Re: [PATCH] Revert "slub: force on no_hash_pointers when slub_debug is enabled"

[Kees Cook]

On Wed, Feb 08, 2023 at 11:58:54PM +0000, Hyeonggon Yoo wrote:
> On Wed, Feb 08, 2023 at 11:47:17AM -0800, Kees Cook wrote:
> > This reverts commit 792702911f581f7793962fbeb99d5c3a1b28f4c3.
> > 
> > Linking no_hash_pointers() to slub_debug has had a chilling effect
> > on using slub_debug features for security hardening, since system
> > builders are forced to choose between redzoning and heap address location
> > exposures. Instead, just require that the "no_hash_pointers" boot param
> > needs to be used to expose pointers during slub_debug reports.
> > 
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> > Cc: concord@xxxxxxxxxx
> > Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: stable@xxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/lkml/202109200726.2EFEDC5@keescook/
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> in the commit message:
> 
> > Obscuring the pointers that slub shows when debugging makes for some
> > confusing slub debug messages:
> >
> > Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17
> >
> > Those addresses are hashed for kernel security reasons. If we're trying
> > to be secure with slub_debug on the commandline we have some big
> > problems given that we dump whole chunks of kernel memory to the kernel
> > logs.
> 
> it dumps parts of kernel memory anyway and I'm not sure if slub_debug is
> supposed to be used for security hardening.
> 
> what about introducing new boot parameter like, slub_hardening,
> which does not print anything?

But it would be parsed for the same options? Redzoning, for example, is
the common thing used for folks interested in detecting memory corruption
attacks, etc.

-- 
Kees Cook