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

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 3/1/20 7:52 PM, Christian Brauner wrote:
> On Sun, Mar 01, 2020 at 07:21:03PM +0100, Jann Horn wrote:
>> On Sun, Mar 1, 2020 at 12:27 PM Bernd Edlinger
>> <bernd.edlinger@hotmail.de> wrote:
>>> The proposed solution is to have a second mutex that is
>>> used in mm_access, so it is allowed to continue while the
>>> dying threads are not yet terminated.
>>
>> Just for context: When I proposed something similar back in 2016,
>> https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>> was the resulting discussion thread. At least back then, I looked
>> through the various existing users of cred_guard_mutex, and the only
>> places that couldn't be converted to the new second mutex were
>> PTRACE_ATTACH and SECCOMP_FILTER_FLAG_TSYNC.
>>
>>
>> The ideal solution would IMO be something like this: Decide what the
>> new task's credentials should be *before* reaching de_thread(),
>> install them into a second cred* on the task (together with the new
>> dumpability), drop the cred_guard_mutex, and let ptrace_may_access()
>> check against both. After that, some further restructuring might even
> 
> Hm, so essentially a private ptrace_access_cred member in task_struct?
> That would presumably also involve altering various LSM hooks to look at
> ptrace_access_cred.
> 
> (Minor side-note, de_thread() takes a struct task_struct argument but
>  only ever is passed current.)
> 
>> allow the cred_guard_mutex to not be held across all of the VFS
>> operations that happen early on in execve, which may block
>> indefinitely. But that would be pretty complicated, so I think your
>> proposed solution makes sense for now, given that nobody has managed
>> to implement anything better in the last few years.
> 
> Reading through the old threads and how often this issue came up, I tend
> to agree.
> 

Okay, fine.

I managed to change Oleg's test case, into one that shows what exactly
is changed with this patch:


$ cat t.c
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/signal.h>
#include <sys/ptrace.h>

void *thread(void *arg)
{
	ptrace(PTRACE_TRACEME, 0,0,0);
	return NULL;
}

int main(void)
{
	int f, pid = fork();
	char mm[64];

	if (!pid) {
		pthread_t pt;
		pthread_create(&pt, NULL, thread, NULL);
		pthread_join(pt, NULL);
		execlp("echo", "echo", "passed", NULL);
	}

	sleep(1);
	sprintf(mm, "/proc/%d/mem", pid);
        printf("open(%s)\n", mm);
	f = open(mm, O_RDONLY);
        printf("f = %d\n", f);
	// this is not fixed! ptrace(PTRACE_ATTACH, pid, 0,0);
	kill(pid, SIGCONT);
	if (f >= 0)
		close(f);
	return 0;
}
$ gcc -pthread -Wall t.c
$ ./a.out 
open(/proc/2802/mem)
f = 3
$ passed

previously this did block, how can I make a test case for this?
I am not so experienced in this matter.


Thanks
Bernd.