ||
+ !kvm_ghcb_rcx_is_valid(svm))
goto vmgexit_err;
break;
case SVM_VMGEXIT_MMIO_READ:
case SVM_VMGEXIT_MMIO_WRITE:
- if (!ghcb_sw_scratch_is_valid(ghcb))
+ if (!kvm_ghcb_sw_scratch_is_valid(svm))
goto vmgexit_err;
break;
case SVM_VMGEXIT_NMI_COMPLETE:
@@ -2563,9 +2567,6 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
dump_ghcb(svm);
}

- /* Clear the valid entries fields */
- memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
-
ghcb_set_sw_exit_info_1(ghcb, 2);
ghcb_set_sw_exit_info_2(ghcb, reason);

@@ -2586,7 +2587,7 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm)
*/
if (svm->sev_es.ghcb_sa_sync) {
kvm_write_guest(svm->vcpu.kvm,
- ghcb_get_sw_scratch(svm->sev_es.ghcb),
+ svm->sev_es.sw_scratch,
svm->sev_es.ghcb_sa,
svm->sev_es.ghcb_sa_len);
svm->sev_es.ghcb_sa_sync = false;
@@ -2637,7 +2638,7 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
u64 scratch_gpa_beg, scratch_gpa_end;
void *scratch_va;

- scratch_gpa_beg = ghcb_get_sw_scratch(ghcb);
+ scratch_gpa_beg = svm->sev_es.sw_scratch;
if (!scratch_gpa_beg) {
pr_err("vmgexit: scratch gpa not provided\n");
goto e_scratch;
@@ -2853,11 +2854,11 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)

exit_code = ghcb_get_sw_exit_code(ghcb);

+ sev_es_sync_from_ghcb(svm);
ret = sev_es_validate_vmgexit(svm);
if (ret)
return ret;

- sev_es_sync_from_ghcb(svm);
ghcb_set_sw_exit_info_1(ghcb, 0);
ghcb_set_sw_exit_info_2(ghcb, 0);

diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 18af7e712a5a..8239c8de45ac 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -190,10 +190,12 @@ struct vcpu_sev_es_state {
/* SEV-ES support */
struct sev_es_save_area *vmsa;
struct ghcb *ghcb;
+ u8 valid_bitmap[16];
struct kvm_host_map ghcb_map;
bool received_first_sipi;

/* SEV-ES scratch area support */
+ u64 sw_scratch;
void *ghcb_sa;
u32 ghcb_sa_len;
bool ghcb_sa_sync;
@@ -744,4 +746,28 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm);
void __svm_sev_es_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted);
void __svm_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted);

+#define DEFINE_KVM_GHCB_ACCESSORS(field) \
+ static __always_inline bool kvm_ghcb_##field##_is_valid(const struct vcpu_svm *svm) \
+ { \
+ return test_bit(GHCB_BITMAP_IDX(field), \
+ (unsigned long *)&svm->sev_es.valid_bitmap); \
+ } \
+ \
+ static __always_inline u64 kvm_ghcb_get_##field##_if_valid(struct vcpu_svm *svm, struct ghcb *ghcb) \
+ { \
+ return kvm_ghcb_##field##_is_valid(svm) ? ghcb->save.field : 0; \
+ } \
+
+DEFINE_KVM_GHCB_ACCESSORS(cpl)
+DEFINE_KVM_GHCB_ACCESSORS(rax)
+DEFINE_KVM_GHCB_ACCESSORS(rcx)
+DEFINE_KVM_GHCB_ACCESSORS(rdx)
+DEFINE_KVM_GHCB_ACCESSORS(rbx)
+DEFINE_KVM_GHCB_ACCESSORS(rsi)
+DEFINE_KVM_GHCB_ACCESSORS(sw_exit_code)
+DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_1)
+DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_2)
+DEFINE_KVM_GHCB_ACCESSORS(sw_scratch)
+DEFINE_KVM_GHCB_ACCESSORS(xcr0)
+
#endif
--
2.39.0

Next message: Li, Xin3: "RE: [RFC PATCH 1/1] x86/traps: Get rid of exception handlers' second argument error code"
Previous message: Paolo Bonzini: "[PATCH 2/3] KVM: SEV: only access GHCB fields once"
In reply to: Paolo Bonzini: "[PATCH 2/3] KVM: SEV: only access GHCB fields once"
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