[PATCH] fbdev: fix divide error in fb_var_to_videomode

[Shile Zhang]

On 2019/4/1 23:25, Bartlomiej Zolnierkiewicz wrote:
> On 03/29/2019 09:46 PM, Mukesh Ojha wrote:
>> On 3/19/2019 5:50 PM, shile.zhang at linux.alibaba.com wrote:
>>> From: Shile Zhang <shile.zhang at linux.alibaba.com>
>>>
>>> To fix following divide-by-zero error found by Syzkaller:
>>>
>>>     divide error: 0000 [#1] SMP PTI
>>>     CPU: 7 PID: 8447 Comm: test Kdump: loaded Not tainted 4.19.24-8.al7.x86_64 #1
>>>     Hardware name: Alibaba Cloud Alibaba Cloud ECS, BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
>>>     RIP: 0010:fb_var_to_videomode+0xae/0xc0
>>>     Code: 04 44 03 46 78 03 4e 7c 44 03 46 68 03 4e 70 89 ce d1 ee 69 c0 e8 03 00 00 f6 c2 01 0f 45 ce 83 e2 02 8d 34 09 0f 45 ce 31 d2 <41> f7 f0 31 d2 f7 f1 89 47 08 f3 c3 66 0f 1f 44 00 00 0f 1f 44 00
>>>     RSP: 0018:ffffb7e189347bf0 EFLAGS: 00010246
>>>     RAX: 00000000e1692410 RBX: ffffb7e189347d60 RCX: 0000000000000000
>>>     RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffb7e189347c10
>>>     RBP: ffff99972a091c00 R08: 0000000000000000 R09: 0000000000000000
>>>     R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000100
>>>     R13: 0000000000010000 R14: 00007ffd66baf6d0 R15: 0000000000000000
>>>     FS:  00007f2054d11740(0000) GS:ffff99972fbc0000(0000) knlGS:0000000000000000
>>>     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>     CR2: 00007f205481fd20 CR3: 00000004288a0001 CR4: 00000000001606a0
>>>     Call Trace:
>>>      fb_set_var+0x257/0x390
>>>      ? lookup_fast+0xbb/0x2b0
>>>      ? fb_open+0xc0/0x140
>>>      ? chrdev_open+0xa6/0x1a0
>>>      do_fb_ioctl+0x445/0x5a0
>>>      do_vfs_ioctl+0x92/0x5f0
>>>      ? __alloc_fd+0x3d/0x160
>>>      ksys_ioctl+0x60/0x90
>>>      __x64_sys_ioctl+0x16/0x20
>>>      do_syscall_64+0x5b/0x190
>>>      entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>     RIP: 0033:0x7f20548258d7
>>>     Code: 44 00 00 48 8b 05 b9 15 2d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 89 15 2d 00 f7 d8 64 89 01 48
>>>
>>> It can be triggered easily with following test code:
>>>
>>>     #include <linux/fb.h>
>>>     #include <fcntl.h>
>>>     #include <sys/ioctl.h>
>>>     int main(void)
>>>     {
>>>             struct fb_var_screeninfo var = {.activate = 0x100, .pixclock = 60};
>>>             int fd = open("/dev/fb0", O_RDWR);
>>>             if (fd < 0)
>>>                     return 1;
>>>
>>>             if (ioctl(fd, FBIOPUT_VSCREENINFO, &var))
>>>                     return 1;
>>>
>>>             return 0;
>>>     }
>>>
>>> Signed-off-by: Shile Zhang <shile.zhang at linux.alibaba.com>
>> To avoid any exception we should include the check.
>> Looks fine to me.
>> Reviewed-by: Mukesh Ojha <mojha at codeaurora.org>
> Patch queued for v5.2, thanks.
Thank you all!
>
> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics