linux-kernel - Re: [PATCH] mm: slub: reinitialize random sequence cache on slab object update

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Date:   Thu, 5 Mar 2020 13:40:17 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     vjitta@...eaurora.org
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        vinmenon@...eaurora.org, kernel-team@...roid.com,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] mm: slub: reinitialize random sequence cache on slab
 object update

On 3/5/20 6:48 AM, vjitta@...eaurora.org wrote:
> On 2020-02-27 22:23, Vlastimil Babka wrote:
>> 
>> This is even more nasty as it doesn't seem to require that no objects 
>> exist.
>> Also there is no synchronization against concurrent allocations/frees? 
>> Gasp.
> 
> Since, random sequence cache is only used to update the freelist in 
> shuffle_freelist
> which is done only when a new slab is created incase if objects 
> allocations are
> done without a need of new slab creation they will use the existing 
> freelist which
> should be fine as object size doesn't change after order_store() and 
> incase if a new
> slab is created we will get the updated freelist. so in both cases i 
> think it should
> be fine.

I have some doubts. With reinit_cache_random_seq() for SLUB, s->random_seq will
in turn:
cache_random_seq_destroy()
- point to an object that's been kfree'd
- point to NULL
init_cache_random_seq()
  cache_random_seq_create()
  - point to freshly allocated zeroed out object
    freelist_randomize()
    - the object is gradually initialized
- the indices are gradually transformed to page offsets

At any point of this, new slab can be allocated in parallel and observe
s->random_seq in shuffle_freelist(), and it's only ok if it's currently NULL.

Could it be fixed? In the reinit part you would need to
- atomically update a valid s->random_seq to another valid s->random_seq
(perhaps with NULL in between which means some freelist won't be perhaps randomized)
- write barrier
- call calculate_sizes() with updated flags / new order, make sure all the
fields of s-> are updated in a safe order and with write barries (i.e. update
s->oo and s->flags would be probably last, but maybe that's not all) so that
anyone allocating a new slab will always get something valid (maybe that path
would need also new read barriers?)

No, I don't think it's worth the trouble?