[PATCH] fbdev: pm3fb: Fix potential divide by zero

Subsystems: framebuffer layer, the rest

STALE407d

6 messages, 3 authors, 2025-07-27 · open the first message on its own page

[PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Alex Guo <hidden>
Date: 2025-06-07 19:50:07

variable var->pixclock can be set by user. In case it equals to
 zero, divide by zero would occur in pm3fb_check_var. Similar
crashes have happened in other fbdev drivers. There is no check
and modification on var->pixclock along the call chain to
pm3fb_check_var. So we fix this by checking whether 'pixclock'
is zero.

Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
Error out if 'pixclock' equals zero")

Signed-off-by: Alex Guo <redacted>
---
 drivers/video/fbdev/pm3fb.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/video/fbdev/pm3fb.c b/drivers/video/fbdev/pm3fb.c
index 6e55e42514d6..d9b3f1937ce6 100644
--- a/drivers/video/fbdev/pm3fb.c
+++ b/drivers/video/fbdev/pm3fb.c

@@ -998,6 +998,9 @@ static int pm3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
 		return -EINVAL;
 	}
 
+	if (!var->pixclock)
+		return -EINVAL;
+		
 	if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
 		DPRINTK("pixclock too high (%ldKHz)\n",
 			PICOS2KHZ(var->pixclock));

-- 
2.34.1

Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Helge Deller <deller@gmx.de>
Date: 2025-06-08 13:12:18

On 6/7/25 21:49, Alex Guo wrote:

variable var->pixclock can be set by user. In case it equals to
  zero, divide by zero would occur in pm3fb_check_var. Similar
crashes have happened in other fbdev drivers. There is no check
and modification on var->pixclock along the call chain to
pm3fb_check_var. So we fix this by checking whether 'pixclock'
is zero.

Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
Error out if 'pixclock' equals zero")

Signed-off-by: Alex Guo <redacted>
---
  drivers/video/fbdev/pm3fb.c | 3 +++
  1 file changed, 3 insertions(+)

applied.

Thanks!
Helge

Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: 2025-06-10 07:42:18

Hi Alex,

On Sat, 7 Jun 2025 at 22:14, Alex Guo [off-list ref] wrote:

variable var->pixclock can be set by user. In case it equals to
 zero, divide by zero would occur in pm3fb_check_var. Similar
crashes have happened in other fbdev drivers. There is no check
and modification on var->pixclock along the call chain to
pm3fb_check_var. So we fix this by checking whether 'pixclock'
is zero.

Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
Error out if 'pixclock' equals zero")

Signed-off-by: Alex Guo <redacted>

Thanks for your patch, which is now commit 59d1fc7b3e1ae9d4
("fbdev: pm3fb: fix potential divide by zero") in fbdev/for-next.

quoted hunk

--- a/drivers/video/fbdev/pm3fb.c
+++ b/drivers/video/fbdev/pm3fb.c

@@ -998,6 +998,9 @@ static int pm3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
                return -EINVAL;
        }

+       if (!var->pixclock)
+               return -EINVAL;

While this fixes the crash, this is correct behavior for an fbdev driver.
When a value is invalid, it should be rounded up to a valid value instead,
if possible.

+
        if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
                DPRINTK("pixclock too high (%ldKHz)\n",
                        PICOS2KHZ(var->pixclock));

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Alex Guo <hidden>
Date: 2025-06-11 16:12:09

Hi Greet,

Thanks for your confirmation and suggestions.

I added this patch based on existing checks on var->pixclock in other drivers, such as savagefb_check_var, nvidiafb_check_var, etc.
Are you suggesting that it is better to replace an invalid value (var->pixclock == 0) with a default valid value, instead of returning -EINVAL? If so, could you advise what a suitable default value would be for this case?

Actually, I have found a few similar issues in other functions as well. I would like to make sure I am addressing them in the correct way.

Best,
Alex

Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: 2025-06-12 09:29:22

Hi Alex,

On Wed, 11 Jun 2025 at 18:12, Alex Guo [off-list ref] wrote:

quoted

On Sat, 7 Jun 2025 at 22:14, Alex Guo [off-list ref] wrote:

quoted

variable var->pixclock can be set by user. In case it equals to
 zero, divide by zero would occur in pm3fb_check_var. Similar
crashes have happened in other fbdev drivers. There is no check
and modification on var->pixclock along the call chain to
pm3fb_check_var. So we fix this by checking whether 'pixclock'
is zero.

Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
Error out if 'pixclock' equals zero")

Signed-off-by: Alex Guo <redacted>

Thanks for your patch, which is now commit 59d1fc7b3e1ae9d4
("fbdev: pm3fb: fix potential divide by zero") in fbdev/for-next.

quoted

--- a/drivers/video/fbdev/pm3fb.c
+++ b/drivers/video/fbdev/pm3fb.c

@@ -998,6 +998,9 @@ static int pm3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
                return -EINVAL;
        }

+       if (!var->pixclock)
+               return -EINVAL;

While this fixes the crash, this is correct behavior for an fbdev driver.
When a value is invalid, it should be rounded up to a valid value instead,
if possible.

Thanks for your confirmation and suggestions.

I added this patch based on existing checks on var->pixclock in other drivers, such as savagefb_check_var, nvidiafb_check_var, etc.
Are you suggesting that it is better to replace an invalid value (var->pixclock == 0) with a default valid value, instead of returning -EINVAL?

Indeed.

If so, could you advise what a suitable default value would be for this case?

The answer is hidden in the existing check below:

quoted

+
        if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
                DPRINTK("pixclock too high (%ldKHz)\n",
                        PICOS2KHZ(var->pixclock));
                return -EINVAL;
        }

It can be replaced by:

    if (var->pixclock <= KHZ2PICOS(PM3_MAX_PIXCLOCK))
            var->pixclock = KHZ2PICOS(PM3_MAX_PIXCLOCK) + 1;

The "+ 1" is needed because of rounding.

Actually, I have found a few similar issues in other functions as well. I would like to make sure I am addressing them in the correct way.

That would be great. Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

From: Helge Deller <deller@gmx.de>
Date: 2025-07-27 16:28:35

Hi Alex,

On 6/12/25 11:29, Geert Uytterhoeven wrote:

On Wed, 11 Jun 2025 at 18:12, Alex Guo [off-list ref] wrote:

quoted

On Sat, 7 Jun 2025 at 22:14, Alex Guo [off-list ref] wrote:

quoted

variable var->pixclock can be set by user. In case it equals to
  zero, divide by zero would occur in pm3fb_check_var. Similar
crashes have happened in other fbdev drivers. There is no check
and modification on var->pixclock along the call chain to
pm3fb_check_var. So we fix this by checking whether 'pixclock'
is zero.

Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
Error out if 'pixclock' equals zero")

Signed-off-by: Alex Guo <redacted>

Thanks for your patch, which is now commit 59d1fc7b3e1ae9d4
("fbdev: pm3fb: fix potential divide by zero") in fbdev/for-next.

quoted

--- a/drivers/video/fbdev/pm3fb.c
+++ b/drivers/video/fbdev/pm3fb.c

@@ -998,6 +998,9 @@ static int pm3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
                 return -EINVAL;
         }

+       if (!var->pixclock)
+               return -EINVAL;

While this fixes the crash, this is correct behavior for an fbdev driver.
When a value is invalid, it should be rounded up to a valid value instead,
if possible.

Thanks for your confirmation and suggestions.

I added this patch based on existing checks on var->pixclock in other drivers, such as savagefb_check_var, nvidiafb_check_var, etc.
Are you suggesting that it is better to replace an invalid value (var->pixclock == 0) with a default valid value, instead of returning -EINVAL?

Indeed.

quoted

If so, could you advise what a suitable default value would be for this case?

The answer is hidden in the existing check below:

quoted

+
         if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
                 DPRINTK("pixclock too high (%ldKHz)\n",
                         PICOS2KHZ(var->pixclock));
                 return -EINVAL;
         }

It can be replaced by:

     if (var->pixclock <= KHZ2PICOS(PM3_MAX_PIXCLOCK))
             var->pixclock = KHZ2PICOS(PM3_MAX_PIXCLOCK) + 1;

The "+ 1" is needed because of rounding.

You sent a whole bunch of patches [1] which check pixclock against
zero, but you don't set the default value as Geert pointed out
above. Can you maybe revise your patches accordingly?

Helge

[1] https://patchwork.kernel.org/project/linux-fbdev/list/

`h`	back out one level
`j`	next message in thread
`k`	previous message in thread
`l`	drill in
`Esc`	close help / fold thread tree
`?`	toggle this help