From: Greg KH <gregkh@linuxfoundation.org>
To: Adrian Salido <salidoa@google.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] driver core: platform: fix race condition with driver_override
Date: Wed, 26 Apr 2017 17:48:12 +0200 [thread overview]
Message-ID: <20170426154812.GA18399@kroah.com> (raw)
In-Reply-To: <CAEP91RET3sKH_77W7=D0Dcj-itN-2eP=qv3+jNndeSvrVk6xWw@mail.gmail.com>
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
prev parent reply other threads:[~2017-04-26 15:48 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-25 23:55 [PATCH] driver core: platform: fix race condition with driver_override Adrian Salido
2017-04-26 11:57 ` Greg KH
2017-04-26 14:51 ` Adrian Salido
2017-04-26 15:48 ` Greg KH [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170426154812.GA18399@kroah.com \
--to=gregkh@linuxfoundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=salidoa@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).