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>

4 files changed, 28 insertions, 8 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 599203876c32..3d2b6b6fb20c 100644
--- 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 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 6074aa064b54..14ec3bdad9a9 100644
--- 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 0b9aa8ab89f0..2b2845ae983e 100644
--- 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)