runtime: run libc SIGSETXID and SIGCANCEL handlers on signal stack

These signals are used by glibc to broadcast setuid/setgid to all
threads and to send pthread cancellations.  Unlike other signals, the
Go runtime does not intercept these because they must invoke the libc
handlers (see issues #3871 and #6997).  However, because 1) these
signals may be issued asynchronously by a thread running C code to
another thread running Go code and 2) glibc does not set SA_ONSTACK
for its handlers, glibc's signal handler may be run on a Go stack.
Signal frames range from 1.5K on amd64 to many kilobytes on ppc64, so
this may overflow the Go stack and corrupt heap (or other stack) data.

Fix this by ensuring that these signal handlers have the SA_ONSTACK
flag (but not otherwise taking over the handler).

This has been a problem since Go 1.1, but it's likely that people
haven't encountered it because it only affects setuid/setgid and
pthread_cancel.

Fixes #9600.

Change-Id: I6cf5f5c2d3aa48998d632f61f1ddc2778dcfd300
Reviewed-on: https://go-review.googlesource.com/1887Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/cgo_linux_test.go

@@ -9,3 +9,4 @@ import "testing"
 func TestSetgid(t *testing.T)  { testSetgid(t) }
 func Test6997(t *testing.T)    { test6997(t) }
 func TestBuildID(t *testing.T) { testBuildID(t) }
+func Test9400(t *testing.T)    { test9400(t) }

misc/cgo/test/issue9400/asm_386.s

+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$0-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADDL	$(1024 * 8), SP
+
+	// Ask signaller to setgid
+	MOVL	$1, ·Baton(SB)
+
+	// Wait for setgid completion
+loop:
+	PAUSE
+	MOVL	·Baton(SB), AX
+	CMPL	AX, $0
+	JNE	loop
+
+	// Restore stack
+	SUBL	$(1024 * 8), SP
+	RET

misc/cgo/test/issue9400/asm_amd64x.s

+// +build amd64 amd64p32
+
+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$0-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADDQ	$(1024 * 8), SP
+
+	// Ask signaller to setgid
+	MOVL	$1, ·Baton(SB)
+
+	// Wait for setgid completion
+loop:
+	PAUSE
+	MOVL	·Baton(SB), AX
+	CMPL	AX, $0
+	JNE	loop
+
+	// Restore stack
+	SUBQ	$(1024 * 8), SP
+	RET

misc/cgo/test/issue9400/asm_arm.s

+#include "textflag.h"
+
+TEXT cas<>(SB),NOSPLIT,$0
+	MOVW	$0xffff0fc0, PC
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$-4-0
+	// Save link register
+	MOVW	R14, R4
+
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADD	$(1024 * 8), R13
+
+	// Ask signaller to setgid
+	MOVW	$·Baton(SB), R2
+storeloop:
+	MOVW	0(R2), R0
+	MOVW	$1, R1
+	BL	cas<>(SB)
+	BCC	storeloop
+
+	// Wait for setgid completion
+loop:
+	MOVW	$0, R0
+	MOVW	$0, R1
+	BL	cas<>(SB)
+	BCC	loop
+
+	// Restore stack
+	SUB	$(1024 * 8), R13
+
+	MOVW	R4, R14
+	RET

misc/cgo/test/issue9400/asm_ppc64x.s

+// +build ppc64 ppc64le
+
+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$-8-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADD	$(1024 * 8), R1
+
+	// Ask signaller to setgid
+	MOVW	$1, R3
+	SYNC
+	MOVW	R3, ·Baton(SB)
+
+	// Wait for setgid completion
+loop:
+	SYNC
+	MOVW	·Baton(SB), R3
+	CMP	R3, $0
+	// Hint that we're in a spin loop
+	OR	R1, R1, R1
+	BNE	loop
+	ISYNC
+
+	// Restore stack
+	SUB	$(1024 * 8), R1
+	RET

misc/cgo/test/issue9400/stubs.go

+// Copyright 2014 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.
+
+package issue9400
+
+var Baton int32
+
+func RewindAndSetgid()

misc/cgo/test/issue9400_linux.go

+// Copyright 2014 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.
+
+// Test that SIGSETXID runs on signal stack, since it's likely to
+// overflow if it runs on the Go stack.
+
+package cgotest
+
+/*
+#include <sys/types.h>
+#include <unistd.h>
+*/
+import "C"
+
+import (
+	"runtime"
+	"sync/atomic"
+	"testing"
+
+	"./issue9400"
+)
+
+func test9400(t *testing.T) {
+	// We synchronize through a shared variable, so we need two procs
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(2))
+
+	// Start signaller
+	atomic.StoreInt32(&issue9400.Baton, 0)
+	go func() {
+		// Wait for RewindAndSetgid
+		for atomic.LoadInt32(&issue9400.Baton) == 0 {
+			runtime.Gosched()
+		}
+		// Broadcast SIGSETXID
+		runtime.LockOSThread()
+		C.setgid(0)
+		// Indicate that signalling is done
+		atomic.StoreInt32(&issue9400.Baton, 0)
+	}()
+
+	// Grow the stack and put down a test pattern
+	const pattern = 0x123456789abcdef
+	var big [1024]uint64 // len must match assmebly
+	for i := range big {
+		big[i] = pattern
+	}
+
+	// Temporarily rewind the stack and trigger SIGSETXID
+	issue9400.RewindAndSetgid()
+
+	// Check test pattern
+	for i := range big {
+		if big[i] != pattern {
+			t.Fatalf("entry %d of test pattern is wrong; %#x != %#x", i, big[i], pattern)
+		}
+	}
+}

src/runtime/os1_darwin.go

@@ -394,6 +394,10 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sigaction(uint32(i), &sa, nil)
 }
 
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	memclr(unsafe.Pointer(&sa), unsafe.Sizeof(sa))

src/runtime/os1_dragonfly.go

@@ -189,6 +189,10 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sigaction(i, &sa, nil)
 }
 
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	sigaction(i, nil, &sa)

src/runtime/os1_freebsd.go

@@ -190,6 +190,11 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sa.sa_handler = fn
 	sigaction(i, &sa, nil)
 }
+
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	sigaction(i, nil, &sa)

src/runtime/os1_linux.go

@@ -246,6 +246,20 @@ func setsig(i int32, fn uintptr, restart bool) {
 	}
 }
 
+func setsigstack(i int32) {
+	var sa sigactiont
+	if rt_sigaction(uintptr(i), nil, &sa, unsafe.Sizeof(sa.sa_mask)) != 0 {
+		gothrow("rt_sigaction failure")
+	}
+	if sa.sa_handler == 0 || sa.sa_handler == _SIG_DFL || sa.sa_handler == _SIG_IGN || sa.sa_flags&_SA_ONSTACK != 0 {
+		return
+	}
+	sa.sa_flags |= _SA_ONSTACK
+	if rt_sigaction(uintptr(i), &sa, nil, unsafe.Sizeof(sa.sa_mask)) != 0 {
+		gothrow("rt_sigaction failure")
+	}
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 

src/runtime/os1_netbsd.go

@@ -233,6 +233,10 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sigaction(i, &sa, nil)
 }
 
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	sigaction(i, nil, &sa)

src/runtime/os1_openbsd.go

@@ -203,6 +203,10 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sigaction(i, &sa, nil)
 }
 
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	sigaction(i, nil, &sa)

src/runtime/os3_solaris.go

@@ -252,6 +252,10 @@ func setsig(i int32, fn uintptr, restart bool) {
 	sigaction(i, &sa, nil)
 }
 
+func setsigstack(i int32) {
+	gothrow("setsigstack")
+}
+
 func getsig(i int32) uintptr {
 	var sa sigactiont
 	sigaction(i, nil, &sa)

src/runtime/runtime2.go

@@ -372,6 +372,7 @@ const (
 	_SigHandling = 1 << 5 // our signal handler is registered
 	_SigIgnored  = 1 << 6 // the signal was ignored before we registered for it
 	_SigGoExit   = 1 << 7 // cause all runtime procs to exit (only used on Plan 9).
+	_SigSetStack = 1 << 8 // add SA_ONSTACK to libc handler
 )
 
 // Layout of in-memory per-function information prepared by linker

src/runtime/signal1_unix.go

@@ -37,6 +37,11 @@ func initsig() {
 			}
 		}
 
+		if t.flags&_SigSetStack != 0 {
+			setsigstack(i)
+			continue
+		}
+
 		t.flags |= _SigHandling
 		setsig(i, funcPC(sighandler), true)
 	}

src/runtime/signal_linux.go

@@ -42,8 +42,8 @@ var sigtable = [...]sigTabT{
 	/* 29 */ {_SigNotify, "SIGIO: i/o now possible"},
 	/* 30 */ {_SigNotify, "SIGPWR: power failure restart"},
 	/* 31 */ {_SigNotify, "SIGSYS: bad system call"},
-	/* 32 */ {0, "signal 32"}, /* SIGCANCEL; see issue 6997 */
-	/* 33 */ {0, "signal 33"}, /* SIGSETXID; see issue 3871 */
+	/* 32 */ {_SigSetStack, "signal 32"}, /* SIGCANCEL; see issue 6997 */
+	/* 33 */ {_SigSetStack, "signal 33"}, /* SIGSETXID; see issue 3871, 9400 */
 	/* 34 */ {_SigNotify, "signal 34"},
 	/* 35 */ {_SigNotify, "signal 35"},
 	/* 36 */ {_SigNotify, "signal 36"},