[PATCH] drm/ast: fix using freed memory

[PATCH] drm/ast: fix using freed memory

[trix]

From: Tom Rix <trix@redhat.com>

clang static analysis reports this problem
ast_mode.c:1235:3: warning: Use of memory after it is freed
  drm_connector_update_edid_property(&ast_connector->base, edid);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The second condition on

  if (!flags && ast_connector->i2c)

Means that the edid is not always set.  If the previous block
fails the freed edid value will be used.  So set edid to NULL
after freeing.

Fixes: 55dc449a7c60 ("drm/ast: Handle failed I2C initialization gracefully")
Signed-off-by: Tom Rix <trix@redhat.com>
---
 drivers/gpu/drm/ast/ast_mode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
index ab52efb15670e..9131dc8a1a2fc 100644
--- a/drivers/gpu/drm/ast/ast_mode.c
+++ b/drivers/gpu/drm/ast/ast_mode.c
@@ -1224,10 +1224,12 @@ static int ast_get_modes(struct drm_connector *connector)
 			return -ENOMEM;
 
 		flags = ast_dp501_read_edid(connector->dev, (u8 *)edid);
-		if (flags)
+		if (flags) {
 			ast->dp501_maxclk = ast_get_dp501_max_clk(connector->dev);
-		else
+		} else {
 			kfree(edid);
+			edid = NULL;
+		}
 	}
 	if (!flags && ast_connector->i2c)
 		edid = drm_get_edid(connector, &ast_connector->i2c->adapter);
-- 
2.26.3

Re: [PATCH] drm/ast: fix using freed memory

[Nick Desaulniers]

On Thu, Feb 3, 2022 at 7:23 AM <trix@redhat.com> wrote:
>
> From: Tom Rix <trix@redhat.com>
>
> clang static analysis reports this problem
> ast_mode.c:1235:3: warning: Use of memory after it is freed
>   drm_connector_update_edid_property(&ast_connector->base, edid);
>   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The second condition on
>
>   if (!flags && ast_connector->i2c)
>
> Means that the edid is not always set.  If the previous block
> fails the freed edid value will be used.  So set edid to NULL
> after freeing.
>
> Fixes: 55dc449a7c60 ("drm/ast: Handle failed I2C initialization gracefully")
> Signed-off-by: Tom Rix <trix@redhat.com>
> ---
>  drivers/gpu/drm/ast/ast_mode.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> index ab52efb15670e..9131dc8a1a2fc 100644
> --- a/drivers/gpu/drm/ast/ast_mode.c
> +++ b/drivers/gpu/drm/ast/ast_mode.c
> @@ -1224,10 +1224,12 @@ static int ast_get_modes(struct drm_connector *connector)
>                         return -ENOMEM;
>
>                 flags = ast_dp501_read_edid(connector->dev, (u8 *)edid);
> -               if (flags)
> +               if (flags) {
>                         ast->dp501_maxclk = ast_get_dp501_max_clk(connector->dev);
> -               else
> +               } else {
>                         kfree(edid);
> +                       edid = NULL;
> +               }

You probably didn't need to add the {} for the if, but it doesn't really matter.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>         }
>         if (!flags && ast_connector->i2c)
>                 edid = drm_get_edid(connector, &ast_connector->i2c->adapter);
> --
> 2.26.3
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] drm/ast: fix using freed memory

[Thomas Zimmermann]

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Hi

Am 03.02.22 um 16:23 schrieb trix@redhat.com:
> From: Tom Rix <trix@redhat.com>
> 
> clang static analysis reports this problem
> ast_mode.c:1235:3: warning: Use of memory after it is freed
>    drm_connector_update_edid_property(&ast_connector->base, edid);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> The second condition on
> 
>    if (!flags && ast_connector->i2c)
> 
> Means that the edid is not always set.  If the previous block
> fails the freed edid value will be used.  So set edid to NULL
> after freeing.

Thanks for your patch. We have meanwhile merges a change that replaces 
the code entirely.

Best regards
Thomas

> 
> Fixes: 55dc449a7c60 ("drm/ast: Handle failed I2C initialization gracefully")
> Signed-off-by: Tom Rix <trix@redhat.com>
> ---
>   drivers/gpu/drm/ast/ast_mode.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> index ab52efb15670e..9131dc8a1a2fc 100644
> --- a/drivers/gpu/drm/ast/ast_mode.c
> +++ b/drivers/gpu/drm/ast/ast_mode.c
> @@ -1224,10 +1224,12 @@ static int ast_get_modes(struct drm_connector *connector)
>   			return -ENOMEM;
>   
>   		flags = ast_dp501_read_edid(connector->dev, (u8 *)edid);
> -		if (flags)
> +		if (flags) {
>   			ast->dp501_maxclk = ast_get_dp501_max_clk(connector->dev);
> -		else
> +		} else {
>   			kfree(edid);
> +			edid = NULL;
> +		}
>   	}
>   	if (!flags && ast_connector->i2c)
>   		edid = drm_get_edid(connector, &ast_connector->i2c->adapter);

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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