platform/syscall: use syscall + int3 to execute a system call in a stub process

Right now, we need to call ptrace(PTRACE_SYSCALL) and wait() twice to execute
one system call in a stub process. With these changes, we will need to call
ptrace + wait only once.

In addition, this allows to workaround the kernel bug when a stub process
doesn't stop on syscall-exit-stop and starts executing the next system call.

Reported-by: syzbot+37143cafa8dc3b5008ee@syzkaller.appspotmail.com
PiperOrigin-RevId: 288393029

pkg/sentry/platform/ptrace/stub_amd64.s

@@ -64,6 +64,8 @@ begin:
 	CMPQ AX, $0
 	JL error
 
+	MOVQ $0, BX
+
 	// SIGSTOP to wait for attach.
 	//
 	// The SYSCALL instruction will be used for future syscall injection by
@@ -73,23 +75,26 @@ begin:
 	MOVQ $SIGSTOP, SI
 	SYSCALL
 
-	// The tracer may "detach" and/or allow code execution here in three cases:
-	//
-	// 1. New (traced) stub threads are explicitly detached by the
-	// goroutine in newSubprocess. However, they are detached while in
-	// group-stop, so they do not execute code here.
-	//
-	// 2. If a tracer thread exits, it implicitly detaches from the stub,
-	// potentially allowing code execution here. However, the Go runtime
-	// never exits individual threads, so this case never occurs.
-	//
-	// 3. subprocess.createStub clones a new stub process that is untraced,
+	// The sentry sets BX to 1 when creating stub process.
+	CMPQ BX, $1
+	JE clone
+
+	// Notify the Sentry that syscall exited.
+done:
+	INT $3
+	// Be paranoid.
+	JMP done
+clone:
+	// subprocess.createStub clones a new stub process that is untraced,
 	// thus executing this code. We setup the PDEATHSIG before SIGSTOPing
 	// ourselves for attach by the tracer.
 	//
 	// R15 has been updated with the expected PPID.
-	JMP begin
+	CMPQ AX, $0
+	JE begin
 
+	// The clone syscall returns a non-zero value.
+	JMP done
 error:
 	// Exit with -errno.
 	MOVQ AX, DI

pkg/sentry/platform/ptrace/stub_arm64.s

@@ -59,6 +59,8 @@ begin:
 	CMP $0x0, R0
 	BLT error
 
+	MOVD $0, R9
+
 	// SIGSTOP to wait for attach.
 	//
 	// The SYSCALL instruction will be used for future syscall injection by
@@ -66,22 +68,26 @@ begin:
 	MOVD $SYS_KILL, R8
 	MOVD $SIGSTOP, R1
 	SVC
-	// The tracer may "detach" and/or allow code execution here in three cases:
-	//
-	// 1. New (traced) stub threads are explicitly detached by the
-	// goroutine in newSubprocess. However, they are detached while in
-	// group-stop, so they do not execute code here.
-	//
-	// 2. If a tracer thread exits, it implicitly detaches from the stub,
-	// potentially allowing code execution here. However, the Go runtime
-	// never exits individual threads, so this case never occurs.
-	//
-	// 3. subprocess.createStub clones a new stub process that is untraced,
+
+	// The sentry sets R9 to 1 when creating stub process.
+	CMP $1, R9
+	BEQ clone
+
+done:
+	// Notify the Sentry that syscall exited.
+	BRK $3
+	B done // Be paranoid.
+clone:
+	// subprocess.createStub clones a new stub process that is untraced,
 	// thus executing this code. We setup the PDEATHSIG before SIGSTOPing
 	// ourselves for attach by the tracer.
 	//
 	// R7 has been updated with the expected PPID.
-	B begin
+	CMP $0, R0
+	BEQ begin
+
+	// The clone system call returned a non-zero value.
+	B done
 
 error:
 	// Exit with -errno.

pkg/sentry/platform/ptrace/subprocess.go

@@ -430,13 +430,15 @@ func (t *thread) syscall(regs *syscall.PtraceRegs) (uintptr, error) {
 	}
 
 	for {
-		// Execute the syscall instruction.
-		if _, _, errno := syscall.RawSyscall6(syscall.SYS_PTRACE, syscall.PTRACE_SYSCALL, uintptr(t.tid), 0, 0, 0, 0); errno != 0 {
+		// Execute the syscall instruction. The task has to stop on the
+		// trap instruction which is right after the syscall
+		// instruction.
+		if _, _, errno := syscall.RawSyscall6(syscall.SYS_PTRACE, syscall.PTRACE_CONT, uintptr(t.tid), 0, 0, 0, 0); errno != 0 {
 			panic(fmt.Sprintf("ptrace syscall-enter failed: %v", errno))
 		}
 
 		sig := t.wait(stopped)
-		if sig == (syscallEvent | syscall.SIGTRAP) {
+		if sig == syscall.SIGTRAP {
 			// Reached syscall-enter-stop.
 			break
 		} else {
@@ -448,18 +450,6 @@ func (t *thread) syscall(regs *syscall.PtraceRegs) (uintptr, error) {
 		}
 	}
 
-	// Complete the actual system call.
-	if _, _, errno := syscall.RawSyscall6(syscall.SYS_PTRACE, syscall.PTRACE_SYSCALL, uintptr(t.tid), 0, 0, 0, 0); errno != 0 {
-		panic(fmt.Sprintf("ptrace syscall-enter failed: %v", errno))
-	}
-
-	// Wait for syscall-exit-stop. "[Signal-delivery-stop] never happens
-	// between syscall-enter-stop and syscall-exit-stop; it happens *after*
-	// syscall-exit-stop.)" - ptrace(2), "Syscall-stops"
-	if sig := t.wait(stopped); sig != (syscallEvent | syscall.SIGTRAP) {
-		t.dumpAndPanic(fmt.Sprintf("wait failed: expected SIGTRAP, got %v [%d]", sig, sig))
-	}
-
 	// Grab registers.
 	if err := t.getRegs(regs); err != nil {
 		panic(fmt.Sprintf("ptrace get regs failed: %v", err))

pkg/sentry/platform/ptrace/subprocess_amd64.go

@@ -141,9 +141,11 @@ func (t *thread) adjustInitRegsRip() {
 	t.initRegs.Rip -= initRegsRipAdjustment
 }
 
-// Pass the expected PPID to the child via R15 when creating stub process
+// Pass the expected PPID to the child via R15 when creating stub process.
 func initChildProcessPPID(initregs *syscall.PtraceRegs, ppid int32) {
 	initregs.R15 = uint64(ppid)
+	// Rbx has to be set to 1 when creating stub process.
+	initregs.Rbx = 1
 }
 
 // patchSignalInfo patches the signal info to account for hitting the seccomp

pkg/sentry/platform/ptrace/subprocess_arm64.go

@@ -127,6 +127,8 @@ func (t *thread) adjustInitRegsRip() {
 // Pass the expected PPID to the child via X7 when creating stub process
 func initChildProcessPPID(initregs *syscall.PtraceRegs, ppid int32) {
 	initregs.Regs[7] = uint64(ppid)
+	// R9 has to be set to 1 when creating stub process.
+	initregs.Regs[9] = 1
 }
 
 // patchSignalInfo patches the signal info to account for hitting the seccomp