[PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()

The expression htotal * vtotal can have a zero value on
overflow. It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
 V1 -> V2: Replaced the code of the first version with a check.

 drivers/video/fbdev/core/fbmon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c

@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
 	vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
 		 vm->vsync_len;
 	/* prevent division by zero */
-	if (htotal && vtotal) {
+	if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
 		fbmode->refresh = vm->pixelclock / (htotal * vtotal);
 	/* a mode must have htotal and vtotal != 0 or it is invalid */
 	} else {

-- 
2.34.1

On 3/5/24 14:51, Roman Smirnov wrote:

The expression htotal * vtotal can have a zero value on
overflow.

I'm not sure if thos always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

why don't you then simply check for
	if .. ((htotal * vtotal) == 0) ...
instead?

Helge

  		fbmode->refresh = vm->pixelclock / (htotal * vtotal);
  	/* a mode must have htotal and vtotal != 0 or it is invalid */
  	} else {

On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:

On 3/5/24 14:51, Roman Smirnov wrote:

quoted

The expression htotal * vtotal can have a zero value on
overflow.

I'm not sure if thos always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

quoted

It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
  V1 -> V2: Replaced the code of the first version with a check.

  drivers/video/fbdev/core/fbmon.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c

@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
       vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
                vm->vsync_len;
       /* prevent division by zero */
-     if (htotal && vtotal) {
+     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {

why don't you then simply check for
        if .. ((htotal * vtotal) == 0) ...
instead?

Helge

Thomas Zimmermann from the previous discussion said:

On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:

Maybe use

   if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))

for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.

This prevents overflow, which is also a problematic case.

On 3/18/24 09:11, Roman Smirnov wrote:

On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:

quoted

On 3/5/24 14:51, Roman Smirnov wrote:

quoted

The expression htotal * vtotal can have a zero value on
overflow.

I'm not sure if those always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

quoted

It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
   V1 -> V2: Replaced the code of the first version with a check.

   drivers/video/fbdev/core/fbmon.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c

@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
        vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
                 vm->vsync_len;
        /* prevent division by zero */
-     if (htotal && vtotal) {
+     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {

why don't you then simply check for
         if .. ((htotal * vtotal) == 0) ...
instead?

Helge

Thomas Zimmermann from the previous discussion said:

On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:

quoted

Maybe use

    if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))

for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.

This prevents overflow, which is also a problematic case.

I don't like adding another division here and I doubt we have
a problem with possible overflow.
So, I suggest to keep it simple, something like:
	...
	total = htotal * vtotal;
	if (total)
		fbmode->refresh = vm->pixelclock / total;
	else...
Helge

On Mon, 18 Mar 2024 20:15:55 +0100 Helge Deller wrote:

On 3/18/24 09:11, Roman Smirnov wrote:

quoted

On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:

quoted

On 3/5/24 14:51, Roman Smirnov wrote:

quoted

The expression htotal * vtotal can have a zero value on
overflow.

I'm not sure if those always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

quoted

It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
   V1 -> V2: Replaced the code of the first version with a check.

   drivers/video/fbdev/core/fbmon.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c

@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
        vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
                 vm->vsync_len;
        /* prevent division by zero */
-     if (htotal && vtotal) {
+     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {

why don't you then simply check for
         if .. ((htotal * vtotal) == 0) ...
instead?

Helge

Thomas Zimmermann from the previous discussion said:
On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:

quoted

Maybe use

    if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))

for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.

This prevents overflow, which is also a problematic case.

I don't like adding another division here and I doubt we have
a problem with possible overflow.
So, I suggest to keep it simple, something like:
       ...
       total = htotal * vtotal;
       if (total)
               fbmode->refresh = vm->pixelclock / total;
       else...

Okay, I'll prepare a third version with that change:

    if (htotal && vtotal && (htotal * vtotal))

I think that will be enough.

On 3/19/24 11:12 AM, Roman Smirnov wrote:
[...]

quoted

quoted

quoted

On 3/5/24 14:51, Roman Smirnov wrote:

quoted

The expression htotal * vtotal can have a zero value on
overflow.

I'm not sure if those always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

quoted

It is necessary to prevent division by zero like in
fb_var_to_videomode().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
   V1 -> V2: Replaced the code of the first version with a check.

   drivers/video/fbdev/core/fbmon.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c

@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
        vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
                 vm->vsync_len;
        /* prevent division by zero */
-     if (htotal && vtotal) {
+     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {

why don't you then simply check for
         if .. ((htotal * vtotal) == 0) ...
instead?

Helge

Thomas Zimmermann from the previous discussion said:
On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:

quoted

Maybe use

    if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))

for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.

This prevents overflow, which is also a problematic case.

I don't like adding another division here and I doubt we have
a problem with possible overflow.
So, I suggest to keep it simple, something like:
       ...
       total = htotal * vtotal;
       if (total)
               fbmode->refresh = vm->pixelclock / total;
       else...

Okay, I'll prepare a third version with that change:

    if (htotal && vtotal && (htotal * vtotal))

   I think the 1st 2 checks here are now redundant. Also, the inner
parens are not necessary...

I think that will be enough.

   More than enough. :-)

MBR, Sergey