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45217e9898
If userspace passes a negative timeout to PPS_FETCH, it triggers a kernel
splat from schedule_timeout():
schedule_timeout: wrong timeout value fffffffffff0bfb4
CPU: 17 UID: 0 PID: 4720 Comm: a.out Not tainted 7.1.0-rc5-x86-kvm-00150-g331d97e36b37 #1 PREEMPT_RT
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-20240910_120124-localhost 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x4b/0x70
schedule_timeout+0xb7/0xe0
pps_cdev_pps_fetch.isra.0+0x93/0x150
pps_cdev_ioctl+0x70/0x310
__x64_sys_ioctl+0x7b/0xc0
do_syscall_64+0xb6/0xfc0
entry_SYSCALL_64_after_hwframe+0x4b/0x53
Here is a trivial reproducer that works with the PPS_CLIENT_KTIMER test
device enabled in the kernel:
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/pps.h>
#include <err.h>
int main() {
struct pps_fdata fdata;
int fd;
fd = open("/dev/pps0", O_RDWR);
if (fd == -1)
err(1, "Failed to open /dev/pps0");
fdata.timeout.sec = -1;
fdata.timeout.nsec = 0;
if (ioctl(fd, PPS_FETCH, &fdata))
err(2, "PPS_FETCH failed");
close(fd);
return 0;
}
Sashiko imagines this to be some sort of security problem, which is
obviously really silly. But I think it is still worth fixing, so buggy
userspace code can't trigger the splat.
Silence the splat by using timespec64_to_jiffies(), which hard limits the
timeout to LONG_MAX jiffies. To be safe, explicitly preserve the
-ETIMEDOUT return value userspace sees today if it passes a negative
timeout.
If you really squint, this is still a slight behavior change in that there
are "denormalized" combinations of tv_sec and tv_nsec which used to work
but will now return -ETIMEDOUT. I can't imagine anybody will care about
that...
Link: https://lore.kernel.org/c5c97c3b3c9d66010382094fd538e59a38f4aacf.1781289959.git.calvin@wbinvd.org
Fixes: eae9d2ba0c ("LinuxPPS: core support")
Signed-off-by: Calvin Owens <calvin@wbinvd.org>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/cover.1779733602.git.calvin%40wbinvd.org?part=3
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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