linux-kernel - Re: [PATCH 1/6] livepatch v5: avoid race when checking for state of the patch

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Message-ID: <20141210152506.GA32670@treble.redhat.com>
Date:	Wed, 10 Dec 2014 09:25:06 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Miroslav Benes <mbenes@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christoph Hellwig <hch@...radead.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	live-patching@...r.kernel.org, x86@...nel.org, kpatch@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] livepatch v5: avoid race when checking for state of
 the patch

On Wed, Dec 10, 2014 at 11:11:47AM +0100, Petr Mladek wrote:
> On Tue 2014-12-09 12:32:49, Josh Poimboeuf wrote:
> > On Tue, Dec 09, 2014 at 07:05:02PM +0100, Petr Mladek wrote:
> > > klp_patch_enable() and klp_patch_disable() should check the current state
> > > of the patch under the klp_lock. Otherwise, it might detect that the operation
> > > is valid but the situation might change before it takes the lock.
> > 
> > Hi Petr,
> > 
> > Thanks for the patches.
> > 
> > I don't think this patch is necessary.  klp_is_enabled() doesn't check
> > the state of the patch.  It checks the initialization state of the core
> > module (klp_root_kobj), which can only be set in klp_init().  It's not
> > protected by the lock, so I don't see the point of this patch.
> 
> Ah, I have misread the name and expected that it checked whether
> the patch was enabled or disabled. The original code is OK then.
> 
> Well, Jiri Kosina pointed out that the check did not make much sense.
> klp_is_enabled() could not be called if the livepatch module is not
> loaded. And the later check for klp_patch_is_registered() is enough
> to check whether the klp_enable_patch()/klp_disable_patch() calls
> are allowed or not.

But livepatch isn't a module, it's part of the kernel.  Even if the init
function returns an error, that doesn't prevent any of the other
exported functions from getting called.


> 
> So, I suggest to remove the checks at all.
> 
> Best Regards,
> Petr
> 
> 
> > > 
> > > Signed-off-by: Petr Mladek <pmladek@...e.cz>
> > > ---
> > >  kernel/livepatch/core.c | 16 ++++++++++------
> > >  1 file changed, 10 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > > index d6d0f50e81f8..b848069e44cc 100644
> > > --- a/kernel/livepatch/core.c
> > > +++ b/kernel/livepatch/core.c
> > > @@ -425,11 +425,13 @@ int klp_disable_patch(struct klp_patch *patch)
> > >  {
> > >  	int ret;
> > >  
> > > -	if (!klp_is_enabled())
> > > -		return -ENODEV;
> > > -
> > >  	mutex_lock(&klp_mutex);
> > >  
> > > +	if (!klp_is_enabled()) {
> > > +		ret = -ENODEV;
> > > +		goto err;
> > > +	}
> > > +
> > >  	if (!klp_patch_is_registered(patch)) {
> > >  		ret = -EINVAL;
> > >  		goto err;
> > > @@ -489,11 +491,13 @@ int klp_enable_patch(struct klp_patch *patch)
> > >  {
> > >  	int ret;
> > >  
> > > -	if (!klp_is_enabled())
> > > -		return -ENODEV;
> > > -
> > >  	mutex_lock(&klp_mutex);
> > >  
> > > +	if (!klp_is_enabled()) {
> > > +		ret = -ENODEV;
> > > +		goto err;
> > > +	}
> > > +
> > >  	if (!klp_patch_is_registered(patch)) {
> > >  		ret = -EINVAL;
> > >  		goto err;
> > > -- 
> > > 1.8.5.2
> > > 
> > 
> > -- 
> > Josh

-- 
Josh
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