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SubjectRe: [PATCH v6 08/10] x86: avoid W^X being broken during modules loading
On Wed, 28 Nov 2018 18:59:30 +0000
Nadav Amit <namit@vmware.com> wrote:

> > 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().

Oops, right. It should be easy to fix for kprobe :) what you need is below.

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index c33b06f..51818f3 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -431,8 +431,10 @@ void *alloc_insn_page(void)
void *page;

page = module_alloc(PAGE_SIZE);
- if (page)
+ if (page) {
set_memory_ro((unsigned long)page & PAGE_MASK, 1);
+ set_memory_x((unsigned long)page & PAGE_MASK, 1);
+ }

return page;
}

>
> 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().

right.

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

Thank you,

>
> Regards,
> Nadav
>
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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