Re: [PATCHv5] exec: Fix a deadlock in ptrace

On 3/3/20 9:08 PM, Eric W. Biederman wrote:

Bernd Edlinger [off-list ref] writes:

quoted

On 3/3/20 4:18 PM, Eric W. Biederman wrote:

quoted

Bernd Edlinger [off-list ref] writes:

quoted

diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c
new file mode 100644
index 0000000..6d8a048
--- /dev/null
+++ b/tools/testing/selftests/ptrace/vmaccess.c

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (c) 2020 Bernd Edlinger <bernd.edlinger@hotmail.de>
+ * All rights reserved.
+ *
+ * Check whether /proc/$pid/mem can be accessed without causing deadlocks
+ * when de_thread is blocked with ->cred_guard_mutex held.
+ */
+
+#include "../kselftest_harness.h"
+#include <stdio.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <signal.h>
+#include <unistd.h>
+#include <sys/ptrace.h>
+
+static void *thread(void *arg)
+{
+	ptrace(PTRACE_TRACEME, 0, 0L, 0L);
+	return NULL;
+}
+
+TEST(vmaccess)
+{
+	int f, pid = fork();
+	char mm[64];
+
+	if (!pid) {
+		pthread_t pt;
+
+		pthread_create(&pt, NULL, thread, NULL);
+		pthread_join(pt, NULL);
+		execlp("true", "true", NULL);
+	}
+
+	sleep(1);
+	sprintf(mm, "/proc/%d/mem", pid);
+	f = open(mm, O_RDONLY);
+	ASSERT_LE(0, f);
+	close(f);
+	f = kill(pid, SIGCONT);
+	ASSERT_EQ(0, f);
+}
+
+TEST(attach)
+{
+	int f, pid = fork();
+
+	if (!pid) {
+		pthread_t pt;
+
+		pthread_create(&pt, NULL, thread, NULL);
+		pthread_join(pt, NULL);
+		execlp("true", "true", NULL);
+	}
+
+	sleep(1);
+	f = ptrace(PTRACE_ATTACH, pid, 0L, 0L);

To be meaningful this code needs to learn to loop when
ptrace returns -EAGAIN.

Because that is pretty much what any self respecting user space
process will do.

At which point I am not certain we can say that the behavior has
sufficiently improved not to be a deadlock.

In this special dead-duck test it won't work, but it would
still be lots more transparent what is going on, since previously
you had two zombie process, and no way to even output debug
messages, which also all self respecting user space processes
should do.

Agreed it is more transparent.  So if you are going to deadlock
it is better.

My previous proposal (which I admit is more work to implement) would
actually allow succeeding in this case and so it would not be subject to
a dead lock (even via -EGAIN) at this point.

quoted

So yes, I can at least give a good example and re-try it several
times together with wait4 which a tracer is expected to do.

Thank you,

Eric

Okay, I think it can be done with minimal API changes,
but it needs two mutexes, one that guards the execve,
and one that guards only the credentials.

If no traced sibling thread exists, the mutexes are used this way:
lock(exec_guard_mutex)
cred_locked_in_execve = true;
de_thread()
lock(cred_guard_mutex)
unlock(cred_guard_mutex)
cred_locked_in_execve = false;
unlock(exec_guard_mutex)

so effectively no API change at all.

If a traced sibling thread exists, the mutexes are used differently:
lock(exec_guard_mutex)
cred_locked_in_execve = true;
unlock(exec_guard_mutex)
de_thread()
lock(cred_guard_mutex)
unlock(cred_guard_mutex)
lock(exec_guard_mutex)
cred_locked_in_execve = false;
unlock(exec_guard_mutex)

Only the case changes that would deadlock anyway.


Bernd.