Re: [RFC PATCH 2/5] mm, slub: make some slub_debug related attributes read-only

[Kees Cook]

On Tue, Jun 02, 2020 at 04:15:16PM +0200, Vlastimil Babka wrote:
> SLUB_DEBUG creates several files under /sys/kernel/slab/<cache>/ that can be
> read to check if the respective debugging options are enabled for given cache.
> The options can be also toggled at runtime by writing into the files. Some of
> those, namely red_zone, poison, and store_user can be toggled only when no
> objects yet exist in the cache.
> 
> Vijayanand reports [1] that there is a problem with freelist randomization if
> changing the debugging option's state results in different number of objects
> per page, and the random sequence cache needs thus needs to be recomputed.
> 
> However, another problem is that the check for "no objects yet exist in the
> cache" is racy, as noted by Jann [2] and fixing that would add overhead or
> otherwise complicate the allocation/freeing paths. Thus it would be much
> simpler just to remove the runtime toggling support. The documentation
> describes it's "In case you forgot to enable debugging on the kernel command
> line", but the neccessity of having no objects limits its usefulness anyway for
> many caches.
> 
> Vijayanand describes an use case [3] where debugging is enabled for all but
> zram caches for memory overhead reasons, and using the runtime toggles was the
> only way to achieve such configuration. After the previous patch it's now
> possible to do that directly from the kernel boot option, so we can remove the
> dangerous runtime toggles by making the /sys attribute files read-only.
> 
> While updating it, also improve the documentation of the debugging /sys files.
> 
> [1] https://lkml.kernel.org/r/1580379523-32272-1-git-send-email-vjitta@xxxxxxxxxxxxxx
> [2] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@xxxxxxxxxxxxxx
> [3] https://lore.kernel.org/r/1383cd32-1ddc-4dac-b5f8-9c42282fa81c@xxxxxxxxxxxxxx
> 
> Reported-by: Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx>
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook