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If userspace holds its file descriptor open, it can call PPS_KC_BIND on a
device which has been unplugged, leaving pps_kc_hardpps_dev as a dangling
pointer after close().
After that sequence, PPS_KC_BIND is broken until the system is rebooted,
because the pointer comparison in pps_kc_bind() can never be true.
calling pps_ktimer_init+0x0/0x1000 [pps_ktimer] @ 1081
initcall pps_ktimer_init+0x0/0x1000 [pps_ktimer] returned 0 after 811 usecs
pps pps0: bound kernel consumer: edge=0x1
pps pps0: unbound kernel consumer on device removal
pps pps0: bound kernel consumer: edge=0x1
calling pps_ktimer_init+0x0/0x1000 [pps_ktimer] @ 1085
initcall pps_ktimer_init+0x0/0x1000 [pps_ktimer] returned 0 after 340 usecs
pps pps0: another kernel consumer is already bound
Here is a short reproducer, which uses rmmod of the pps-ktimer testcase
to simulate a device being unplugged:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/pps.h>
#include <errno.h>
#include <err.h>
int main(void)
{
while (1) {
int fd;
if (system("insmod ./pps-ktimer.ko"))
err(1, "insmod failed");
fd = open("/dev/pps0", O_RDWR);
if (fd == -1)
err(1, "open failed");
struct pps_bind_args args = {
.tsformat = PPS_TSFMT_TSPEC,
.edge = PPS_CAPTUREASSERT,
.consumer = PPS_KC_HARDPPS,
};
if (ioctl(fd, PPS_KC_BIND, &args))
err(1, "first PPS_KC_BIND failed");
if (system("rmmod pps-ktimer"))
err(1, "rmmod failed");
if (ioctl(fd, PPS_KC_BIND, &args)) {
if (errno != ENODEV)
err(1, "second PPS_KC_BIND failed");
else
puts("Got ENODEV, kernel is patched");
}
close(fd);
}
}
Fix this by setting a flag when the device is unplugged, returning -ENODEV
from PPS_KC_BIND if the flag is set.
For userspace to encounter this new behavior, it must do something which
breaks the interface today, so this fix shouldn't cause any observable
behavior change for working programs.
Link: https://lore.kernel.org/672778c177ac9b6fdcb445e35c97ac4ca7d1149f.1780506611.git.calvin@wbinvd.org
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=1
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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