x86/modules: Avoid breaking W^X while loading modules (f2c65fb3) · Commits · 戴 / test

When modules and BPF filters are loaded, there is a time window in which some memory is both writable and executable. An attacker that has already found another vulnerability (e.g., a dangling pointer) might be able to exploit this behavior to overwrite kernel code. Prevent having writable executable PTEs in this stage. In addition, avoiding having W+X mappings can also slightly simplify the patching of modules code on initialization (e.g., by alternatives and static-key), as would be done in the next patch. This was actually the main motivation for this patch. To avoid having W+X mappings, set them initially as RW (NX) and after they are set as RO set them as X as well. Setting them as executable is done as a separate step to avoid one core in which the old PTE is cached (hence writable), and another which sees the updated PTE (executable), which would break the W^X protection. Suggested-by:

Thomas Gleixner <tglx@linutronix.de> Suggested-by:

Andy Lutomirski <luto@amacapital.net> Signed-off-by:

Nadav Amit <namit@vmware.com> Signed-off-by:

Rick Edgecombe <rick.p.edgecombe@intel.com> Signed-off-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Cc: <akpm@linux-foundation.org> Cc: <ard.biesheuvel@linaro.org> Cc: <deneen.t.dock@intel.com> Cc: <kernel-hardening@lists.openwall.com> Cc: <kristen@linux.intel.com> Cc: <linux_dti@icloud.com> Cc: <will.deacon@arm.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@intel.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jessica Yu <jeyu@kernel.org> Cc: Kees Cook <keescook@chromium.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Rik van Riel <riel@surriel.com> Link: https://lkml.kernel.org/r/20190426001143.4983-12-namit@vmware.com Signed-off-by:

Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/alternative.c

+21 −7

Original line number	Diff line number	Diff line
		@@ -671,12 +671,26 @@ void __init_or_module text_poke_early(void addr, const void *opcode,
		size_t len)
		{
		unsigned long flags;

		if (boot_cpu_has(X86_FEATURE_NX) &&
		is_module_text_address((unsigned long)addr)) {
		/*
		* Modules text is marked initially as non-executable, so the
		* code cannot be running and speculative code-fetches are
		* prevented. Just change the code.
		*/
		memcpy(addr, opcode, len);
		} else {
		local_irq_save(flags);
		memcpy(addr, opcode, len);
		local_irq_restore(flags);
		sync_core();
		/* Could also do a CLFLUSH here to speed up CPU recovery; but
		that causes hangs on some VIA CPUs. */

		/*
		* Could also do a CLFLUSH here to speed up CPU recovery; but
		* that causes hangs on some VIA CPUs.
		*/
		}
		return addr;
		}

arch/x86/kernel/module.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -87,7 +87,7 @@ void *module_alloc(unsigned long size)
		p = __vmalloc_node_range(size, MODULE_ALIGN,
		MODULES_VADDR + get_module_load_offset(),
		MODULES_END, GFP_KERNEL,
		PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
		PAGE_KERNEL, 0, NUMA_NO_NODE,
		__builtin_return_address(0));
		if (p && (kasan_module_alloc(p, size) < 0)) {
		vfree(p);

include/linux/filter.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -746,6 +746,7 @@ static inline void bpf_prog_unlock_ro(struct bpf_prog *fp)
		static inline void bpf_jit_binary_lock_ro(struct bpf_binary_header *hdr)
		{
		set_memory_ro((unsigned long)hdr, hdr->pages);
		set_memory_x((unsigned long)hdr, hdr->pages);
		}

		static inline void bpf_jit_binary_unlock_ro(struct bpf_binary_header *hdr)

kernel/module.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -1950,8 +1950,13 @@ void module_enable_ro(const struct module *mod, bool after_init)
		return;

		frob_text(&mod->core_layout, set_memory_ro);
		frob_text(&mod->core_layout, set_memory_x);

		frob_rodata(&mod->core_layout, set_memory_ro);

		frob_text(&mod->init_layout, set_memory_ro);
		frob_text(&mod->init_layout, set_memory_x);

		frob_rodata(&mod->init_layout, set_memory_ro);

		if (after_init)

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