Re: [RFC PATCH 3/5] mm, slub: remove runtime allocation order changes

[Kees Cook]

On Tue, Jun 02, 2020 at 04:15:17PM +0200, Vlastimil Babka wrote:
> SLUB allows runtime changing of page allocation order by writing into the
> /sys/kernel/slab/<cache>/order file. Jann has reported [1] that this interface
> allows the order to be set too small, leading to crashes.
> 
> While it's possible to fix the immediate issue, closer inspection reveals
> potential races. Storing the new order calls calculate_sizes() which
> non-atomically updates a lot of kmem_cache fields while the cache is still in
> use. Unexpected behavior might occur even if the fields are set to the same
> value as they were.
> 
> This could be fixed by splitting out the part of calculate_sizes() that depends
> on forced_order, so that we only update kmem_cache.oo field. This could still
> race with init_cache_random_seq(), shuffle_freelist(), allocate_slab(). Perhaps
> it's possible to audit and e.g. add some READ_ONCE/WRITE_ONCE accesses, it
> might be easier just to remove the runtime order changes, which is what this
> patch does. If there are valid usecases for per-cache order setting, we could
> e.g. extend the boot parameters to do that.
> 
> [1] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@xxxxxxxxxxxxxx
> 
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook