x86/modules: Avoid breaking W^X while loading modules

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>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 599203876c32fa2946cf9aef69b801c752d40f69..3d2b6b6fb20c4feb04895e3feb36087a41805970 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -668,15 +668,29 @@ 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 (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.
+                */
+       }
        return addr;
 }
 

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index b052e883dd8cc35df741a4e18c5236e38a234936..cfa3106faee42ee4879d512d698149a399f21ac8 100644 (file)
--- 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 6074aa064b540929ce076781884d436a7598f533..14ec3bdad9a90ab67931e172d2d76c20a1d1cb18 100644 (file)
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -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)

diff --git a/kernel/module.c b/kernel/module.c
index 0b9aa8ab89f08a3cf8474a7b56a46325a05b0ed5..2b2845ae983ed6c143384af00e6e4a43f8dd382a 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -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)