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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH 2/2] x86/AMD: Fix handling of x87 exception pointers on Fam17h hardware
AMD Pre-Fam17h CPUs "optimise" {F,}X{SAVE,RSTOR} by not saving/restoring
FOP/FIP/FDP if an x87 exception isn't pending. This causes an information
leak, CVE-2006-1056, and worked around by several OSes, including Xen. AMD
Fam17h CPUs no longer have this leak, and advertise so in a CPUID bit.
Introduce the RSTR_FP_ERR_PTRS feature, as specified by AMD, and expose to all
guests by default. While adjusting libxl's cpuid table, add CLZERO which
looks to have been omitted previously.
Also introduce an X86_BUG bit to trigger the (F)XRSTOR workaround, and set it
on AMD hardware where RSTR_FP_ERR_PTRS is not advertised. Optimise the
workaround path by dropping the data-dependent unpredictable conditions which
will evalute to true for all 64bit OSes and most 32bit ones.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
v2:
* Use the AMD naming, not that I am convinced this is a sensible name to use.
* Adjust the i387 codepaths as well as the xstate ones.
* Add xen-cpuid/libxl data for the CPUID bit.
---
tools/libxl/libxl_cpuid.c | 3 +++
tools/misc/xen-cpuid.c | 1 +
xen/arch/x86/cpu/amd.c | 7 +++++++
xen/arch/x86/i387.c | 14 +++++---------
xen/arch/x86/xstate.c | 6 ++----
xen/include/asm-x86/cpufeature.h | 3 +++
xen/include/asm-x86/cpufeatures.h | 2 ++
xen/include/public/arch-x86/cpufeatureset.h | 1 +
8 files changed, 24 insertions(+), 13 deletions(-)
diff --git a/tools/libxl/libxl_cpuid.c b/tools/libxl/libxl_cpuid.c
index a8d07fac50..acc92fd46c 100644
--- a/tools/libxl/libxl_cpuid.c
+++ b/tools/libxl/libxl_cpuid.c
@@ -256,7 +256,10 @@ int libxl_cpuid_parse_config(libxl_cpuid_policy_list
*cpuid, const char* str)
{"invtsc", 0x80000007, NA, CPUID_REG_EDX, 8, 1},
+ {"clzero", 0x80000008, NA, CPUID_REG_EBX, 0, 1},
+ {"rstr-fp-err-ptrs", 0x80000008, NA, CPUID_REG_EBX, 2, 1},
{"ibpb", 0x80000008, NA, CPUID_REG_EBX, 12, 1},
+
{"nc", 0x80000008, NA, CPUID_REG_ECX, 0, 8},
{"apicidsize", 0x80000008, NA, CPUID_REG_ECX, 12, 4},
diff --git a/tools/misc/xen-cpuid.c b/tools/misc/xen-cpuid.c
index b0db0525a9..04cdd9aa95 100644
--- a/tools/misc/xen-cpuid.c
+++ b/tools/misc/xen-cpuid.c
@@ -145,6 +145,7 @@ static const char *const str_e7d[32] =
static const char *const str_e8b[32] =
{
[ 0] = "clzero",
+ [ 2] = "rstr-fp-err-ptrs",
[12] = "ibpb",
};
diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index a2f83c79a5..463f9776c7 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -580,6 +580,13 @@ static void init_amd(struct cpuinfo_x86 *c)
}
/*
+ * Older AMD CPUs don't save/load FOP/FIP/FDP unless an FPU exception
+ * is pending. Xen works around this at (F)XRSTOR time.
+ */
+ if ( !cpu_has(c, X86_FEATURE_RSTR_FP_ERR_PTRS) )
+ setup_force_cpu_cap(X86_BUG_FPU_PTR_LEAK);
+
+ /*
* Attempt to set lfence to be Dispatch Serialising. This MSR almost
* certainly isn't virtualised (and Xen at least will leak the real
* value in but silently discard writes), as well as being per-core
diff --git a/xen/arch/x86/i387.c b/xen/arch/x86/i387.c
index 88178485cb..82dbc461c3 100644
--- a/xen/arch/x86/i387.c
+++ b/xen/arch/x86/i387.c
@@ -43,20 +43,17 @@ static inline void fpu_fxrstor(struct vcpu *v)
const typeof(v->arch.xsave_area->fpu_sse) *fpu_ctxt = v->arch.fpu_ctxt;
/*
- * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
+ * Some CPUs don't save/restore FDP/FIP/FOP unless an exception
* is pending. Clear the x87 state here by setting it to fixed
* values. The hypervisor data segment can be sometimes 0 and
* sometimes new user value. Both should be ok. Use the FPU saved
* data block as a safe address because it should be in L1.
*/
- if ( !(fpu_ctxt->fsw & ~fpu_ctxt->fcw & 0x003f) &&
- boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
- {
+ if ( cpu_bug_fpu_ptr_leak )
asm volatile ( "fnclex\n\t"
"ffree %%st(7)\n\t" /* clear stack tag */
"fildl %0" /* load to clear state */
: : "m" (*fpu_ctxt) );
- }
/*
* FXRSTOR can fault if passed a corrupted data block. We handle this
@@ -169,11 +166,10 @@ static inline void fpu_fxsave(struct vcpu *v)
: "=m" (*fpu_ctxt) : "R" (fpu_ctxt) );
/*
- * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
- * is pending.
+ * Some CPUs don't save/restore FDP/FIP/FOP unless an exception is
+ * pending. The restore code fills in suitable defaults.
*/
- if ( !(fpu_ctxt->fsw & 0x0080) &&
- boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+ if ( cpu_bug_fpu_ptr_leak && !(fpu_ctxt->fsw & 0x0080) )
return;
/*
diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 3293ef834f..fd3c0c5a36 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -369,15 +369,13 @@ void xrstor(struct vcpu *v, uint64_t mask)
unsigned int faults, prev_faults;
/*
- * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
+ * Some CPUs don't save/restore FDP/FIP/FOP unless an exception
* is pending. Clear the x87 state here by setting it to fixed
* values. The hypervisor data segment can be sometimes 0 and
* sometimes new user value. Both should be ok. Use the FPU saved
* data block as a safe address because it should be in L1.
*/
- if ( (mask & ptr->xsave_hdr.xstate_bv & X86_XCR0_FP) &&
- !(ptr->fpu_sse.fsw & ~ptr->fpu_sse.fcw & 0x003f) &&
- boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+ if ( cpu_bug_fpu_ptr_leak )
asm volatile ( "fnclex\n\t" /* clear exceptions */
"ffree %%st(7)\n\t" /* clear stack tag */
"fildl %0" /* load to clear state */
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 906dd59c4b..5d7b819314 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -136,6 +136,9 @@
#define cpu_has_msr_tsc_aux (cpu_has_rdtscp || cpu_has_rdpid)
+/* Bugs. */
+#define cpu_bug_fpu_ptr_leak boot_cpu_has(X86_BUG_FPU_PTR_LEAK)
+
enum _cache_type {
CACHE_TYPE_NULL = 0,
CACHE_TYPE_DATA = 1,
diff --git a/xen/include/asm-x86/cpufeatures.h
b/xen/include/asm-x86/cpufeatures.h
index ab3650f73b..afb861f588 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -43,5 +43,7 @@ XEN_CPUFEATURE(SC_VERW_IDLE, X86_SYNTH(25)) /* VERW used
by Xen for idle */
#define X86_NR_BUG 1
#define X86_BUG(x) ((FSCAPINTS + X86_NR_SYNTH) * 32 + (x))
+#define X86_BUG_FPU_PTR_LEAK X86_BUG( 0) /* (F)XRSTOR doesn't load
FOP/FIP/FDP. */
+
/* Total number of capability words, inc synth and bug words. */
#define NCAPINTS (FSCAPINTS + X86_NR_SYNTH + X86_NR_BUG) /* N 32-bit words
worth of info */
diff --git a/xen/include/public/arch-x86/cpufeatureset.h
b/xen/include/public/arch-x86/cpufeatureset.h
index e2c82a4554..babaf4b375 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -243,6 +243,7 @@ XEN_CPUFEATURE(EFRO, 7*32+10) /* APERF/MPERF
Read Only interface */
/* AMD-defined CPU features, CPUID level 0x80000008.ebx, word 8 */
XEN_CPUFEATURE(CLZERO, 8*32+ 0) /*A CLZERO instruction */
+XEN_CPUFEATURE(RSTR_FP_ERR_PTRS, 8*32+ 2) /*A (F)X{SAVE,RSTOR} always
saves/restores FPU Error pointers. */
XEN_CPUFEATURE(IBPB, 8*32+12) /*A IBPB support only (no IBRS, used
by AMD) */
/* Intel-defined CPU features, CPUID level 0x00000007:0.edx, word 9 */
--
2.11.0
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