From: Nadav Amit <na...@vmware.com>
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. 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.
Cc: Kees Cook <keesc...@chromium.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Dave Hansen <dave.han...@intel.com>
Cc: Masami Hiramatsu <mhira...@kernel.org>
Suggested-by: Thomas Gleixner <t...@linutronix.de>
Suggested-by: Andy Lutomirski <l...@amacapital.net>
Signed-off-by: Nadav Amit <na...@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgeco...@intel.com>
---
arch/x86/kernel/alternative.c | 28 +++++++++++++++++++++-------
arch/x86/kernel/module.c | 2 +-
include/linux/filter.h | 4 ++--
kernel/module.c | 5 +++++
4 files changed, 29 insertions(+), 10 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 ad106d845b22..f18cd317faf8 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -483,7 +483,7 @@ struct bpf_prog {
u16 pages; /* Number of allocated pages */
u16 jited:1, /* Is our filter JIT'ed? */
jit_requested:1,/* archs need to JIT the prog */
- undo_set_mem:1, /* Passed set_memory_ro()
checkpoint */
+ undo_set_mem:1, /* Passed set_memory_ro()
checkpoint */
gpl_compatible:1, /* Is filter GPL compatible?
*/
cb_access:1, /* Is control block accessed? */
dst_needed:1, /* Do we need dst entry? */
@@ -681,7 +681,6 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32
size_default)
static inline void bpf_prog_lock_ro(struct bpf_prog *fp)
{
- fp->undo_set_mem = 1;
set_memory_ro((unsigned long)fp, fp->pages);
}
@@ -694,6 +693,7 @@ 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 2ad1b5239910..ae1b77da6a20 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)
--
2.17.1
