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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: [PATCH v3] slab: Decouple slab_debug and no_hash_pointers Date: Mon, 21 Apr 2025 09:55:12 -0700 Message-Id: <20250421165508.make.689-kees@kernel.org> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=8609; i=kees@kernel.org; h=from:subject:message-id; bh=iS0I9IX2sFCSsXL/t1+wx5auONRvDGG0fRjH5jXkuQk=; b=owGbwMvMwCVmps19z/KJym7G02pJDBlsFfkRtxzucF1bXNfZ/STmaC3/poZAvv5nTEZlv2dce +y1uLito5SFQYyLQVZMkSXIzj3OxeNte7j7XEWYOaxMIEMYuDgFYCI95owMs+6sMnQuY/21ZUXp Rd3/ihzcx47ey1x4+d5ML991+a8yghkZ9imJL9dZ8n7zFNnXnHVFb7ecfH4l9xEz///YV7FNXz8 ZswAA X-Developer-Key: i=kees@kernel.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: A0D214000B X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 8b9is57gtekcsabmggpwrannxgk3tdw4 X-HE-Tag: 1745254518-63038 X-HE-Meta: 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 xSi5tgCW Q9VeUiN84XTFlbjHoEvuOko9y91eFgNz1WBmK01SF41QUP2TFKIrBH24YInOOX2bt521Vdu2UwfRmaC6OmPFivnR7wRZDq4BJST5rpo6cnhtDsw6L0vFrCVk3fnAp4WeWd3rpJSIdS38Ra/Oib/LXDujv5I2wXxpMx85lDCHDiyYuk0OVfdcUJcqMEUD/pWo5Mog0voVNfPaYcph1AVCvVXP2XO/mLMuUiD0LOBg/6mTbaikvDjxg+nT8lC3nR6O/carYOydMv4/hsenVAijaOmTeh8GIOfmq9HRnInVNu1TN7AZDHHFYV4uChDWstJEZ6epOamL5v+58DqdjoL5LBu9IyAbC7J0jd1MqdYyZpAhaW2FQ3M8WbWH9zQNzHVZ/ufHsJDQVBJmMxefbgR5ZLaRdh/1Kni4NvwxgroThDbSfynyZhU7uqBIvt8qn9LHt1fAUPnxpckyyKig= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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