[PATCH v6 08/10] x86: avoid W^X being broken during modules loading

[Nadav Amit]

> On Nov 20, 2018, at 12:35 PM, Nadav Amit <namit at vmware.com> wrote:
> 
> 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. This patch
> prevents having writable executable PTEs in this stage.
> 
> In addition, avoiding having R+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.
> 
> 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.
> 
> Cc: Andy Lutomirski <luto at kernel.org>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Dave Hansen <dave.hansen at intel.com>
> Cc: Masami Hiramatsu <mhiramat at kernel.org>
> Suggested-by: Thomas Gleixner <tglx at linutronix.de>
> Suggested-by: Andy Lutomirski <luto at amacapital.net>
> Signed-off-by: Nadav Amit <namit at vmware.com>
> ---
> arch/x86/kernel/alternative.c | 28 +++++++++++++++++++++-------
> arch/x86/kernel/module.c      |  2 +-
> include/linux/filter.h        |  6 ++++++
> kernel/module.c               | 10 ++++++++++
> 4 files changed, 38 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 8fc4685f3117..18415e3b6000 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -667,15 +667,29 @@ void __init alternative_instructions(void)
>  * handlers seeing an inconsistent instruction while you patch.
>  */
> void *__init_or_module text_poke_early(void *addr, const void *opcode,
> -					      size_t len)
> +				       size_t len)
> {
> 	unsigned long flags;
> -	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. */
> +
> +	if (static_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. We can 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.
> +		 */
> +	}
> 	return addr;
> }
> 
> diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
> index b052e883dd8c..cfa3106faee4 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -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);
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index de629b706d1d..ee9ae03c5f56 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -704,7 +704,13 @@ static inline void bpf_prog_unlock_ro(struct bpf_prog *fp)
> 
> static inline void bpf_jit_binary_lock_ro(struct bpf_binary_header *hdr)
> {
> +	/*
> +	 * Perform mapping changes in two stages to avoid opening a time-window
> +	 * in which a PTE is cached in any TLB as writable, but marked as
> +	 * executable in the memory-resident mappings (e.g., page-tables).
> +	 */
> 	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)
> diff --git a/kernel/module.c b/kernel/module.c
> index 49a405891587..7cb207249437 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1946,9 +1946,19 @@ void module_enable_ro(const struct module *mod, bool after_init)
> 	if (!rodata_enabled)
> 		return;
> 
> +	/*
> +	 * Perform mapping changes in two stages to avoid opening a time-window
> +	 * in which a PTE is cached in any TLB as writable, but marked as
> +	 * executable in the memory-resident mappings (e.g., page-tables).
> +	 */
> 	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)
> -- 
> 2.17.1

Rick pointed out that I screwed up ftrace and kprobes.

For kprobes, I think I need to add set_memory_x() to alloc_insn_page() and
change arch_ftrace_update_trampoline().

For arch_ftrace_update_trampoline(), I think I should remove not use
set_memory_rw() when patching in __probe_kernel_write() should be done
through text_poke().

I’ll give it another look and send another version later.

Regards,
Nadav