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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id b142sm7613439pfb.17.2021.09.20.07.29.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Sep 2021 07:29:55 -0700 (PDT) Date: Mon, 20 Sep 2021 07:29:54 -0700 From: Kees Cook To: Stephen Boyd Cc: Andrew Morton , linux-kernel@vger.kernel.org, Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , linux-mm@kvack.org, Petr Mladek , Joe Perches Subject: Re: [PATCH v3 4/4] slub: Force on no_hash_pointers when slub_debug is enabled Message-ID: <202109200726.2EFEDC5@keescook> References: <20210601182202.3011020-1-swboyd@chromium.org> <20210601182202.3011020-5-swboyd@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210601182202.3011020-5-swboyd@chromium.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 01, 2021 at 11:22:02AM -0700, Stephen Boyd wrote: > 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. Let's force on the no_hash_pointers commandline flag when > slub_debug is on the commandline. This makes slub debug messages more > meaningful and if by chance a kernel address is in some slub debug > object dump we will have a better chance of figuring out what went > wrong. > > Note that we don't use %px in the slub code because we want to reduce > the number of places that %px is used in the kernel. This also nicely > prints a big fat warning at kernel boot if slub_debug is on the > commandline so that we know that this kernel shouldn't be used on > production systems. Eeeek. I missed this patch. NAK NAK. People use slub_debug for production systems to gain redzoning, etc, as a layer of defense, and they absolutely do not want %p-hashing disabled. %p hashing is controlled by the no_hash_pointers boot param (since v5.12), and needs to stay separate from slub_debug. Can we please revert this in Linus's tree and in v5.14? -Kees > > Signed-off-by: Stephen Boyd > --- > include/linux/kernel.h | 2 ++ > lib/vsprintf.c | 2 +- > mm/slub.c | 4 ++++ > 3 files changed, 7 insertions(+), 1 deletion(-) > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index 15d8bad3d2f2..bf950621febf 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -357,6 +357,8 @@ int sscanf(const char *, const char *, ...); > extern __scanf(2, 0) > int vsscanf(const char *, const char *, va_list); > > +extern int no_hash_pointers_enable(char *str); > + > extern int get_option(char **str, int *pint); > extern char *get_options(const char *str, int nints, int *ints); > extern unsigned long long memparse(const char *ptr, char **retptr); > diff --git a/lib/vsprintf.c b/lib/vsprintf.c > index f0c35d9b65bf..cc281f5895f9 100644 > --- a/lib/vsprintf.c > +++ b/lib/vsprintf.c > @@ -2186,7 +2186,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode, > bool no_hash_pointers __ro_after_init; > EXPORT_SYMBOL_GPL(no_hash_pointers); > > -static int __init no_hash_pointers_enable(char *str) > +int __init no_hash_pointers_enable(char *str) > { > if (no_hash_pointers) > return 0; > diff --git a/mm/slub.c b/mm/slub.c > index bf4949115412..a722794f1dbd 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -4460,6 +4460,10 @@ void __init kmem_cache_init(void) > if (debug_guardpage_minorder()) > slub_max_order = 0; > > + /* Print slub debugging pointers without hashing */ > + if (static_branch_unlikely(&slub_debug_enabled)) > + no_hash_pointers_enable(NULL); > + > kmem_cache_node = &boot_kmem_cache_node; > kmem_cache = &boot_kmem_cache; > > -- > https://chromeos.dev > -- Kees Cook