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

[Fredrik Noring]

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@xxxxxxxxxxxxxxxxx wrote:
> > From: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxx>
> > ---
> >   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;
> >   }