Re: [PATCH] fbdev: fix divide error in fb_var_to_videomode

Hi Shile,

Thank you for your report and patch.

On Tue, Mar 26, 2019 at 01:54:13PM +0800, Shile Zhang wrote:

Hi, Bartlomiej, Fredrik, Daniel,

Could you please help to have a look at this patch?

I have tested your patch with a MIPS R5900 system and indeed there is
a division by zero, although my kernel inhibits the exception that you
observed.

The statements

	mode->refresh = 0;

	if (!var->pixclock)
		return;

a bit further up in fb_var_to_videomode seem to suggest that setting
mode->refresh to zero signals an invalid value. Your patch makes sense
to me in that having htotal or vtotal zero is equally absurd.

The note above the function could be improved to clarify the meaning
of assigning zero to mode->refresh. The refresh field is marked as
optional in the fb_videomode structure definition; perhaps zero is a
special value for this purpose.

Interestingly, the division by zero on the MIPS R5900 assigns -1 to
mode->refresh, which isn't zero as one might prefer for a video mode
that is invalid. Your patch would correct this anomaly.

Hopefully Bartlomiej or someone else with authority to approve your
patch submission will have a look at it.

Fredrik

On 2019/3/19 20:20, shile.zhang@linux.alibaba.com wrote:

quoted

From: Shile Zhang <redacted>

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 <redacted>
---
  drivers/video/fbdev/core/modedb.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/drivers/video/fbdev/core/modedb.c b/drivers/video/fbdev/core/modedb.c
index 283d9307..ac04987 100644
--- a/drivers/video/fbdev/core/modedb.c
+++ b/drivers/video/fbdev/core/modedb.c

@@ -935,6 +935,9 @@ void fb_var_to_videomode(struct fb_videomode *mode,
  	if (var->vmode & FB_VMODE_DOUBLE)
  		vtotal *= 2;
  
+	if (!htotal || !vtotal)
+		return;
+
  	hfreq = pixclock/htotal;
  	mode->refresh = hfreq/vtotal;
  }