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([2001:af0:8000:1409:193:86:92:181]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4956a509901sm130525395e9.8.2026.07.22.06.57.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Jul 2026 06:57:55 -0700 (PDT) Message-ID: <2cada244-171d-45cb-a6f0-46e6a09375c7@suse.com> Date: Wed, 22 Jul 2026 15:57:54 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 1/2] module: Extend module_blacklist parameter to built-in modules To: Aaron Tomlin Cc: arnd@arndb.de, mcgrof@kernel.org, da.gomez@kernel.org, samitolvanen@google.com, peterz@infradead.org, akpm@linux-foundation.org, mhiramat@kernel.org, neelx@suse.com, da.anzani@gmail.com, sean@ashe.io, chjohnst@mail.com, steve@abita.co, mproche@mail.com, nick.lane@mail.com, linux-arch@vger.kernel.org, linux-modules@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260718190121.378314-1-atomlin@atomlin.com> <20260718190121.378314-2-atomlin@atomlin.com> Content-Language: en-US From: Petr Pavlu In-Reply-To: <20260718190121.378314-2-atomlin@atomlin.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/18/26 9:01 PM, Aaron Tomlin wrote: > Currently, the "module_blacklist=" command-line parameter only applies > to loadable modules. If a module is built-in, the parameter is silently > ignored. This patch extends the blacklisting functionality to built-in > modules by intercepting their initialisation routines during early boot. > > To achieve this, we introduce a new ".initcall.modnames" memory section. > For each built-in module, we use a standard C structure (i.e., struct > initcall_modname) to map its initcall function pointer to its associated > KBUILD_MODNAME string. This mapping is restricted only to files implementing > built-in modules via module_init() to avoid mapping core kernel subsystems > and save memory. > > During boot, built-in initcalls are executed sequentially via > do_initcall_level() and do_pre_smp_initcalls(). We introduce a new > wrapper function, do_one_initcall_builtin(), to cross-reference the > initcall function pointer against the ".initcall.modnames" table. If > a match is found and the module is present in the blacklist, the > initcall is skipped. > > To make the blacklist functional on monolithic kernels, the command-line > parameter parsing and the module_is_blacklisted() lookup function are > decoupled from the loadable module subsystem and moved to init/main.c. > This enables "module_blacklist=" to intercept built-in modules even on > kernels built with CONFIG_MODULES=n. > > Design Considerations and Trade-offs: > > 1. LTO and CFI Compatibility vs. PREL32 > > Previous iterations of this patch attempted to use top-level > inline assembly to generate 32-bit relative offsets (PREL32) to > save memory. However, raw inline assembly operates blindly > outside of the C compiler's visibility. When compiled with > CONFIG_LTO_CLANG or CONFIG_CFI_CLANG, the compiler applies > symbol renaming and generates Control Flow Integrity stubs. > The raw assembly string-matching fails to track these changes, > resulting in undefined references or runtime address mismatches. > > To resolve this, we strictly use standard C structures to hold > the function pointers. This natively allows the compiler to > resolve LTO renaming and map CFI stubs correctly. We trade the > minor spatial optimisation of PREL32 (using absolute 64-bit > pointers instead) to guarantee architectural safety under modern > compiler protections. Because this metadata is placed in an > ".init" section and freed entirely after boot, the temporary > memory overhead is negligible. > > 2. Architectural Safety and Elimination of Runtime Vulnerabilities: > > By embedding the boot-time blacklist check inside the > do_one_initcall_builtin() __init wrapper function, we ensure > the metadata lookup logic is exclusively invoked during early > boot. This approach provides strict structural guarantees: > - It inherently eliminates Use-After-Free (UAF) and race conditions > since loadable modules (which execute post-boot and invoke > do_one_initcall() directly) bypass this __init wrapper entirely. > - It prevents modpost section mismatch warnings since the __init > metadata is strictly accessed by other __init functions. > - It mitigates Spectre v1 speculative execution vulnerabilities > by guaranteeing the unprivileged runtime module loading path > cannot speculatively branch into reclaimed .init.text instructions. > > Signed-off-by: Aaron Tomlin > --- > include/asm-generic/vmlinux.lds.h | 4 ++- > include/linux/init.h | 23 ++++++++++++- > include/linux/module.h | 5 ++- > init/main.c | 55 +++++++++++++++++++++++++++++-- > kernel/module/main.c | 21 +----------- > 5 files changed, 83 insertions(+), 25 deletions(-) > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index 5659f4b5a125..799d912dcbc0 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -734,7 +734,9 @@ > EARLYCON_TABLE() \ > LSM_TABLE() \ > EARLY_LSM_TABLE() \ > - KUNIT_INIT_TABLE() > + KUNIT_INIT_TABLE() \ > + STRUCT_ALIGN(); \ > + BOUNDED_SECTION_BY(.initcall.modnames, _initcall_modnames) > > #define INIT_TEXT \ > *(.init.text .init.text.*) \ > diff --git a/include/linux/init.h b/include/linux/init.h > index 40331923b9f4..1cf163715264 100644 > --- a/include/linux/init.h > +++ b/include/linux/init.h > @@ -271,7 +271,28 @@ extern struct module __this_module; > __initcall_name(initcall, __iid, id), \ > __initcall_section(__sec, __iid)) > > -#define ___define_initcall(fn, id, __sec) \ > +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS > +#define __initcall_fn_ptr(fn, __iid, id) __initcall_stub(fn, __iid, id) > +#else > +#define __initcall_fn_ptr(fn, __iid, id) fn > +#endif Nit: I suggest moving the __initcall_fn_ptr() definition into the existing '#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS' block that defines ____define_initcall() to make the code shorter and group the related implementations. > + > +struct initcall_modname { > + initcall_t initcall_fn; > + const char *modname; > +}; > + > +#define ____define_initcall_modname(fn, id, __sec, __iid) \ > + __unique_initcall(fn, id, __sec, __iid) \ > + static const char __initstr_##fn[] __used __aligned(1) \ > + __section(".init.rodata") = KBUILD_MODNAME; \ > + static const struct initcall_modname __modname_##fn __used \ > + __section(".initcall.modnames") = { \ > + .initcall_fn = __initcall_fn_ptr(fn, __iid, id), \ > + .modname = __initstr_##fn \ > + }; > + > +#define ___define_initcall(fn, id, __sec) \ > __unique_initcall(fn, id, __sec, __initcall_id(fn)) > > #define __define_initcall(fn, id) ___define_initcall(fn, id, .initcall##id) > diff --git a/include/linux/module.h b/include/linux/module.h > index 7566815fabbe..fc1525e8f63c 100644 > --- a/include/linux/module.h > +++ b/include/linux/module.h > @@ -86,7 +86,8 @@ extern void cleanup_module(void); > * builtin) or at module insertion time (if a module). There can only > * be one per module. > */ > -#define module_init(x) __initcall(x); > +#define module_init(initfn) \ > + ____define_initcall_modname(initfn, 6, .initcall6, __initcall_id(initfn)) The initcall numbering and section naming should remain defined only in one place, in include/linux/init.h. Could something like the following work? In include/linux/module.h: #define module_init(initfn) __builtin_module_initcall(initfn); In include/linux/init.h: #define __define_initcall_modname(fn, id) \ __define_initcall(fn, id) \ static const char __initstr_##fn[] __used __aligned(1) \ __section(".init.rodata") = KBUILD_MODNAME; \ static const struct initcall_modname __modname_##fn __used \ __section(".initcall.modnames") = { \ .initcall_fn = __initcall_fn_ptr( \ fn, .initcall##id, id), \ .modname = __initstr_##fn \ }; #define __builtin_module_initcall(fn) __define_initcall_modname(fn, 6) > > /** > * module_exit() - driver exit entry point > @@ -883,6 +884,8 @@ static inline void module_for_each_mod(int(*func)(struct module *mod, void *data > } > #endif /* CONFIG_MODULES */ > > +bool module_is_blacklisted(const char *module_name); > + > #ifdef CONFIG_SYSFS > extern struct kset *module_kset; > extern const struct kobj_type module_ktype; > diff --git a/init/main.c b/init/main.c > index e363232b428b..ee6d7db212d5 100644 > --- a/init/main.c > +++ b/init/main.c > @@ -1334,6 +1334,57 @@ static inline void do_trace_initcall_level(const char *level) > } > #endif /* !TRACEPOINTS_ENABLED */ > > +extern struct initcall_modname __start_initcall_modnames[]; > +extern struct initcall_modname __stop_initcall_modnames[]; > + > +/* module_blacklist is a comma-separated list of module names */ > +static char *module_blacklist; > +bool __init_or_module module_is_blacklisted(const char *module_name) > +{ > + const char *p; > + size_t len; > + > + if (!module_blacklist) > + return false; > + > + for (p = module_blacklist; *p; p += len) { > + len = strcspn(p, ","); > + if (strlen(module_name) == len && !memcmp(module_name, p, len)) > + return true; > + if (p[len] == ',') > + len++; > + } > + return false; > +} > +core_param(module_blacklist, module_blacklist, charp, 0400); > + > +static const char *__init get_builtin_modname(initcall_t fn) > +{ > + struct initcall_modname *p; > + > + for (p = __start_initcall_modnames; p < __stop_initcall_modnames; p++) { > + if (dereference_function_descriptor(p->initcall_fn) == > + dereference_function_descriptor(fn)) Using dereference_function_descriptor() looks unnecessary. > + return p->modname; > + } > + return NULL; > +} > + > +static void __init do_one_initcall_builtin(initcall_t fn) > +{ > + const char *modname; > + > + if (module_blacklist) { > + modname = get_builtin_modname(fn); > + if (modname && module_is_blacklisted(modname)) { > + pr_info("Skipping initcall for blacklisted built-in module %s\n", > + modname); > + return; > + } > + } > + do_one_initcall(fn); > +} > + > int __init_or_module do_one_initcall(initcall_t fn) > { > int count = preempt_count(); > @@ -1406,7 +1457,7 @@ static void __init do_initcall_level(int level, char *command_line) > > do_trace_initcall_level(initcall_level_names[level]); > for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++) > - do_one_initcall(initcall_from_entry(fn)); > + do_one_initcall_builtin(initcall_from_entry(fn)); > } > > static void __init do_initcalls(void) > @@ -1451,7 +1502,7 @@ static void __init do_pre_smp_initcalls(void) > > do_trace_initcall_level("early"); > for (fn = __initcall_start; fn < __initcall0_start; fn++) > - do_one_initcall(initcall_from_entry(fn)); > + do_one_initcall_builtin(initcall_from_entry(fn)); > } > > static int run_init_process(const char *init_filename) > diff --git a/kernel/module/main.c b/kernel/module/main.c > index 46dd8d25a605..a8021039d44b 100644 > --- a/kernel/module/main.c > +++ b/kernel/module/main.c > @@ -2919,26 +2919,7 @@ int __weak module_frob_arch_sections(Elf_Ehdr *hdr, > return 0; > } > > -/* module_blacklist is a comma-separated list of module names */ > -static char *module_blacklist; > -static bool blacklisted(const char *module_name) > -{ > - const char *p; > - size_t len; > - > - if (!module_blacklist) > - return false; > > - for (p = module_blacklist; *p; p += len) { > - len = strcspn(p, ","); > - if (strlen(module_name) == len && !memcmp(module_name, p, len)) > - return true; > - if (p[len] == ',') > - len++; > - } > - return false; > -} > -core_param(module_blacklist, module_blacklist, charp, 0400); Nit: This hunk leaves three consecutive blank lines after module_frob_arch_sections(). > > static struct module *layout_and_allocate(struct load_info *info, int flags) > { > @@ -3391,7 +3372,7 @@ static int early_mod_check(struct load_info *info, int flags) > * Now that we know we have the correct module name, check > * if it's blacklisted. > */ > - if (blacklisted(info->name)) { > + if (module_is_blacklisted(info->name)) { > pr_err("Module %s is blacklisted\n", info->name); > return -EPERM; > } -- Thanks, Petr