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Subject[PATCH V2] ipc/mqueue.c: change __do_notify() to bypass check_kill_permission()
Commit cc731525f26a ("signal: Remove kernel interal si_code magic")
changed the value of SI_FROMUSER(SI_MESGQ), this means that mq_notify()
no longer works if the sender doesn't have rights to send a signal.

Change __do_notify() to use do_send_sig_info() instead of kill_pid_info()
to avoid check_kill_permission().

This needs the additional notify.sigev_signo != 0 check, shouldn't we
change do_mq_notify() to deny sigev_signo == 0 ?

Test-case:

#include <signal.h>
#include <mqueue.h>
#include <unistd.h>
#include <sys/wait.h>
#include <assert.h>

static int notified;

static void sigh(int sig)
{
notified = 1;
}

int main(void)
{
signal(SIGIO, sigh);

int fd = mq_open("/mq", O_RDWR|O_CREAT, 0666, NULL);
assert(fd >= 0);

struct sigevent se = {
.sigev_notify = SIGEV_SIGNAL,
.sigev_signo = SIGIO,
};
assert(mq_notify(fd, &se) == 0);

if (!fork()) {
assert(setuid(1) == 0);
mq_send(fd, "",1,0);
return 0;
}

wait(NULL);
mq_unlink("/mq");
assert(notified);
return 0;
}

Reported-by: Yoji <yoji.fujihar.min@gmail.com>
Fixes: cc731525f26a ("signal: Remove kernel interal si_code magic")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
ipc/mqueue.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 49a05ba3000d..63b164932ffd 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -774,28 +774,40 @@ static void __do_notify(struct mqueue_inode_info *info)
* synchronously. */
if (info->notify_owner &&
info->attr.mq_curmsgs == 1) {
- struct kernel_siginfo sig_i;
switch (info->notify.sigev_notify) {
case SIGEV_NONE:
break;
- case SIGEV_SIGNAL:
- /* sends signal */
+ case SIGEV_SIGNAL: {
+ struct kernel_siginfo sig_i;
+ struct task_struct *task;
+
+ /* do_mq_notify() accepts sigev_signo == 0, why?? */
+ if (!info->notify.sigev_signo)
+ break;

clear_siginfo(&sig_i);
sig_i.si_signo = info->notify.sigev_signo;
sig_i.si_errno = 0;
sig_i.si_code = SI_MESGQ;
sig_i.si_value = info->notify.sigev_value;
- /* map current pid/uid into info->owner's namespaces */
rcu_read_lock();
+ /* map current pid/uid into info->owner's namespaces */
sig_i.si_pid = task_tgid_nr_ns(current,
ns_of_pid(info->notify_owner));
- sig_i.si_uid = from_kuid_munged(info->notify_user_ns, current_uid());
+ sig_i.si_uid = from_kuid_munged(info->notify_user_ns,
+ current_uid());
+ /*
+ * We can't use kill_pid_info(), this signal should
+ * bypass check_kill_permission(). It is from kernel
+ * but si_fromuser() can't know this.
+ */
+ task = pid_task(info->notify_owner, PIDTYPE_PID);
+ if (task)
+ do_send_sig_info(info->notify.sigev_signo,
+ &sig_i, task, PIDTYPE_TGID);
rcu_read_unlock();
-
- kill_pid_info(info->notify.sigev_signo,
- &sig_i, info->notify_owner);
break;
+ }
case SIGEV_THREAD:
set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
netlink_sendskb(info->notify_sock, info->notify_cookie);
--
2.25.1.362.g51ebf55

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