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SubjectRe: [PATCH 1/1] video: fbdev: fix divide error in fbcon_switch
Date
Attached the Syzkaller C reproducer.

#define _GNU_SOURCE

#include <dirent.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#include <linux/futex.h>

static void sleep_ms(uint64_t ms)
{
usleep(ms * 1000);
}

static uint64_t current_time_ms(void)
{
struct timespec ts;
if (clock_gettime(CLOCK_MONOTONIC, &ts))
exit(1);
return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void thread_start(void* (*fn)(void*), void* arg)
{
pthread_t th;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setstacksize(&attr, 128 << 10);
int i;
for (i = 0; i < 100; i++) {
if (pthread_create(&th, &attr, fn, arg) == 0) {
pthread_attr_destroy(&attr);
return;
}
if (errno == EAGAIN) {
usleep(50);
continue;
}
break;
}
exit(1);
}

typedef struct {
int state;
} event_t;

static void event_init(event_t* ev)
{
ev->state = 0;
}

static void event_reset(event_t* ev)
{
ev->state = 0;
}

static void event_set(event_t* ev)
{
if (ev->state)
exit(1);
__atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
syscall(SYS_futex, &ev->state, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1000000);
}

static void event_wait(event_t* ev)
{
while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0,
0);
}

static int event_isset(event_t* ev)
{
return __atomic_load_n(&ev->state, __ATOMIC_ACQUIRE);
}

static int event_timedwait(event_t* ev, uint64_t timeout)
{
uint64_t start = current_time_ms();
uint64_t now = start;
for (;;) {
uint64_t remain = timeout - (now - start);
struct timespec ts;
ts.tv_sec = remain / 1000;
ts.tv_nsec = (remain % 1000) * 1000 * 1000;
syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0,
&ts);
if (__atomic_load_n(&ev->state, __ATOMIC_RELAXED))
return 1;
now = current_time_ms();
if (now - start > timeout)
return 0;
}
}

static bool write_file(const char* file, const char* what, ...)
{
char buf[1024];
va_list args;
va_start(args, what);
vsnprintf(buf, sizeof(buf), what, args);
va_end(args);
buf[sizeof(buf) - 1] = 0;
int len = strlen(buf);
int fd = open(file, O_WRONLY | O_CLOEXEC);
if (fd == -1)
return false;
if (write(fd, buf, len) != len) {
int err = errno;
close(fd);
errno = err;
return false;
}
close(fd);
return true;
}

static long syz_open_dev(volatile long a0, volatile long a1, volatile long
a2)
{
if (a0 == 0xc || a0 == 0xb) {
char buf[128];
sprintf(buf, "/dev/%s/%d:%d", a0 == 0xc ? "char" : "block",
(uint8_t)a1, (uint8_t)a2);
return open(buf, O_RDWR, 0);
} else {
char buf[1024];
char* hash;
strncpy(buf, (char*)a0, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = 0;
while ((hash = strchr(buf, '#'))) {
*hash = '0' + (char)(a1 % 10);
a1 /= 10;
}
return open(buf, a2, 0);
}
}

static void kill_and_wait(int pid, int* status)
{
kill(-pid, SIGKILL);
kill(pid, SIGKILL);
int i;
for (i = 0; i < 100; i++) {
if (waitpid(-1, status, WNOHANG | __WALL) == pid)
return;
usleep(1000);
}
DIR* dir = opendir("/sys/fs/fuse/connections");
if (dir) {
for (;;) {
struct dirent* ent = readdir(dir);
if (!ent)
break;
if (strcmp(ent->d_name, ".") == 0 || strcmp(ent->d_name, "..") ==
0)
continue;
char abort[300];
snprintf(abort, sizeof(abort),
"/sys/fs/fuse/connections/%s/abort", ent->d_name);
int fd = open(abort, O_WRONLY);
if (fd == -1) {
continue;
}
if (write(fd, abort, 1) < 0) {
}
close(fd);
}
closedir(dir);
} else {
}
while (waitpid(-1, status, __WALL) != pid) {
}
}

static void setup_test()
{
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
setpgrp();
write_file("/proc/self/oom_score_adj", "1000");
}

struct thread_t {
int created, call;
event_t ready, done;
};

static struct thread_t threads[16];
static void execute_call(int call);
static int running;

static void* thr(void* arg)
{
struct thread_t* th = (struct thread_t*)arg;
for (;;) {
event_wait(&th->ready);
event_reset(&th->ready);
execute_call(th->call);
__atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
event_set(&th->done);
}
return 0;
}

static void execute_one(void)
{
int i, call, thread;
int collide = 0;
again:
for (call = 0; call < 6; call++) {
for (thread = 0; thread < (int)(sizeof(threads) /
sizeof(threads[0])); thread++) {
struct thread_t* th = &threads[thread];
if (!th->created) {
th->created = 1;
event_init(&th->ready);
event_init(&th->done);
event_set(&th->done);
thread_start(thr, th);
}
if (!event_isset(&th->done))
continue;
event_reset(&th->done);
th->call = call;
__atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
event_set(&th->ready);
if (collide && (call % 2) == 0)
break;
event_timedwait(&th->done, 45);
break;
}
}
for (i = 0; i < 100 && __atomic_load_n(&running, __ATOMIC_RELAXED); i++)
sleep_ms(1);
if (!collide) {
collide = 1;
goto again;
}
}

static void execute_one(void);

#define WAIT_FLAGS __WALL

static void loop(void)
{
int iter;
for (iter = 0;; iter++) {
int pid = fork();
if (pid < 0)
exit(1);
if (pid == 0) {
setup_test();
execute_one();
exit(0);
}
int status = 0;
uint64_t start = current_time_ms();
for (;;) {
if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
break;
sleep_ms(1);
if (current_time_ms() - start < 5 * 1000)
continue;
kill_and_wait(pid, &status);
break;
}
}
}

uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};

void execute_call(int call)
{
intptr_t res; switch (call) {
case 0:
memcpy((void*)0x20000000, "/dev/fb0\000", 9);
res = syscall(__NR_openat, 0xffffffffffffff9cul, 0x20000000ul, 0ul,
0ul);
if (res != -1)
r[0] = res;
break;
case 1:
*(uint32_t*)0x20000100 = 0;
*(uint32_t*)0x20000104 = 0x10;
*(uint32_t*)0x20000108 = 0;
*(uint32_t*)0x2000010c = 0;
*(uint32_t*)0x20000110 = 0;
*(uint32_t*)0x20000114 = 0;
*(uint32_t*)0x20000118 = 0;
*(uint32_t*)0x2000011c = 0;
*(uint32_t*)0x20000120 = 0;
*(uint32_t*)0x20000124 = 0;
*(uint32_t*)0x20000128 = 0;
*(uint32_t*)0x2000012c = 0;
*(uint32_t*)0x20000130 = 0;
*(uint32_t*)0x20000134 = 0;
*(uint32_t*)0x20000138 = 0;
*(uint32_t*)0x2000013c = 0;
*(uint32_t*)0x20000140 = 0;
*(uint32_t*)0x20000144 = 0;
*(uint32_t*)0x20000148 = 0;
*(uint32_t*)0x2000014c = 0;
*(uint32_t*)0x20000150 = 0;
*(uint32_t*)0x20000154 = 0;
*(uint32_t*)0x20000158 = 0;
*(uint32_t*)0x2000015c = 0;
*(uint32_t*)0x20000160 = 0;
*(uint32_t*)0x20000164 = 0;
*(uint32_t*)0x20000168 = 0;
*(uint32_t*)0x2000016c = 0;
*(uint32_t*)0x20000170 = 0;
*(uint32_t*)0x20000174 = 0;
*(uint32_t*)0x20000178 = 0;
*(uint32_t*)0x2000017c = 0;
*(uint32_t*)0x20000180 = 0;
*(uint32_t*)0x20000184 = 0;
*(uint32_t*)0x20000188 = 0;
*(uint32_t*)0x2000018c = 0;
*(uint32_t*)0x20000190 = 0;
*(uint32_t*)0x20000194 = 0;
*(uint32_t*)0x20000198 = 0;
*(uint32_t*)0x2000019c = 0;
syscall(__NR_ioctl, r[0], 0x4601ul, 0x20000100ul);
break;
case 2:
syscall(__NR_socket, 0xaul, 3ul, 0x3aul);
break;
case 3:
res = syz_open_dev(0xc, 4, 0x14);
if (res != -1)
r[1] = res;
break;
case 4:
syscall(__NR_ioctl, r[1], 0x5437ul, 0ul);
break;
case 5:
syscall(__NR_ioctl, r[1], 0x5606ul, 8ul);
break;
}

}
int main(void)
{
syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 3ul, 0x32ul, -1, 0);
loop();
return 0;
}



> On Oct 21, 2020, at 4:57 PM, saeed.mirzamohammadi@oracle.com wrote:
>
> From: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
>
> This patch fixes the issue due to:
>
> [ 89.572883] divide_error: 0000 [#1] SMP KASAN PTI
> [ 89.572897] CPU: 3 PID: 16083 Comm: repro Not tainted 5.9.0-rc7.20200930.rc1.allarch-19-g3e32d0d.syzk #5
> [ 89.572902] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
> [ 89.572934] RIP: 0010:cirrusfb_check_var+0x84/0x1260
>
> The error happens when the pixels value is calculated before performing the sanity checks on bits_per_pixel.
> A bits_per_pixel set to zero causes divide by zero error.
>
> This patch moves the calculation after the sanity check.
>
> Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
> Tested-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
> ---
> drivers/video/fbdev/cirrusfb.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> index 15a9ee7cd734..a7749101b094 100644
> --- a/drivers/video/fbdev/cirrusfb.c
> +++ b/drivers/video/fbdev/cirrusfb.c
> @@ -531,7 +531,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
> {
> int yres;
> /* memory size in pixels */
> - unsigned pixels = info->screen_size * 8 / var->bits_per_pixel;
> + unsigned int pixels;
> struct cirrusfb_info *cinfo = info->par;
>
> switch (var->bits_per_pixel) {
> @@ -573,6 +573,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
> return -EINVAL;
> }
>
> + pixels = info->screen_size * 8 / var->bits_per_pixel;
> if (var->xres_virtual < var->xres)
> var->xres_virtual = var->xres;
> /* use highest possible virtual resolution */
> --
> 2.27.0
>

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