[PATCH v2 1/2] mm/mm_init: Introduce a boot parameter for check_pages

[Joshua Hahn <joshua.hahnjy@gmail.com>]

Use-after-free and double-free bugs can be very difficult to track down.
The kernel is good at tracking these and preventing bad pages from being
used/created through simple checks gated behind "check_pages_enabled".

Currently, the only ways to enable this flag is by building with
CONFIG_DEBUG_VM, or as a side effect of other checks such as
init_on_{alloc, free}, page_poisoning, or debug_pagealloc among others.
These solutions are powerful, but may often be too coarse in balancing
the performance vs. safety that a user may want, particularly in
latency-sensitive production environments.

Introduce a new boot parameter "check_pages", which enables page checking
with no other side effects. It takes kstrbool-able inputs as an argument
(i.e. 0/1, true/false, on/off, ...). This patch is backwards-compatible;
setting CONFIG_DEBUG_VM still enables page checking.

Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
v1 --> v2:
- Changed check_pages from a build config into a boot config, as suggested
  by Vlastimil.
- Introduced the second patch, which decouples page checking from 
  init_on_page_alloc and init_on_page_free.
---

 Documentation/admin-guide/kernel-parameters.txt |  8 ++++++++
 mm/mm_init.c                                    | 11 ++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6c42061ca20e..0ba9561440a7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -669,6 +669,14 @@
 			nokmem -- Disable kernel memory accounting.
 			nobpf -- Disable BPF memory accounting.
 
+	check_pages=	[MM,EARLY] Enable sanity checking of pages after
+			allocations / before freeing. This adds checks to catch
+			double-frees, use-after-frees, and other sources of
+			page corruption by inspecting page internals (flags,
+			mapcount/refcount, memcg_data, etc.).
+			Format: { "0" | "1" }
+			Default: 0 (1 if CONFIG_DEBUG_VM is set)
+
 	checkreqprot=	[SELINUX] Set initial checkreqprot flag value.
 			Format: { "0" | "1" }
 			See security/selinux/Kconfig help text.
diff --git a/mm/mm_init.c b/mm/mm_init.c
index c6812b4dbb2e..01d46efc42b4 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -2525,6 +2525,14 @@ early_param("init_on_free", early_init_on_free);
 
 DEFINE_STATIC_KEY_MAYBE(CONFIG_DEBUG_VM, check_pages_enabled);
 
+static bool _check_pages_enabled_early __initdata;
+
+static int __init early_check_pages(char *buf)
+{
+	return kstrtobool(buf, &_check_pages_enabled_early);
+}
+early_param("check_pages", early_check_pages);
+
 /*
  * Enable static keys related to various memory debugging and hardening options.
  * Some override others, and depend on early params that are evaluated in the
@@ -2591,7 +2599,8 @@ static void __init mem_debugging_and_hardening_init(void)
 	 * of struct pages being allocated or freed. With CONFIG_DEBUG_VM it's
 	 * enabled already.
 	 */
-	if (!IS_ENABLED(CONFIG_DEBUG_VM) && want_check_pages)
+	if (!IS_ENABLED(CONFIG_DEBUG_VM) && (_check_pages_enabled_early ||
+					     want_check_pages))
 		static_branch_enable(&check_pages_enabled);
 }
 
-- 
2.47.3