This patch adds SMAP handling logic when setting CR4 for guests
Thanks a lot to Paolo Bonzini for his suggestion to use the branchless
way to detect SMAP violation.
Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
---
arch/x86/kvm/cpuid.h | 8 ++++++++
arch/x86/kvm/mmu.c | 35 ++++++++++++++++++++++++++++++++---
arch/x86/kvm/mmu.h | 44 ++++++++++++++++++++++++++++++++++++--------
arch/x86/kvm/paging_tmpl.h | 2 +-
arch/x86/kvm/x86.c | 9 ++++++++-
5 files changed, 85 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index a2a1bb7..eeecbed 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -48,6 +48,14 @@ static inline bool guest_cpuid_has_smep(struct kvm_vcpu *vcpu)
return best && (best->ebx & bit(X86_FEATURE_SMEP));
}
+static inline bool guest_cpuid_has_smap(struct kvm_vcpu *vcpu)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = kvm_find_cpuid_entry(vcpu, 7, 0);
+ return best && (best->ebx & bit(X86_FEATURE_SMAP));
+}
+
static inline bool guest_cpuid_has_fsgsbase(struct kvm_vcpu *vcpu)
{
struct kvm_cpuid_entry2 *best;
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9b53135..5a1ed38 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3601,20 +3601,28 @@ static void reset_rsvds_bits_mask_ept(struct kvm_vcpu *vcpu,
}
}
-static void update_permission_bitmask(struct kvm_vcpu *vcpu,
+void update_permission_bitmask(struct kvm_vcpu *vcpu,
struct kvm_mmu *mmu, bool ept)
{
unsigned bit, byte, pfec;
u8 map;
- bool fault, x, w, u, wf, uf, ff, smep;
+ bool fault, x, w, u, wf, uf, ff, smapf, cr4_smap, smep, smap = 0;
smep = kvm_read_cr4_bits(vcpu, X86_CR4_SMEP);
+ cr4_smap = kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
for (byte = 0; byte < ARRAY_SIZE(mmu->permissions); ++byte) {
pfec = byte << 1;
map = 0;
wf = pfec & PFERR_WRITE_MASK;
uf = pfec & PFERR_USER_MASK;
ff = pfec & PFERR_FETCH_MASK;
+ /*
+ * PFERR_RSVD_MASK bit is used to detect SMAP violation.
+ * We will check it in permission_fault(), this bit is
+ * set in pfec for normal fault, while it is cleared for
+ * SMAP violations.
+ */
+ smapf = !(pfec & PFERR_RSVD_MASK);
for (bit = 0; bit < 8; ++bit) {
x = bit & ACC_EXEC_MASK;
w = bit & ACC_WRITE_MASK;
@@ -3627,11 +3635,32 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
w |= !is_write_protection(vcpu) && !uf;
/* Disallow supervisor fetches of user code if cr4.smep */
x &= !(smep && u && !uf);
+
+ /*
+ * SMAP:kernel-mode data accesses from user-mode
+ * mappings should fault. A fault is considered
+ * as a SMAP violation if all of the following
+ * conditions are ture:
+ * - X86_CR4_SMAP is set in CR4
+ * - An user page is accessed
+ * - Page fault in kernel mode
+ * - !(CPL<3 && X86_EFLAGS_AC is set)
+ *
+ * Here, we cover the first three conditions,
+ * The CPL and X86_EFLAGS_AC is in smapf,which
+ * permission_fault() computes dynamically.
+ *
+ * Also, SMAP does not affect instruction
+ * fetches, add the !ff check here to make it
+ * clearer.
+ */
+ smap = cr4_smap && u && !uf && !ff;
} else
/* Not really needed: no U/S accesses on ept */
u = 1;
- fault = (ff && !x) || (uf && !u) || (wf && !w);
+ fault = (ff && !x) || (uf && !u) || (wf && !w) ||
+ (smapf && smap);
map |= fault << bit;
}
mmu->permissions[byte] = map;
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 2926152..822190f 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -44,11 +44,17 @@
#define PT_DIRECTORY_LEVEL 2
#define PT_PAGE_TABLE_LEVEL 1
-#define PFERR_PRESENT_MASK (1U << 0)
-#define PFERR_WRITE_MASK (1U << 1)
-#define PFERR_USER_MASK (1U << 2)
-#define PFERR_RSVD_MASK (1U << 3)
-#define PFERR_FETCH_MASK (1U << 4)
+#define PFERR_PRESENT_BIT 0
+#define PFERR_WRITE_BIT 1
+#define PFERR_USER_BIT 2
+#define PFERR_RSVD_BIT 3
+#define PFERR_FETCH_BIT 4
+
+#define PFERR_PRESENT_MASK (1U << PFERR_PRESENT_BIT)
+#define PFERR_WRITE_MASK (1U << PFERR_WRITE_BIT)
+#define PFERR_USER_MASK (1U << PFERR_USER_BIT)
+#define PFERR_RSVD_MASK (1U << PFERR_RSVD_BIT)
+#define PFERR_FETCH_MASK (1U << PFERR_FETCH_BIT)
int kvm_mmu_get_spte_hierarchy(struct kvm_vcpu *vcpu, u64 addr, u64 sptes[4]);
void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask);
@@ -73,6 +79,8 @@ int handle_mmio_page_fault_common(struct kvm_vcpu *vcpu, u64 addr, bool direct);
void kvm_init_shadow_mmu(struct kvm_vcpu *vcpu, struct kvm_mmu *context);
void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, struct kvm_mmu *context,
bool execonly);
+void update_permission_bitmask(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
+ bool ept);
static inline unsigned int kvm_mmu_available_pages(struct kvm *kvm)
{
@@ -110,10 +118,30 @@ static inline bool is_write_protection(struct kvm_vcpu *vcpu)
* Will a fault with a given page-fault error code (pfec) cause a permission
* fault with the given access (in ACC_* format)?
*/
-static inline bool permission_fault(struct kvm_mmu *mmu, unsigned pte_access,
- unsigned pfec)
+static inline bool permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
+ unsigned pte_access, unsigned pfec)
{
- return (mmu->permissions[pfec >> 1] >> pte_access) & 1;
+ int cpl = kvm_x86_ops->get_cpl(vcpu);
+ unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
+
+ /*
+ * If CPL < 3, SMAP prevention are disabled if EFLAGS.AC = 1.
+ *
+ * If CPL = 3, SMAP applies to all supervisor-mode data accesses
+ * (these are implicit supervisor accesses) regardless of the value
+ * of EFLAGS.AC.
+ *
+ * This computes (cpl < 3) && (rflags & X86_EFLAGS_AC), leaving
+ * the result in X86_EFLAGS_AC. We then insert it in place of
+ * the PFERR_RSVD_MASK bit; this bit will always be zero in pfec,
+ * but it will be one in index if SMAP checks are being overridden.
+ * It is important to keep this branchless.
+ */
+ unsigned long smap = (cpl-3) & (rflags & X86_EFLAGS_AC);
+ int index = (pfec >> 1) +
+ (smap >> (X86_EFLAGS_AC_BIT - PFERR_RSVD_BIT + 1));
+
+ return (mmu->permissions[index] >> pte_access) & 1;
}
void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index cba218a..4107765 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -353,7 +353,7 @@ retry_walk:
walker->ptes[walker->level - 1] = pte;
} while (!is_last_gpte(mmu, walker->level, pte));
- if (unlikely(permission_fault(mmu, pte_access, access))) {
+ if (unlikely(permission_fault(vcpu, mmu, pte_access, access))) {
errcode |= PFERR_PRESENT_MASK;
goto error;
}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2b85784..5869c6d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -646,6 +646,9 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
if (!guest_cpuid_has_smep(vcpu) && (cr4 & X86_CR4_SMEP))
return 1;
+ if (!guest_cpuid_has_smap(vcpu) && (cr4 & X86_CR4_SMAP))
+ return 1;
+
if (!guest_cpuid_has_fsgsbase(vcpu) && (cr4 & X86_CR4_FSGSBASE))
return 1;
@@ -674,6 +677,9 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
(!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE)))
kvm_mmu_reset_context(vcpu);
+ if ((cr4 ^ old_cr4) & X86_CR4_SMAP)
+ update_permission_bitmask(vcpu, vcpu->arch.walk_mmu, false);
+
if ((cr4 ^ old_cr4) & X86_CR4_OSXSAVE)
kvm_update_cpuid(vcpu);
@@ -4108,7 +4114,8 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
| (write ? PFERR_WRITE_MASK : 0);
if (vcpu_match_mmio_gva(vcpu, gva)
- && !permission_fault(vcpu->arch.walk_mmu, vcpu->arch.access, access)) {
+ && !permission_fault(vcpu, vcpu->arch.walk_mmu,
+ vcpu->arch.access, access)) {
*gpa = vcpu->arch.mmio_gfn << PAGE_SHIFT |
(gva & (PAGE_SIZE - 1));
trace_vcpu_match_mmio(gva, *gpa, write, false);
--
1.8.3.1
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