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Message-ID: <CAGXu5j+3pD7Ss_PBY9H_A6B5-Ers2wYqFJ1y4iryKzqc=jCxXg@mail.gmail.com> Date: Tue, 15 Nov 2016 09:37:37 -0800 From: Kees Cook <keescook@...omium.org> To: Michael Ellerman <mpe@...erman.id.au> Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR On Tue, Nov 15, 2016 at 2:57 AM, Michael Ellerman <mpe@...erman.id.au> wrote: > POISON_POINTER_DELTA is defined in poison.h, and is intended to be used > to shift poison values so that they don't alias userspace. > > We should add it to ZERO_SIZE_PTR so that attackers can't use > ZERO_SIZE_PTR as a way to get a pointer to userspace. Ah, when dealing with a 0-sized malloc or similar? Do you have pointers to exploits that rely on this? Regardless, normally PAN/SMAP-like things should be sufficient to protect against this. Additionally, on everything but x86_64 and arm64, POISON_POINTER_DELTA == 0, if I'm reading correctly: #ifdef CONFIG_ILLEGAL_POINTER_VALUE # define POISON_POINTER_DELTA _AC(CONFIG_ILLEGAL_POINTER_VALUE, UL) #else # define POISON_POINTER_DELTA 0 #endif ... config ILLEGAL_POINTER_VALUE hex default 0 if X86_32 default 0xdead000000000000 if X86_64 ... config ILLEGAL_POINTER_VALUE hex default 0xdead000000000000 Is the plan to add ILLEGAL_POINTER_VALUE for powerpc too? And either way, this patch, IIUC, will break the ZERO_OR_NULL_PTR() check, since suddenly all of userspace will match it. (Though maybe that's okay?) -Kees > > Signed-off-by: Michael Ellerman <mpe@...erman.id.au> > --- > include/linux/slab.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 084b12bad198..17ddd7aea2dd 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -12,6 +12,7 @@ > #define _LINUX_SLAB_H > > #include <linux/gfp.h> > +#include <linux/poison.h> > #include <linux/types.h> > #include <linux/workqueue.h> > > @@ -109,7 +110,7 @@ > * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can. > * Both make kfree a no-op. > */ > -#define ZERO_SIZE_PTR ((void *)16) > +#define ZERO_SIZE_PTR ((void *)(16 + POISON_POINTER_DELTA)) > > #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ > (unsigned long)ZERO_SIZE_PTR) > -- > 2.7.4 > -- Kees Cook Nexus Security
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