linux-kernel - Re: [PATCH v2] mm/slub: Fix incorrect interpretation of s->offset

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Date:   Mon, 27 Apr 2020 13:42:12 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Changbin Du <changbin.du@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] mm/slub: Fix incorrect interpretation of s->offset

On 4/27/20 12:10 PM, Christopher Lameter wrote:
> On Mon, 27 Apr 2020, Waiman Long wrote:
>
>> To fix it, use the check "s->offset == s->inuse" in the new helper
>> function freeptr_after_object() instead. Also add another helper function
>> get_info_end() to return the end of info block (inuse + free pointer
>> if not overlapping with object).
>>
>> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   mm/slub.c | 37 ++++++++++++++++++++++---------------
>>   1 file changed, 22 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0e736d66bb42..68f1b4b1c309 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -551,15 +551,29 @@ static void print_section(char *level, char *text, u8 *addr,
>>   	metadata_access_disable();
>>   }
>>
>> +static inline bool freeptr_after_object(struct kmem_cache *s)
> bool freeptr_outside_of_object()?
>
I can change to that name. It doesn't really matter to me.
>> +{
>> +	return s->offset == s->inuse;
> s->offset >= s->inuse?
>
> There may be a redzone after the object.
>
Technically inuse is object + red zone. According to calculate_sizes():

         s->inuse = size;

         if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
                 s->ctor)) {
                 /*
                  * Relocate free pointer after the object if it is not
                  * permitted to overwrite the first word of the object on
                  * kmem_cache_free.
                  *
                  * This is the case if we do RCU, have a constructor or
                  * destructor or are poisoning the objects.
                  */
                 s->offset = size;
                 size += sizeof(void *);

So (s->offset == s->inuse) when the free pointer is outside of the object.

>> +static inline unsigned int get_info_end(struct kmem_cache *s)
> static inline track_offset()?
>
The main reason why I don't use that is because there is a track data 
structure in slub. There are functions name get_track() and set_track(). 
I don't want to confuse with them.

Cheers,
Longman