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SubjectRe: [PATCH v6 08/10] x86: avoid W^X being broken during modules loading
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> On Nov 20, 2018, at 12:35 PM, Nadav Amit <namit@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@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Suggested-by: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: Nadav Amit <namit@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


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