linux-kernel - Re: [PATCH] kref: Avoid null pointer dereference after WARN

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Date:   Tue, 27 Jun 2017 11:34:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Daniel Micay <danielmicay@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kref: Avoid null pointer dereference after WARN

On Tue, Jun 27, 2017 at 12:06 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Mon, Jun 26, 2017 at 08:52:15PM -0700, Kees Cook wrote:
>> From: Daniel Micay <danielmicay@...il.com>
>>
>> The WARN_ON() checking for a NULL release pointer should be a BUG()
>> since continuing with a NULL release pointer will lead to a NULL
>> pointer dereference anyway.
>>
>> The kref_put() case is extracted from PaX, and Kees Cook noted it should
>> be extended to the other two cases.
>>
>> Signed-off-by: Daniel Micay <danielmicay@...il.com>
>> [kees: clarify commit log]
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  include/linux/kref.h | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/kref.h b/include/linux/kref.h
>> index f4156f88f557..82a2c225eae3 100644
>> --- a/include/linux/kref.h
>> +++ b/include/linux/kref.h
>> @@ -66,7 +66,7 @@ static inline void kref_get(struct kref *kref)
>>   */
>>  static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref))
>>  {
>> -     WARN_ON(release == NULL);
>> +     BUG_ON(release == NULL);
>
> I remember one complaint was that WARN_ON was "huge" and this bloated
> the kernel code a lot.  But then that got fixed up.  Is BUG_ON going to
> cause the same complaint again?

It used to be that WARN was much larger than BUG, but Peter Z fixed[1]
that recently. BUG is very small (on x86 a single instruction). I'll
send a v2 with the .text comparison.

-Kees

[1] 9a93848fe787 ("x86/debug: Implement __WARN() using UD0")

-- 
Kees Cook
Pixel Security