hpet: Fix division by zero in hpet_time_div() [Linux 4.9.187]

This Linux kernel change "hpet: Fix division by zero in hpet_time_div()" is included in the Linux 4.9.187 release. This change is authored by Kefeng Wang <wangkefeng.wang [at] huawei.com> on Thu Jul 11 21:27:57 2019 +0800. The commit for this change in Linux stable tree is 0cbed4f (patch) which is from upstream commit 0c7d37f. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream 0c7d37f.

hpet: Fix division by zero in hpet_time_div()

commit 0c7d37f4d9b8446956e97b7c5e61173cdb7c8522 upstream.

The base value in do_div() called by hpet_time_div() is truncated from
unsigned long to uint32_t, resulting in a divide-by-zero exception.

UBSAN: Undefined behaviour in ../drivers/char/hpet.c:572:2
division by zero
CPU: 1 PID: 23682 Comm: syz-executor.3 Not tainted 4.4.184.x86_64+ #4
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
 0000000000000000 b573382df1853d00 ffff8800a3287b98 ffffffff81ad7561
 ffff8800a3287c00 ffffffff838b35b0 ffffffff838b3860 ffff8800a3287c20
 0000000000000000 ffff8800a3287bb0 ffffffff81b8f25e ffffffff838b35a0
Call Trace:
 [<ffffffff81ad7561>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81ad7561>] dump_stack+0xc1/0x120 lib/dump_stack.c:51
 [<ffffffff81b8f25e>] ubsan_epilogue+0x12/0x8d lib/ubsan.c:166
 [<ffffffff81b900cb>] __ubsan_handle_divrem_overflow+0x282/0x2c8 lib/ubsan.c:262
 [<ffffffff823560dd>] hpet_time_div drivers/char/hpet.c:572 [inline]
 [<ffffffff823560dd>] hpet_ioctl_common drivers/char/hpet.c:663 [inline]
 [<ffffffff823560dd>] hpet_ioctl_common.cold+0xa8/0xad drivers/char/hpet.c:577
 [<ffffffff81e63d56>] hpet_ioctl+0xc6/0x180 drivers/char/hpet.c:676
 [<ffffffff81711590>] vfs_ioctl fs/ioctl.c:43 [inline]
 [<ffffffff81711590>] file_ioctl fs/ioctl.c:470 [inline]
 [<ffffffff81711590>] do_vfs_ioctl+0x6e0/0xf70 fs/ioctl.c:605
 [<ffffffff81711eb4>] SYSC_ioctl fs/ioctl.c:622 [inline]
 [<ffffffff81711eb4>] SyS_ioctl+0x94/0xc0 fs/ioctl.c:613
 [<ffffffff82846003>] tracesys_phase2+0x90/0x95

The main C reproducer autogenerated by syzkaller,

  syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
  memcpy((void*)0x20000100, "/dev/hpet\000", 10);
  syscall(__NR_openat, 0xffffffffffffff9c, 0x20000100, 0, 0);
  syscall(__NR_ioctl, r[0], 0x40086806, 0x40000000000000);

Fix it by using div64_ul().

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zhang HongJun <zhanghongjun2@huawei.com>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190711132757.130092-1-wangkefeng.wang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

There are 3 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/char/hpet.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 818a8d4..bedfd24 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -569,8 +569,7 @@ static inline unsigned long hpet_time_div(struct hpets *hpets,
    unsigned long long m;

    m = hpets->hp_tick_freq + (dis >> 1);
-   do_div(m, dis);
-   return (unsigned long)m;
+   return div64_ul(m, dis);
 }

 static int

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