APPLIED: [PATCH][ARTFUL]UBUNTU SAUCE: exec: ensure file system accounting in check_unsafe_exec is correct

[Seth Forshee]

On Fri, May 12, 2017 at 03:49:53PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1672819
> 
> There are two very race windows that need to be taken into consideration
> when check_unsafe_exec  performs the file system accounting comparing the
> number of fs->users against the number threads that share the same fs.
> 
> The first race can occur when a pthread creates a new pthread and the
> the fs->users count is incremented before the new pthread is associated with
> the pthread performing the exec. When this occurs, the pthread performing
> the exec has flags with bit PF_FORKNOEXEC set.
> 
> The second race can occur when a pthread is terminating and the fs->users
> count has been decremented by the pthread is still associated with the
> pthread that is performing the exec. When this occurs, the pthread
> peforming the exec has flags with bit PF_EXITING set.
> 
> This fix keeps track of any pthreads that may be in the race window
> (when PF_FORKNOEXEC or PF_EXITING) are set and if the fs count does
> not match the expected count we retry the count as we may have hit
> this small race windows.  Tests on an 8 thread server with the
> reproducer (see below) show that this retry occurs rarely, so the
> overhead of the retry is very small.
> 
> Below is a reproducer of the race condition.
> 
> The bug manifests itself because the current check_unsafe_exec
> hits this race and indicates it is not a safe exec, and the
> exec'd suid program fails to setuid.
> 
> $ cat Makefile
> ALL=a b
> all: $(ALL)
> 
> a: LDFLAGS=-pthread
> 
> b: b.c
> 	$(CC) b.c -o b
> 	sudo chown root:root b
> 	sudo chmod u+s b
> 
> test:
> 	for I in $$(seq 1000); do echo $I; ./a ; done
> 
> clean:
> 	rm -vf $(ALL)
> 
> $ cat a.c
> 
> #include <stdio.h>
> #include <unistd.h>
> #include <pthread.h>
> #include <sys/types.h>
> 
> void *nothing(void *p)
> {
> 	return NULL;
> }
> 
> void *target(void *p) {
> 	for (;;) {
> 		pthread_t t;
> 		if (pthread_create(&t, NULL, nothing, NULL) == 0)
> 			pthread_join(t, NULL);
>     	}
> 	return NULL;
> }
> 
> int main(void)
> {
> 	struct timespec tv;
> 	int i;
> 
> 	for (i = 0; i < 10; i++) {
> 		pthread_t t;
> 		pthread_create(&t, NULL, target, NULL);
> 	}
> 	tv.tv_sec = 0;
> 	tv.tv_nsec = 100000;
> 	nanosleep(&tv, NULL);
> 	if (execl("./b", "./b", NULL) < 0)
> 		perror("execl");
> 	return 0;
> }
> 
> $ cat b.c
> 
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/types.h>
> 
> int main(void)
> {
> 	const uid_t euid = geteuid();
> 	if (euid != 0) {
> 		printf("Failed, got euid %d (expecting 0)\n", euid);
>         	return 1;
> 	}
> 	return 0;
> }
> 
> $ make
> make
> cc   -pthread  a.c   -o a
> cc b.c -o b
> sudo chown root:root b
> sudo chmod u+s b
> $ for i in $(seq 1000); do ./a; done
> 
> Without the fix, one will see 'Failed, got euid 1000 (expecting 0)' messages
> 
> Signed-off-by: Colin Ian King <colin.king at canonical.com>

Applied to artful master-next.