[PATCH] mm/mm_init: Pull CONFIG_DEBUG_CHECK_PAGES out of CONFIG_DEBUG_VM

[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-sensitie production environments.

Introduce CONFIG_DEBUG_CHECK_PAGES, which sets is_check_pages_enabled
with no other side effects. Setting CONFIG_DEBUG_VM automatically
enables this as well as to have backwards compatibility.

Developed on top of 7f1dae318f81e508ef59835bc82bdf33e4cb1021 "mm: swap:
remove scan_swap_map_slots() references from comments" of mm-new.

Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 mm/Kconfig.debug | 12 ++++++++++++
 mm/internal.h    |  2 +-
 mm/mm_init.c     |  8 ++++----
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index 32b65073d0cc..366abde25026 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -45,6 +45,18 @@ config DEBUG_PAGEALLOC_ENABLE_DEFAULT
 	  Enable debug page memory allocations by default? This value
 	  can be overridden by debug_pagealloc=off|on.
 
+config DEBUG_CHECK_PAGES
+	bool "Debug VM page allocation/free sanity checks"
+	depends on DEBUG_KERNEL
+	default y if DEBUG_VM
+	help
+	  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.).
+
+	  This is automatically enabled if CONFIG_DEBUG_VM is set.
+
 config SLUB_DEBUG
 	default y
 	bool "Enable SLUB debugging support" if EXPERT
diff --git a/mm/internal.h b/mm/internal.h
index 04c307ee33ae..b8decdfc0930 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -562,7 +562,7 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
 extern char * const zone_names[MAX_NR_ZONES];
 
 /* perform sanity checks on struct pages being allocated or freed */
-DECLARE_STATIC_KEY_MAYBE(CONFIG_DEBUG_VM, check_pages_enabled);
+DECLARE_STATIC_KEY_MAYBE(CONFIG_DEBUG_CHECK_PAGES, check_pages_enabled);
 
 extern int min_free_kbytes;
 extern int defrag_mode;
diff --git a/mm/mm_init.c b/mm/mm_init.c
index c6812b4dbb2e..7f47b22864dd 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -2523,7 +2523,7 @@ static int __init early_init_on_free(char *buf)
 }
 early_param("init_on_free", early_init_on_free);
 
-DEFINE_STATIC_KEY_MAYBE(CONFIG_DEBUG_VM, check_pages_enabled);
+DEFINE_STATIC_KEY_MAYBE(CONFIG_DEBUG_CHECK_PAGES, check_pages_enabled);
 
 /*
  * Enable static keys related to various memory debugging and hardening options.
@@ -2588,10 +2588,10 @@ static void __init mem_debugging_and_hardening_init(void)
 
 	/*
 	 * Any page debugging or hardening option also enables sanity checking
-	 * of struct pages being allocated or freed. With CONFIG_DEBUG_VM it's
-	 * enabled already.
+	 * of struct pages being allocated or freed. With CONFIG_DEBUG_VM or
+	 * CONFIG_DEBUG_CHECK_PAGES it's enabled already.
 	 */
-	if (!IS_ENABLED(CONFIG_DEBUG_VM) && want_check_pages)
+	if (!IS_ENABLED(CONFIG_DEBUG_CHECK_PAGES) && want_check_pages)
 		static_branch_enable(&check_pages_enabled);
 }
 

base-commit: 7f1dae318f81e508ef59835bc82bdf33e4cb1021
-- 
2.47.3