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

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