sentry: don't try to dump/restore unsupported FPU states

If a restore area contains any unsupported state, xrstor triggers a GP
exception. With the KVM platform, it crashes the Sentry.

PiperOrigin-RevId: 834524036

Author: avagin

pkg/ring0/entry_amd64.s

@@ -88,13 +88,8 @@
 #define PTRACE_FS_BASE  168 // +checkoffset linux PtraceRegs.Fs_base
 #define PTRACE_GS_BASE  176 // +checkoffset linux PtraceRegs.Gs_base
 
-// The value for XCR0 is defined to xsave/xrstor everything except for PKRU and
-// AMX regions.
-// TODO(gvisor.dev/issues/9896): Implement AMX support.
-// TODO(gvisor.dev/issues/10087): Implement PKRU support.
-#define XCR0_DISABLED_MASK ((1 << 9) | (1 << 17) | (1 << 18))
-#define XCR0_EAX (0xffffffff ^ XCR0_DISABLED_MASK)
-#define XCR0_EDX 0xffffffff
+#define XCR0_EAX 231 // +checkconst fpu SupportedXFeatureStates
+#define XCR0_EDX 0
 
 // Saves a register set.
 //

pkg/sentry/arch/fpu/fpu_amd64.go

@@ -18,6 +18,7 @@
 package fpu
 
 import (
+	"errors"
 	"fmt"
 	"io"
 
@@ -60,12 +61,37 @@ const (
 const (
 	// XFEATURE_MASK_FPSSE is xsave features that are always enabled in
 	// signal frame fpstate.
-	XFEATURE_MASK_FPSSE = 0x3
+	XFEATURE_MASK_FPSSE = cpuid.XSAVEFeatureX87 | cpuid.XSAVEFeatureSSE
 
 	// FXSAVE_AREA_SIZE is the size of the FXSAVE area.
 	FXSAVE_AREA_SIZE = 512
 )
 
+// LINT.IfChange
+const (
+	// SupportedXFeatureStates contains all xfeatures supported right now.
+	//
+	// TODO(gvisor.dev/issues/9896): Implement AMX support.
+	// TODO(gvisor.dev/issues/10087): Implement PKRU support.
+	SupportedXFeatureStates = cpuid.XSAVEFeatureX87 |
+		cpuid.XSAVEFeatureSSE |
+		cpuid.XSAVEFeatureAVX |
+		cpuid.XSAVEFeatureAVX512op |
+		cpuid.XSAVEFeatureAVX512zmm0 |
+		cpuid.XSAVEFeatureAVX512zmm16
+
+	// ignoredXFeatureStates contains those xfeatures that may be
+	// present in a buffer but are safe to skip during restore.
+	//
+	// Intel MPX deprecated in the Linux kernel.
+	// PKRU states could be leaked. This issue was fixed in the 6.6 kernel.
+	ignoredXFeatureStates = cpuid.XSAVEFeatureBNDREGS |
+		cpuid.XSAVEFeatureBNDCSR |
+		cpuid.XSAVEFeaturePKRU
+)
+
+// LINT.ThenChange(../../platform/systrap/sysmsg/sysmsg_offsets.h)
+
 // initX86FPState (defined in asm files) sets up initial state.
 func initX86FPState(data *byte, useXsave bool)
 
@@ -261,8 +287,13 @@ func (s *State) PtraceSetXstateRegs(src io.Reader, maxlen int, featureSet cpuid.
 	return n, nil
 }
 
+// ErrUnsupportedStateCleared is reported if an userspace state contains any
+// unsupported features.
+var ErrUnsupportedStateCleared = errors.New("contains unsupported states")
+
 // SanitizeUser mutates s to ensure that restoring it is safe.
-func (s *State) SanitizeUser(featureSet cpuid.FeatureSet) {
+func (s *State) SanitizeUser(featureSet cpuid.FeatureSet) error {
+	var err error
 	f := *s
 
 	// Force reserved bits in MXCSR to 0. This is consistent with Linux.
@@ -271,12 +302,17 @@ func (s *State) SanitizeUser(featureSet cpuid.FeatureSet) {
 	if len(f) >= minXstateBytes {
 		// Users can't enable *more* XCR0 bits than what we, and the CPU, support.
 		xstateBV := hostarch.ByteOrder.Uint64(f[xstateBVOffset:])
-		xstateBV &= featureSet.ValidXCR0Mask()
+		xcr0Mask := featureSet.ValidXCR0Mask() & (SupportedXFeatureStates | ignoredXFeatureStates)
+		if xstateBV&xcr0Mask != xstateBV {
+			err = ErrUnsupportedStateCleared
+		}
+		xstateBV &= featureSet.ValidXCR0Mask() & SupportedXFeatureStates
 		hostarch.ByteOrder.PutUint64(f[xstateBVOffset:], xstateBV)
 		// Force XCOMP_BV and reserved bytes in the XSAVE header to 0.
 		reserved := f[xsaveHeaderZeroedOffset : xsaveHeaderZeroedOffset+xsaveHeaderZeroedBytes]
 		clear(reserved)
 	}
+	return err
 }
 
 var (
@@ -357,7 +393,7 @@ func (s *State) AfterLoad() {
 	supportedBV := fxsaveBV
 	hostFeatureSet := cpuid.HostFeatureSet()
 	if hostFeatureSet.UseXsave() {
-		supportedBV = hostFeatureSet.ValidXCR0Mask()
+		supportedBV = hostFeatureSet.ValidXCR0Mask() & SupportedXFeatureStates
 	}
 
 	// What was in use?
@@ -367,7 +403,7 @@ func (s *State) AfterLoad() {
 	}
 
 	// Supported features must be a superset of saved features.
-	if savedBV&^supportedBV != 0 {
+	if savedBV&^(supportedBV|ignoredXFeatureStates) != 0 {
 		panic(ErrLoadingState{supportedFeatures: supportedBV, savedFeatures: savedBV})
 	}
 

pkg/sentry/arch/signal_amd64.go

@@ -299,7 +299,10 @@ func (c *Context64) SignalRestore(st *Stack, rt bool, featureSet cpuid.FeatureSe
 			c.fpState.Reset()
 			return 0, linux.SignalStack{}, err
 		}
-		c.fpState.SanitizeUser(featureSet)
+		if err := c.fpState.SanitizeUser(featureSet); err != nil {
+			c.fpState.Reset()
+			return 0, linux.SignalStack{}, linuxerr.EFAULT
+		}
 	}
 
 	return uc.Sigset, uc.Stack, nil

pkg/sentry/platform/systrap/sysmsg/sysmsg_offsets.h

@@ -20,13 +20,11 @@
 // for the pkg/sentry/platform/systrap/usertrap package.
 #define FAULT_OPCODE 0x06
 
-// The value for XCR0 is defined to xsave/xrstor everything except for PKRU and
-// AMX regions.
-// TODO(gvisor.dev/issues/9896): Implement AMX support.
-// TODO(gvisor.dev/issues/10087): Implement PKRU support.
-#define XCR0_DISABLED_MASK ((1 << 9) | (1 << 17) | (1 << 18))
-#define XCR0_EAX (0xffffffff ^ XCR0_DISABLED_MASK)
-#define XCR0_EDX 0xffffffff
+// LINT.IfChange
+#define XCR0_EAX 0xe7UL
+#define XCR0_EDX 0
+#define XCR0_DISABLED_MASK (~XCR0_EAX)
+// LINT.ThenChange(../../../arch/fpu/fpu_amd64.go)
 
 // LINT.IfChange
 #define MAX_FPSTATE_LEN 3584

test/syscalls/linux/BUILD

@@ -1001,6 +1001,7 @@ cc_binary(
         "//test/util:test_main",
         "//test/util:test_util",
         "//test/util:thread_util",
+        "@com_google_absl//absl/log",
     ],
 )
 

test/syscalls/linux/fpsig_mut_amd64.cc

@@ -15,10 +15,16 @@
 // This program verifies that application floating point state is visible in
 // signal frames, and that changes to said state is visible after the signal
 // handler returns.
+#include <signal.h>
+#include <stdint.h>
 #include <sys/time.h>
 #include <sys/ucontext.h>
+#include <ucontext.h>
+
+#include <cstdint>
 
 #include "gtest/gtest.h"
+#include "absl/log/log.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
 
@@ -56,6 +62,19 @@ void sigusr1(int s, siginfo_t* siginfo, void* _uc) {
   uc_xmm0->element[0] = static_cast<uint32_t>(kNewFPRegValue);
 }
 
+volatile uint64_t testXsaveFeature;
+void sigusr2(int s, siginfo_t* siginfo, void* _uc) {
+  ucontext_t* uc = reinterpret_cast<ucontext_t*>(_uc);
+  *(uint64_t*)((uint8_t*)uc->uc_mcontext.fpregs + 512) |= 1 << testXsaveFeature;
+}
+
+ucontext_t safe_ctx;
+volatile bool segvTriggered;
+void sigsegv(int s, siginfo_t* siginfo, void* _uc) {
+  segvTriggered = true;
+  setcontext(&safe_ctx);
+}
+
 TEST(FPSigTest, StateInFrame) {
   pid = getpid();
   tid = gettid();
@@ -105,6 +124,50 @@ TEST(FPSigTest, StateInFrame) {
   EXPECT_EQ(got, kNewFPRegValue);
 }
 
+TEST(FPSigTest, XFeaturesInFrame) {
+  pid = getpid();
+  tid = gettid();
+
+  struct sigaction sa = {};
+  sigemptyset(&sa.sa_mask);
+  sa.sa_flags = SA_SIGINFO;
+  sa.sa_sigaction = sigusr2;
+  ASSERT_THAT(sigaction(SIGUSR2, &sa, nullptr), SyscallSucceeds());
+  sa.sa_sigaction = sigsegv;
+  ASSERT_THAT(sigaction(SIGSEGV, &sa, nullptr), SyscallSucceeds());
+
+  for (testXsaveFeature = 0; testXsaveFeature < 64; testXsaveFeature++) {
+    getcontext(&safe_ctx);
+    if (segvTriggered) {
+      LOG(INFO) << testXsaveFeature << " triggered SEGV";
+      segvTriggered = false;
+    } else {
+      asm volatile(
+          "movl %[pid], %%edi;"
+          "movl %[tid], %%esi;"
+          "movl %[sig], %%edx;"
+          "movl %[killnr], %%eax;"
+          "syscall;"
+          :
+          : [killnr] "i"(__NR_tgkill), [pid] "rm"(pid), [tid] "rm"(tid),
+            [sig] "i"(SIGUSR2)
+          : "rax", "rdi", "rsi", "rdx",
+            // Clobbered by syscall.
+            "rcx", "r11");
+      switch (testXsaveFeature) {
+        case 8:   // PT (Processor Trace)
+        case 13:  // HDC (Hardware Debug Controls)
+        case 17:  // LBR (Last Branch Record)
+        case 18:  // CET (Control-flow Enforcement Technology)
+        case 19:  // HRESET (Host Reset)
+          FAIL() << testXsaveFeature << " didn't trigger a fault";
+        default:
+          LOG(INFO) << testXsaveFeature << " has been accepted";
+      }
+    }
+  }
+}
+
 }  // namespace
 
 }  // namespace testing