[PATCH 2/3] gfp: mm: introduce __GFP_NOINIT

[Alexander Potapenko <glider@xxxxxxxxxx>]

When passed to an allocator (either pagealloc or SL[AOU]B), __GFP_NOINIT
tells it to not initialize the requested memory if the init_allocations
boot option is enabled. This can be useful in the cases the newly
allocated memory is going to be initialized by the caller right away.

__GFP_NOINIT basically defeats the hardening against information leaks
provided by the init_allocations feature, so one should use it with
caution.

This patch also adds __GFP_NOINIT to alloc_pages() calls in SL[AOU]B.

Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Sandeep Patil <sspatil@xxxxxxxxxxx>
Cc: Laura Abbott <labbott@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Qian Cai <cai@xxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-security-module@xxxxxxxxxxxxxxx
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
---
 include/linux/gfp.h | 6 +++++-
 include/linux/mm.h  | 2 +-
 kernel/kexec_core.c | 2 +-
 mm/slab.c           | 2 +-
 mm/slob.c           | 1 +
 mm/slub.c           | 1 +
 6 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index fdab7de7490d..66d7f5604fe2 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -44,6 +44,7 @@ struct vm_area_struct;
 #else
 #define ___GFP_NOLOCKDEP	0
 #endif
+#define ___GFP_NOINIT		0x1000000u
 /* If the above are modified, __GFP_BITS_SHIFT may need updating */
 
 /*
@@ -208,16 +209,19 @@ struct vm_area_struct;
  * %__GFP_COMP address compound page metadata.
  *
  * %__GFP_ZERO returns a zeroed page on success.
+ *
+ * %__GFP_NOINIT requests non-initialized memory from the underlying allocator.
  */
 #define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)
 #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
 #define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)
+#define __GFP_NOINIT	((__force gfp_t)___GFP_NOINIT)
 
 /* Disable lockdep for GFP context tracking */
 #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
 
 /* Room for N __GFP_FOO bits */
-#define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (25)
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
 /**
diff --git a/include/linux/mm.h b/include/linux/mm.h
index b38b71a5efaa..8f03334a9033 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2601,7 +2601,7 @@ DECLARE_STATIC_KEY_FALSE(init_allocations);
 static inline bool want_init_memory(gfp_t flags)
 {
 	if (static_branch_unlikely(&init_allocations))
-		return true;
+		return !(flags & __GFP_NOINIT);
 	return flags & __GFP_ZERO;
 }
 
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index be84f5f95c97..f9d1f1236cd0 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -302,7 +302,7 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
 {
 	struct page *pages;
 
-	pages = alloc_pages(gfp_mask & ~__GFP_ZERO, order);
+	pages = alloc_pages((gfp_mask & ~__GFP_ZERO) | __GFP_NOINIT, order);
 	if (pages) {
 		unsigned int count, i;
 
diff --git a/mm/slab.c b/mm/slab.c
index dcc5b73cf767..762cb0e7bcc1 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1393,7 +1393,7 @@ static struct page *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
 	struct page *page;
 	int nr_pages;
 
-	flags |= cachep->allocflags;
+	flags |= (cachep->allocflags | __GFP_NOINIT);
 
 	page = __alloc_pages_node(nodeid, flags, cachep->gfporder);
 	if (!page) {
diff --git a/mm/slob.c b/mm/slob.c
index 18981a71e962..867d2d68a693 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -192,6 +192,7 @@ static void *slob_new_pages(gfp_t gfp, int order, int node)
 {
 	void *page;
 
+	gfp |= __GFP_NOINIT;
 #ifdef CONFIG_NUMA
 	if (node != NUMA_NO_NODE)
 		page = __alloc_pages_node(node, gfp, order);
diff --git a/mm/slub.c b/mm/slub.c
index e4efb6575510..a79b4cb768a2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1493,6 +1493,7 @@ static inline struct page *alloc_slab_page(struct kmem_cache *s,
 	struct page *page;
 	unsigned int order = oo_order(oo);
 
+	flags |= __GFP_NOINIT;
 	if (node == NUMA_NO_NODE)
 		page = alloc_pages(flags, order);
 	else
-- 
2.21.0.392.gf8f6787159e-goog