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Subject[PATCH 4.19 079/276] x86/modules: Avoid breaking W^X while loading modules
Date
[ Upstream commit f2c65fb3221adc6b73b0549fc7ba892022db9797 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kernel/alternative.c | 28 +++++++++++++++++++++-------
arch/x86/kernel/module.c | 2 +-
include/linux/filter.h | 1 +
kernel/module.c | 5 +++++
4 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index b9d5e7c9ef43e..918a23704c0c5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -662,15 +662,29 @@ 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 f58336af095c9..6645f123419c6 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 6ecef32fd8bcd..d52a7484aeb2d 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -684,6 +684,7 @@ 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 38bf28b5cc202..f797c6ace7121 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1949,8 +1949,13 @@ 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)
--
2.20.1


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