Change SIGCHLD to SIGKILL in ptrace stubs.

If the child stubs are killed by any unmaskable signal (e.g. SIGKILL), then
the parent process will similarly be killed, resulting in the death of all
other stubs.

The effect of this is that if the OOM killer selects and kills a stub, the
effect is the same as though the OOM killer selected and killed the sentry.

PiperOrigin-RevId: 202219984
Change-Id: I0b638ce7e59e0a0f4d5cde12a7d05242673049d7

1 files changed, 13 insertions, 3 deletions

diff --git a/pkg/sentry/platform/ptrace/subprocess_linux.go b/pkg/sentry/platform/ptrace/subprocess_linux.go
index 227dd4882..b3f2ebb20 100644
--- a/pkg/sentry/platform/ptrace/subprocess_linux.go
+++ b/pkg/sentry/platform/ptrace/subprocess_linux.go
@@ -41,7 +41,15 @@ func createStub() (*thread, error) {
 
 	// Among other things, beforeFork masks all signals.
 	beforeFork()
-	pid, _, errno = syscall.RawSyscall6(syscall.SYS_CLONE, uintptr(syscall.SIGCHLD)|syscall.CLONE_FILES, 0, 0, 0, 0, 0)
+
+	// When creating the new child process, we specify SIGKILL as the
+	// signal to deliver when the child exits. We never expect a subprocess
+	// to exit; they are pooled and reused. This is done to ensure that if
+	// a subprocess is OOM-killed, this process (and all other stubs,
+	// transitively) will be killed as well. It's simply not possible to
+	// safely handle a single stub getting killed: the exact state of
+	// execution is unknown and not recoverable.
+	pid, _, errno = syscall.RawSyscall6(syscall.SYS_CLONE, uintptr(syscall.SIGKILL)|syscall.CLONE_FILES, 0, 0, 0, 0, 0)
 	if errno != 0 {
 		afterFork()
 		return nil, errno
@@ -103,10 +111,12 @@ func (s *subprocess) createStub() (*thread, error) {
 	//
 	// Instead, we create the child untraced, which will do the PDEATHSIG
 	// setup and then SIGSTOP itself for our attach below.
+	//
+	// See above re: SIGKILL.
 	pid, err := t.syscallIgnoreInterrupt(
 		&regs,
 		syscall.SYS_CLONE,
-		arch.SyscallArgument{Value: uintptr(syscall.SIGCHLD | syscall.CLONE_FILES)},
+		arch.SyscallArgument{Value: uintptr(syscall.SIGKILL | syscall.CLONE_FILES)},
 		arch.SyscallArgument{Value: 0},
 		arch.SyscallArgument{Value: 0},
 		arch.SyscallArgument{Value: 0},
@@ -127,7 +137,7 @@ func (s *subprocess) createStub() (*thread, error) {
 		syscall.SYS_WAIT4,
 		arch.SyscallArgument{Value: uintptr(pid)},
 		arch.SyscallArgument{Value: 0},
-		arch.SyscallArgument{Value: syscall.WUNTRACED},
+		arch.SyscallArgument{Value: syscall.WALL | syscall.WUNTRACED},
 		arch.SyscallArgument{Value: 0},
 		arch.SyscallArgument{Value: 0},
 		arch.SyscallArgument{Value: 0})