[PATCH] driver core: platform: fix race condition with driver_override

[PATCH] driver core: platform: fix race condition with driver_override

[Adrian Salido]

The driver_override implementation is susceptible to race condition when
different threads are reading vs storing a different driver override.
Add locking to avoid race condition.

Fixes: 3d713e0e382e ("driver core: platform: add device binding path 'driver_override'")
Cc: stable@vger.kernel.org
Signed-off-by: Adrian Salido <salidoa@google.com>
---
 drivers/base/platform.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index c2456839214a..493e03fa0e07 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -866,7 +866,7 @@ static ssize_t driver_override_store(struct device *dev,
 				     const char *buf, size_t count)
 {
 	struct platform_device *pdev = to_platform_device(dev);
-	char *driver_override, *old = pdev->driver_override, *cp;
+	char *driver_override, *old, *cp;
 
 	if (count > PATH_MAX)
 		return -EINVAL;
@@ -879,12 +879,15 @@ static ssize_t driver_override_store(struct device *dev,
 	if (cp)
 		*cp = '\0';
 
+	device_lock(dev);
+	old = pdev->driver_override;
 	if (strlen(driver_override)) {
 		pdev->driver_override = driver_override;
 	} else {
 		kfree(driver_override);
 		pdev->driver_override = NULL;
 	}
+	device_unlock(dev);
 
 	kfree(old);
 
@@ -895,8 +898,12 @@ static ssize_t driver_override_show(struct device *dev,
 				    struct device_attribute *attr, char *buf)
 {
 	struct platform_device *pdev = to_platform_device(dev);
+	ssize_t len;
 
-	return sprintf(buf, "%s\n", pdev->driver_override);
+	device_lock(dev);
+	len = sprintf(buf, "%s\n", pdev->driver_override);
+	device_unlock(dev);
+	return len;
 }
 static DEVICE_ATTR_RW(driver_override);
 
-- 
2.13.0.rc0.306.g87b477812d-goog

Re: [PATCH] driver core: platform: fix race condition with driver_override

[Greg KH]

On Tue, Apr 25, 2017 at 04:55:26PM -0700, Adrian Salido wrote:
> The driver_override implementation is susceptible to race condition when
> different threads are reading vs storing a different driver override.
> Add locking to avoid race condition.
> 
> Fixes: 3d713e0e382e ("driver core: platform: add device binding path 'driver_override'")
> Cc: stable@vger.kernel.org
> Signed-off-by: Adrian Salido <salidoa@google.com>
> ---
>  drivers/base/platform.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index c2456839214a..493e03fa0e07 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -866,7 +866,7 @@ static ssize_t driver_override_store(struct device *dev,
>  				     const char *buf, size_t count)
>  {
>  	struct platform_device *pdev = to_platform_device(dev);
> -	char *driver_override, *old = pdev->driver_override, *cp;
> +	char *driver_override, *old, *cp;
>  
>  	if (count > PATH_MAX)
>  		return -EINVAL;
> @@ -879,12 +879,15 @@ static ssize_t driver_override_store(struct device *dev,
>  	if (cp)
>  		*cp = '\0';
>  
> +	device_lock(dev);
> +	old = pdev->driver_override;
>  	if (strlen(driver_override)) {
>  		pdev->driver_override = driver_override;
>  	} else {
>  		kfree(driver_override);
>  		pdev->driver_override = NULL;
>  	}
> +	device_unlock(dev);
>  
>  	kfree(old);

Shouldn't you move the lock until after the kfree()?  Or am I missing
what the lock is trying to protect here?

>  
> @@ -895,8 +898,12 @@ static ssize_t driver_override_show(struct device *dev,
>  				    struct device_attribute *attr, char *buf)
>  {
>  	struct platform_device *pdev = to_platform_device(dev);
> +	ssize_t len;
>  
> -	return sprintf(buf, "%s\n", pdev->driver_override);
> +	device_lock(dev);
> +	len = sprintf(buf, "%s\n", pdev->driver_override);
> +	device_unlock(dev);
> +	return len;

Why does the show function need to be changed at all?  How can anything
"race" here?

thanks,

greg k-h

Re: [PATCH] driver core: platform: fix race condition with driver_override

[Adrian Salido]

> > The driver_override implementation is susceptible to race condition when
> > different threads are reading vs storing a different driver override.
> > Add locking to avoid race condition.
> >
> > Fixes: 3d713e0e382e ("driver core: platform: add device binding path 'driver_override'")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Adrian Salido <salidoa@google.com>
> > ---
> >  drivers/base/platform.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > index c2456839214a..493e03fa0e07 100644
> > --- a/drivers/base/platform.c
> > +++ b/drivers/base/platform.c
> > @@ -866,7 +866,7 @@ static ssize_t driver_override_store(struct device *dev,
> >                                    const char *buf, size_t count)
> >  {
> >       struct platform_device *pdev = to_platform_device(dev);
> > -     char *driver_override, *old = pdev->driver_override, *cp;
> > +     char *driver_override, *old, *cp;
> >
> >       if (count > PATH_MAX)
> >               return -EINVAL;
> > @@ -879,12 +879,15 @@ static ssize_t driver_override_store(struct device *dev,
> >       if (cp)
> >               *cp = '\0';
> >
> > +     device_lock(dev);
> > +     old = pdev->driver_override;
> >       if (strlen(driver_override)) {
> >               pdev->driver_override = driver_override;
> >       } else {
> >               kfree(driver_override);
> >               pdev->driver_override = NULL;
> >       }
> > +     device_unlock(dev);
> >
> >       kfree(old);
>
> Shouldn't you move the lock until after the kfree()?  Or am I missing
> what the lock is trying to protect here?

not really, the lock only protecting the variable
pdev->driver_override. Once the value has changed we no longer care
about "old" variable

> >       if (cp)
> >               *cp = '\0';
> >
> > +     device_lock(dev);
> > +     old = pdev->driver_override;
> >       if (strlen(driver_override)) {
> >               pdev->driver_override = driver_override;
> >       } else {
> >               kfree(driver_override);
> >               pdev->driver_override = NULL;
> >       }
> > +     device_unlock(dev);
> >
> >       kfree(old);
>
> >
> > @@ -895,8 +898,12 @@ static ssize_t driver_override_show(struct device *dev,
> >                                   struct device_attribute *attr, char *buf)
> >  {
> >       struct platform_device *pdev = to_platform_device(dev);
> > +     ssize_t len;
> >
> > -     return sprintf(buf, "%s\n", pdev->driver_override);
> > +     device_lock(dev);
> > +     len = sprintf(buf, "%s\n", pdev->driver_override);
> > +     device_unlock(dev);
> > +     return len;
>
> Why does the show function need to be changed at all?  How can anything
> "race" here?

The lock is trying to protect again race between store and show.
Suppose there are 2 threads:

Thread1:
 while (1) {
   driver_override_store("foo");
   driver_override_store("");
 }

Thread2:
while (1) driver_override_show();

Thread 1                                 |         Thread 2
----------------------------------------|-----------------------
old = pdev->driver_override;   |
                                                | len = sprintf(buf,
"%s\n", pdev->driver_override);
                                                | /* snprintf starts reading */
pdev->driver_override =          |
   driver_override;                    |
kfree(old);                                | /* use after free before
snprintf finishes execution */

Similarly there could be a race between multiple threads doing store
where memory leaks could happen

Thanks,
Adrian

Re: [PATCH] driver core: platform: fix race condition with driver_override

[Greg KH]

On Wed, Apr 26, 2017 at 07:51:32AM -0700, Adrian Salido wrote:
> > > The driver_override implementation is susceptible to race condition when
> > > different threads are reading vs storing a different driver override.
> > > Add locking to avoid race condition.
> > >
> > > Fixes: 3d713e0e382e ("driver core: platform: add device binding path 'driver_override'")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Adrian Salido <salidoa@google.com>
> > > ---
> > >  drivers/base/platform.c | 11 +++++++++--
> > >  1 file changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > > index c2456839214a..493e03fa0e07 100644
> > > --- a/drivers/base/platform.c
> > > +++ b/drivers/base/platform.c
> > > @@ -866,7 +866,7 @@ static ssize_t driver_override_store(struct device *dev,
> > >                                    const char *buf, size_t count)
> > >  {
> > >       struct platform_device *pdev = to_platform_device(dev);
> > > -     char *driver_override, *old = pdev->driver_override, *cp;
> > > +     char *driver_override, *old, *cp;
> > >
> > >       if (count > PATH_MAX)
> > >               return -EINVAL;
> > > @@ -879,12 +879,15 @@ static ssize_t driver_override_store(struct device *dev,
> > >       if (cp)
> > >               *cp = '\0';
> > >
> > > +     device_lock(dev);
> > > +     old = pdev->driver_override;
> > >       if (strlen(driver_override)) {
> > >               pdev->driver_override = driver_override;
> > >       } else {
> > >               kfree(driver_override);
> > >               pdev->driver_override = NULL;
> > >       }
> > > +     device_unlock(dev);
> > >
> > >       kfree(old);
> >
> > Shouldn't you move the lock until after the kfree()?  Or am I missing
> > what the lock is trying to protect here?
> 
> not really, the lock only protecting the variable
> pdev->driver_override. Once the value has changed we no longer care
> about "old" variable

What are you protecting it from?  Being overwritten twice?  Or something
else?

> > >       if (cp)
> > >               *cp = '\0';
> > >
> > > +     device_lock(dev);
> > > +     old = pdev->driver_override;
> > >       if (strlen(driver_override)) {
> > >               pdev->driver_override = driver_override;
> > >       } else {
> > >               kfree(driver_override);
> > >               pdev->driver_override = NULL;
> > >       }
> > > +     device_unlock(dev);
> > >
> > >       kfree(old);
> >
> > >
> > > @@ -895,8 +898,12 @@ static ssize_t driver_override_show(struct device *dev,
> > >                                   struct device_attribute *attr, char *buf)
> > >  {
> > >       struct platform_device *pdev = to_platform_device(dev);
> > > +     ssize_t len;
> > >
> > > -     return sprintf(buf, "%s\n", pdev->driver_override);
> > > +     device_lock(dev);
> > > +     len = sprintf(buf, "%s\n", pdev->driver_override);
> > > +     device_unlock(dev);
> > > +     return len;
> >
> > Why does the show function need to be changed at all?  How can anything
> > "race" here?
> 
> The lock is trying to protect again race between store and show.
> Suppose there are 2 threads:
> 
> Thread1:
>  while (1) {
>    driver_override_store("foo");
>    driver_override_store("");
>  }
> 
> Thread2:
> while (1) driver_override_show();
> 
> Thread 1                                 |         Thread 2
> ----------------------------------------|-----------------------
> old = pdev->driver_override;   |
>                                                 | len = sprintf(buf,
> "%s\n", pdev->driver_override);
>                                                 | /* snprintf starts reading */
> pdev->driver_override =          |
>    driver_override;                    |
> kfree(old);                                | /* use after free before
> snprintf finishes execution */
> 
> Similarly there could be a race between multiple threads doing store
> where memory leaks could happen

Ah, the printing of the string, that makes more sense, thanks, I was
thinking of the assignment of the pointer itself, which is atomic on all
sane platforms.

As writing this is only allowed by root, it's not really a big deal,
I'll queue it up for the next release.

thanks,

greg k-h