[PATCH v5 1/7] mm: Restore init_on_* static branch defaults

[PATCH v5 1/7] mm: Restore init_on_* static branch defaults

[Kees Cook]

Choosing the initial state of static branches changes the assembly layout
(if the condition is expected to be likely, inline, or unlikely, out of
line via a jump). The _TRUE/_FALSE defines for CONFIG_INIT_ON_*_DEFAULT_ON
were accidentally removed. These need to stay so that the CONFIG controls
the pessimization of the resulting static branch NOP/JMP locations.

Fixes: 04013513cc84 ("mm, page_alloc: do not rely on the order of page_poison and init_on_alloc/free parameters")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/mm.h | 8 ++++++++
 mm/page_alloc.c    | 8 ++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..b3317d91ee8e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2871,7 +2871,11 @@ static inline void kernel_poison_pages(struct page *page, int numpages) { }
 static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
 #endif
 
+#ifdef CONFIG_INIT_ON_ALLOC_DEFAULT_ON
+DECLARE_STATIC_KEY_TRUE(init_on_alloc);
+#else
 DECLARE_STATIC_KEY_FALSE(init_on_alloc);
+#endif
 static inline bool want_init_on_alloc(gfp_t flags)
 {
 	if (static_branch_unlikely(&init_on_alloc))
@@ -2879,7 +2883,11 @@ static inline bool want_init_on_alloc(gfp_t flags)
 	return flags & __GFP_ZERO;
 }
 
+#ifdef CONFIG_INIT_ON_FREE_DEFAULT_ON
+DECLARE_STATIC_KEY_TRUE(init_on_free);
+#else
 DECLARE_STATIC_KEY_FALSE(init_on_free);
+#endif
 static inline bool want_init_on_free(void)
 {
 	return static_branch_unlikely(&init_on_free);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3e4b29ee2b1e..f2d474a844cf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -167,10 +167,18 @@ unsigned long totalcma_pages __read_mostly;
 
 int percpu_pagelist_fraction;
 gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
+#ifdef CONFIG_INIT_ON_ALLOC_DEFAULT_ON
+DEFINE_STATIC_KEY_TRUE(init_on_alloc);
+#else
 DEFINE_STATIC_KEY_FALSE(init_on_alloc);
+#endif
 EXPORT_SYMBOL(init_on_alloc);
 
+#ifdef CONFIG_INIT_ON_FREE_DEFAULT_ON
+DEFINE_STATIC_KEY_TRUE(init_on_free);
+#else
 DEFINE_STATIC_KEY_FALSE(init_on_free);
+#endif
 EXPORT_SYMBOL(init_on_free);
 
 static bool _init_on_alloc_enabled_early __read_mostly
-- 
2.25.1

Re: [PATCH v5 1/7] mm: Restore init_on_* static branch defaults

[Andrew Morton]

On Tue,  9 Mar 2021 13:42:55 -0800 Kees Cook <keescook@chromium.org> wrote:

> Choosing the initial state of static branches changes the assembly layout
> (if the condition is expected to be likely, inline, or unlikely, out of
> line via a jump). The _TRUE/_FALSE defines for CONFIG_INIT_ON_*_DEFAULT_ON
> were accidentally removed. These need to stay so that the CONFIG controls
> the pessimization of the resulting static branch NOP/JMP locations.

Changelog doesn't really explain why anyone would want to apply this
patch.  This is especially important for -stable patches.

IOW, what is the user visible effect of the bug?

Re: [PATCH v5 1/7] mm: Restore init_on_* static branch defaults

[Kees Cook]

On Wed, Mar 10, 2021 at 03:56:02PM -0800, Andrew Morton wrote:
> On Tue,  9 Mar 2021 13:42:55 -0800 Kees Cook <keescook@chromium.org> wrote:
> 
> > Choosing the initial state of static branches changes the assembly layout
> > (if the condition is expected to be likely, inline, or unlikely, out of
> > line via a jump). The _TRUE/_FALSE defines for CONFIG_INIT_ON_*_DEFAULT_ON
> > were accidentally removed. These need to stay so that the CONFIG controls
> > the pessimization of the resulting static branch NOP/JMP locations.
> 
> Changelog doesn't really explain why anyone would want to apply this
> patch.  This is especially important for -stable patches.
> 
> IOW, what is the user visible effect of the bug?

Yeah, that's a good point, and in writing more details I decided this
wasn't actually worth a stable patch, and should just get folded into
later patches.

Thanks for the sanity-check!

-- 
Kees Cook