[PATCH v2] KVM: eventfd: fix NULL deref irqbypass consumer

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

Reported syzkaller:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
    IP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    PGD 0 
   
    Oops: 0002 [#1] SMP
    CPU: 1 PID: 125 Comm: kworker/1:1 Not tainted 4.9.0+ #1
    Workqueue: kvm-irqfd-cleanup irqfd_shutdown [kvm]
    task: ffff9bbe0dfbb900 task.stack: ffffb61802014000
    RIP: 0010:irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    Call Trace:
     irqfd_shutdown+0x66/0xa0 [kvm]
     process_one_work+0x16b/0x480
     worker_thread+0x4b/0x500
     kthread+0x101/0x140
     ? process_one_work+0x480/0x480
     ? kthread_create_on_node+0x60/0x60
     ret_from_fork+0x25/0x30
    RIP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass] RSP: ffffb61802017e20
    CR2: 0000000000000008

The syzkaller folks reported a NULL pointer dereference that due to 
unregister an consumer which fails registration before. The syzkaller 
creates two VMs w/ an equal eventfd occasionally. So the second VM 
fails to register an irqbypass consumer. It will make irqfd as inactive 
and queue an workqueue work to shutdown irqfd and unregister the irqbypass 
consumer when eventfd is closed. However, the second consumer has been 
initialized though it fails registration. So the token(same as the first 
VM's) is taken to unregister the consumer through the workqueue, the 
consumer of the first VM is found and unregistered, then NULL deref incurred 
in the path of deleting consumer from the consumers list.

#include <fcntl.h>
#include <pthread.h>
#include <setjmp.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

 __thread int skip_segv;
 __thread jmp_buf segv_env;

static void segv_handler(int sig, siginfo_t* info, void* uctx)
{
	if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED))
		_longjmp(segv_env, 1);
	exit(sig);
}

static void install_segv_handler()
{
	struct sigaction sa;
	memset(&sa, 0, sizeof(sa));
	sa.sa_sigaction = segv_handler;
	sa.sa_flags = SA_NODEFER | SA_SIGINFO;
	sigaction(SIGSEGV, &sa, NULL);
	sigaction(SIGBUS, &sa, NULL);
}

#define NONFAILING(...)                                                \
{                                                                    \
	__atomic_fetch_add(&skip_segv, 1, __ATOMIC_SEQ_CST);               \
	if (_setjmp(segv_env) == 0) {                                      \
		__VA_ARGS__;                                                     \
	}                                                                  \
	__atomic_fetch_sub(&skip_segv, 1, __ATOMIC_SEQ_CST);               \
}

static uintptr_t execute_syscall(int nr, uintptr_t a0, uintptr_t a1,
		uintptr_t a2, uintptr_t a3,
		uintptr_t a4, uintptr_t a5,
		uintptr_t a6, uintptr_t a7,
		uintptr_t a8)
{
	return syscall(nr, a0, a1, a2, a3, a4, a5);
}

long r[28];
void* thr(void* arg)
{
	switch ((long)arg) {
		case 0:
			r[0] =
				execute_syscall(__NR_mmap, 0x20000000ul, 0xd000ul, 0x3ul,
						0x32ul, 0xfffffffffffffffful, 0x0ul, 0, 0, 0);
			break;
		case 1:
			r[2] = syscall(__NR_open, "/dev/kvm", 0x40042ul, 0, 0, 0, 0, 0, 0);
			break;
		case 2:
			r[3] = execute_syscall(__NR_ioctl, r[2], 0xae01ul, 0x0ul, 0, 0, 0,
					0, 0, 0);
			break;
		case 3:
			r[4] = execute_syscall(__NR_ioctl, r[3], 0xae41ul, 0x3fful, 0, 0, 0,
					0, 0, 0);
			break;
		case 4:
			r[5] = execute_syscall(__NR_ioctl, r[4], 0xae9aul, 0, 0, 0, 0, 0, 0,
					0);
			break;
		case 5:
			r[6] = execute_syscall(__NR_eventfd2, 0x8ul, 0x801ul, 0, 0, 0, 0, 0,
					0, 0);
			break;
		case 6:
			NONFAILING(*(uint32_t*)0x2000c000 = r[6]);
			NONFAILING(*(uint32_t*)0x2000c004 = (uint32_t)0x98cd);
			NONFAILING(*(uint32_t*)0x2000c008 = (uint32_t)0x0);
			NONFAILING(*(uint32_t*)0x2000c00c = (uint32_t)0xffffffffffffffff);
			NONFAILING(*(uint8_t*)0x2000c010 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c011 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c012 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c013 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c014 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c015 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c016 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c017 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c018 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c019 = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01a = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01b = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01c = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01d = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01e = (uint8_t)0x0);
			NONFAILING(*(uint8_t*)0x2000c01f = (uint8_t)0x0);
			r[27] = execute_syscall(__NR_ioctl, r[3], 0x4020ae76ul,
					0x2000c000ul, 0, 0, 0, 0, 0, 0);
			break;
	}
	return 0;
}

int main()
{
	long i;
	pthread_t th[14];

	install_segv_handler();
	memset(r, -1, sizeof(r));
	srand(getpid());
	for (i = 0; i < 7; i++) {
		pthread_create(&th[i], 0, thr, (void*)i);
		usleep(10000);
	}
	for (i = 0; i < 7; i++) {
		pthread_create(&th[7 + i], 0, thr, (void*)i);
		if (rand() % 2)
			usleep(rand() % 10000);
	}
	usleep(100000);
	return 0;
}

This patch fixes it by making irq_bypass_register/unregister_consumer() 
looks for the consumer entry based on consumer pointer itself instead of 
token matching.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * make irq_bypass_register/unregister_consumer() looks for the consumer 
   entry based on consumer pointer itself instead of token matching

 virt/lib/irqbypass.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/lib/irqbypass.c b/virt/lib/irqbypass.c
index 52abac4..6d2fcd6 100644
--- a/virt/lib/irqbypass.c
+++ b/virt/lib/irqbypass.c
@@ -195,7 +195,7 @@ int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer)
 	mutex_lock(&lock);
 
 	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->token == consumer->token) {
+		if (tmp->token == consumer->token || tmp == consumer) {
 			mutex_unlock(&lock);
 			module_put(THIS_MODULE);
 			return -EBUSY;
@@ -245,7 +245,7 @@ void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
 	mutex_lock(&lock);
 
 	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->token != consumer->token)
+		if (tmp != consumer)
 			continue;
 
 		list_for_each_entry(producer, &producers, node) {
-- 
2.7.4