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[76.23.219.111]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-47ae9cfa2ccsm45046821cf.67.2025.04.21.11.05.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 21 Apr 2025 11:05:12 -0700 (PDT) Date: Mon, 21 Apr 2025 14:05:10 -0400 From: Rafael Aquini To: Kees Cook Cc: Petr Mladek , Sergio Perez Gonzalez , Vlastimil Babka , David Rientjes , Bagas Sanjaya , Harry Yoo , Jonathan Corbet , Andrew Morton , Steven Rostedt , Andy Shevchenko , Rasmus Villemoes , Sergey Senozhatsky , Christoph Lameter , Pekka Enberg , Joonsoo Kim , Roman Gushchin , "Paul E. McKenney" , Thomas Huth , "Borislav Petkov (AMD)" , Ard Biesheuvel , Greg Kroah-Hartman , Tamir Duberstein , Alice Ryhl , Stephen Boyd , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v3] slab: Decouple slab_debug and no_hash_pointers Message-ID: References: <20250421165508.make.689-kees@kernel.org> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250421165508.make.689-kees@kernel.org> On Mon, Apr 21, 2025 at 09:55:12AM -0700, Kees Cook wrote: > Some system owners use slab_debug=FPZ (or similar) as a hardening option, > but do not want to be forced into having kernel addresses exposed due > to the implicit "no_hash_pointers" boot param setting.[1] > > Introduce the "hash_pointers" boot param, which defaults to "auto" > (the current behavior), but also includes "always" (forcing on hashing > even when "slab_debug=..." is defined), and "never". The existing > "no_hash_pointers" boot param becomes an alias for "hash_pointers=never". > > This makes it possible to boot with "slab_debug=FPZ hash_pointers=always". > > Link: https://github.com/KSPP/linux/issues/368 [1] > Fixes: 792702911f58 ("slub: force on no_hash_pointers when slub_debug is enabled") > Co-developed-by: Sergio Perez Gonzalez > Signed-off-by: Sergio Perez Gonzalez > Acked-by: Vlastimil Babka > Acked-by: David Rientjes > Reviewed-by: Bagas Sanjaya > Reviewed-by: Harry Yoo > Acked-by: Rafael Aquini > Tested-by: Petr Mladek > Reviewed-by: Petr Mladek > Signed-off-by: Kees Cook > --- > v3: add docs about implicit "no_hash_pointers" when using "slab_debug" (harry.yoo) > v2: https://lore.kernel.org/all/20250415170232.it.467-kees@kernel.org/ > v1: https://lore.kernel.org/lkml/20250410174428.work.488-kees@kernel.org/ > --- > .../admin-guide/kernel-parameters.txt | 38 ++++++++---- > include/linux/sprintf.h | 2 +- > lib/vsprintf.c | 61 +++++++++++++++++-- > mm/slub.c | 5 +- > 4 files changed, 86 insertions(+), 20 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index d9fd26b95b34..cb12531b2100 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -1795,6 +1795,27 @@ > backtraces on all cpus. > Format: 0 | 1 > > + hash_pointers= > + [KNL,EARLY] > + By default, when pointers are printed to the console > + or buffers via the %p format string, that pointer is > + "hashed", i.e. obscured by hashing the pointer value. > + This is a security feature that hides actual kernel > + addresses from unprivileged users, but it also makes > + debugging the kernel more difficult since unequal > + pointers can no longer be compared. The choices are: > + Format: { auto | always | never } > + Default: auto > + > + auto - Hash pointers unless slab_debug is enabled. > + always - Always hash pointers (even if slab_debug is > + enabled). > + never - Never hash pointers. This option should only > + be specified when debugging the kernel. Do > + not use on production kernels. The boot > + param "no_hash_pointers" is an alias for > + this mode. > + > hashdist= [KNL,NUMA] Large hashes allocated during boot > are distributed across NUMA nodes. Defaults on > for 64-bit NUMA, off otherwise. > @@ -4117,18 +4138,7 @@ > > no_hash_pointers > [KNL,EARLY] > - Force pointers printed to the console or buffers to be > - unhashed. By default, when a pointer is printed via %p > - format string, that pointer is "hashed", i.e. obscured > - by hashing the pointer value. This is a security feature > - that hides actual kernel addresses from unprivileged > - users, but it also makes debugging the kernel more > - difficult since unequal pointers can no longer be > - compared. However, if this command-line option is > - specified, then all normal pointers will have their true > - value printed. This option should only be specified when > - debugging the kernel. Please do not use on production > - kernels. > + Alias for "hash_pointers=never". > > nohibernate [HIBERNATION] Disable hibernation and resume. > > @@ -6478,6 +6488,10 @@ > Documentation/mm/slub.rst. > (slub_debug legacy name also accepted for now) > > + Using this option implies the "no_hash_pointers" > + option which can be undone by adding the > + "hash_pointers=always" option. > + > slab_max_order= [MM] > Determines the maximum allowed order for slabs. > A high setting may cause OOMs due to memory > diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h > index 51cab2def9ec..521bb2cd2648 100644 > --- a/include/linux/sprintf.h > +++ b/include/linux/sprintf.h > @@ -22,7 +22,7 @@ __scanf(2, 0) int vsscanf(const char *, const char *, va_list); > > /* These are for specific cases, do not use without real need */ > extern bool no_hash_pointers; > -int no_hash_pointers_enable(char *str); > +void hash_pointers_finalize(bool slub_debug); > > /* Used for Rust formatting ('%pA') */ > char *rust_fmt_argument(char *buf, char *end, const void *ptr); > diff --git a/lib/vsprintf.c b/lib/vsprintf.c > index 01699852f30c..22cbd75266ef 100644 > --- a/lib/vsprintf.c > +++ b/lib/vsprintf.c > @@ -60,6 +60,20 @@ > bool no_hash_pointers __ro_after_init; > EXPORT_SYMBOL_GPL(no_hash_pointers); > > +/* > + * Hashed pointers policy selected by "hash_pointers=..." boot param > + * > + * `auto` - Hashed pointers enabled unless disabled by slub_debug_enabled=true > + * `always` - Hashed pointers enabled unconditionally > + * `never` - Hashed pointers disabled unconditionally > + */ > +enum hash_pointers_policy { > + HASH_PTR_AUTO = 0, > + HASH_PTR_ALWAYS, > + HASH_PTR_NEVER > +}; > +static enum hash_pointers_policy hash_pointers_mode __initdata; > + > noinline > static unsigned long long simple_strntoull(const char *startp, char **endp, unsigned int base, size_t max_chars) > { > @@ -2271,12 +2285,23 @@ char *resource_or_range(const char *fmt, char *buf, char *end, void *ptr, > return resource_string(buf, end, ptr, spec, fmt); > } > > -int __init no_hash_pointers_enable(char *str) > +void __init hash_pointers_finalize(bool slub_debug) > { > - if (no_hash_pointers) > - return 0; > + switch (hash_pointers_mode) { > + case HASH_PTR_ALWAYS: > + no_hash_pointers = false; > + break; > + case HASH_PTR_NEVER: > + no_hash_pointers = true; > + break; > + case HASH_PTR_AUTO: > + default: > + no_hash_pointers = slub_debug; > + break; > + } > > - no_hash_pointers = true; > + if (!no_hash_pointers) > + return; > > pr_warn("**********************************************************\n"); > pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n"); > @@ -2289,11 +2314,39 @@ int __init no_hash_pointers_enable(char *str) > pr_warn("** the kernel, report this immediately to your system **\n"); > pr_warn("** administrator! **\n"); > pr_warn("** **\n"); > + pr_warn("** Use hash_pointers=always to force this mode off **\n"); > + pr_warn("** **\n"); > pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n"); > pr_warn("**********************************************************\n"); > +} > + > +static int __init hash_pointers_mode_parse(char *str) > +{ > + if (!str) { > + pr_warn("Hash pointers mode empty; falling back to auto.\n"); > + hash_pointers_mode = HASH_PTR_AUTO; > + } else if (strncmp(str, "auto", 4) == 0) { > + pr_info("Hash pointers mode set to auto.\n"); > + hash_pointers_mode = HASH_PTR_AUTO; > + } else if (strncmp(str, "never", 5) == 0) { > + pr_info("Hash pointers mode set to never.\n"); > + hash_pointers_mode = HASH_PTR_NEVER; > + } else if (strncmp(str, "always", 6) == 0) { > + pr_info("Hash pointers mode set to always.\n"); > + hash_pointers_mode = HASH_PTR_ALWAYS; > + } else { > + pr_warn("Unknown hash_pointers mode '%s' specified; assuming auto.\n", str); > + hash_pointers_mode = HASH_PTR_AUTO; > + } > > return 0; > } > +early_param("hash_pointers", hash_pointers_mode_parse); > + > +static int __init no_hash_pointers_enable(char *str) > +{ > + return hash_pointers_mode_parse("never"); > +} > early_param("no_hash_pointers", no_hash_pointers_enable); > > /* > diff --git a/mm/slub.c b/mm/slub.c > index b46f87662e71..f3d61b330a76 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -6314,9 +6314,8 @@ void __init kmem_cache_init(void) > if (debug_guardpage_minorder()) > slub_max_order = 0; > > - /* Print slub debugging pointers without hashing */ > - if (__slub_debug_enabled()) > - no_hash_pointers_enable(NULL); > + /* Inform pointer hashing choice about slub debugging state. */ > + hash_pointers_finalize(__slub_debug_enabled()); > > kmem_cache_node = &boot_kmem_cache_node; > kmem_cache = &boot_kmem_cache; > -- > 2.34.1 > > Acked-by: Rafael Aquini