runtime: fix crash during VDSO calls on arm

As discussed in #32912, a crash occurs when go runtime calls a VDSO function (say
__vdso_clock_gettime) and a signal arrives to that thread.
Since VDSO functions temporarily destroy the G register (R10),
Go functions asynchronously executed in that thread (i.e. Go's signal
handler) can try to load data from the destroyed G, which causes
segmentation fault.

To fix the issue a guard is inserted in front of sigtrampgo, so that the control escapes from
signal handlers without touching G in case the signal occurred in the VDSO context.
The test case included in the patch is take from discussion in a relevant thread on github:
https://github.com/golang/go/issues/32912#issuecomment-517874531.
This patch not only fixes the issue on AArch64 but also that on 32bit ARM.

Fixes #32912

Change-Id: I657472e54b7aa3c617fabc5019ce63aa4105624a
GitHub-Last-Rev: 28ce42c4a02a060f08c1b0dd1c9a392123fd2ee9
GitHub-Pull-Request: golang/go#34030
Reviewed-on: https://go-review.googlesource.com/c/go/+/192937
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/crash_test.go

@@ -143,6 +143,15 @@ func buildTestProg(t *testing.T, binary string, flags ...string) (string, error)
 	return exe, nil
 }
 
+func TestVDSO(t *testing.T) {
+	t.Parallel()
+	output := runTestProg(t, "testprog", "SignalInVDSO")
+	want := "success\n"
+	if output != want {
+		t.Fatalf("output:\n%s\n\nwanted:\n%s", output, want);
+	}
+}
+
 var (
 	staleRuntimeOnce sync.Once // guards init of staleRuntimeErr
 	staleRuntimeErr  error

src/runtime/signal_unix.go

@@ -274,6 +274,21 @@ func sigpipe() {
 	dieFromSignal(_SIGPIPE)
 }
 
+// sigFetchG fetches the value of G safely when running in a signal handler.
+// On some architectures, the g value may be clobbered when running in a VDSO.
+// See issue #32912.
+//
+//go:nosplit
+func sigFetchG(c *sigctxt) *g {
+    switch GOARCH {
+    case "arm", "arm64", "ppc64", "ppc64le":
+        if inVDSOPage(c.sigpc()) {
+            return nil
+        }
+    }
+    return getg()
+}
+
 // sigtrampgo is called from the signal handler function, sigtramp,
 // written in assembly code.
 // This is called by the signal handler, and the world may be stopped.
@@ -289,9 +304,9 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 	if sigfwdgo(sig, info, ctx) {
 		return
 	}
-	g := getg()
+	c := &sigctxt{info, ctx}
+	g := sigFetchG(c)
 	if g == nil {
-		c := &sigctxt{info, ctx}
 		if sig == _SIGPROF {
 			sigprofNonGoPC(c.sigpc())
 			return
@@ -347,7 +362,6 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 		signalDuringFork(sig)
 	}
 
-	c := &sigctxt{info, ctx}
 	c.fixsigcode(sig)
 	sighandler(sig, info, ctx, g)
 	setg(g)
@@ -657,9 +671,10 @@ func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
 		return false
 	}
 	// Determine if the signal occurred inside Go code. We test that:
-	//   (1) we were in a goroutine (i.e., m.curg != nil), and
-	//   (2) we weren't in CGO.
-	g := getg()
+	//   (1) we weren't in VDSO page,
+	//   (2) we were in a goroutine (i.e., m.curg != nil), and
+	//   (3) we weren't in CGO.
+	g := sigFetchG(c)
 	if g != nil && g.m != nil && g.m.curg != nil && !g.m.incgo {
 		return false
 	}

src/runtime/testdata/testprog/vdso.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Invoke signal hander in the VDSO context (see issue 32912).
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"runtime/pprof"
+	"time"
+)
+
+func init() {
+	register("SignalInVDSO", signalInVDSO)
+}
+
+func signalInVDSO() {
+        f, err := ioutil.TempFile("", "timeprofnow")
+        if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+        }
+
+        if err := pprof.StartCPUProfile(f); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+        }
+
+        t0 := time.Now()
+        t1 := t0
+        // We should get a profiling signal 100 times a second,
+        // so running for 1 second should be sufficient.
+        for t1.Sub(t0) < time.Second {
+                t1 = time.Now()
+        }
+
+        pprof.StopCPUProfile()
+
+        name := f.Name()
+        if err := f.Close(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+        }
+
+        if err := os.Remove(name); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+        }
+
+	fmt.Println("success");
+}

src/runtime/vdso_linux.go

@@ -281,6 +281,7 @@ func vdsoauxv(tag, val uintptr) {
 }
 
 // vdsoMarker reports whether PC is on the VDSO page.
+//go:nosplit
 func inVDSOPage(pc uintptr) bool {
 	for _, k := range vdsoSymbolKeys {
 		if *k.ptr != 0 {