[077/118] kasan, mm: optimize krealloc poisoning

Commit Message

Andrew Morton Feb. 26, 2021, 1:20 a.m. UTC

From: Andrey Konovalov <[e-mail]>
Subject: kasan, mm: optimize krealloc poisoning

Currently, krealloc() always calls ksize(), which unpoisons the whole
object including the redzone.  This is inefficient, as kasan_krealloc()
repoisons the redzone for objects that fit into the same buffer.

This patch changes krealloc() instrumentation to use uninstrumented
__ksize() that doesn't unpoison the memory.  Instead, kasan_kreallos() is
changed to unpoison the memory excluding the redzone.

For objects that don't fit into the old allocation, this patch disables
KASAN accessibility checks when copying memory into a new object instead
of unpoisoning it.

Link: https://lkml.kernel.org/r/9b[e-mail]
Signed-off-by: Andrey Konovalov <[e-mail]>
Reviewed-by: Marco Elver <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Cc: Branislav Rankov <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Evgenii Stepanov <[e-mail]>
Cc: Kevin Brodsky <[e-mail]>
Cc: Peter Collingbourne <[e-mail]>
Cc: Vincenzo Frascino <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/kasan/common.c |   12 ++++++++++--
 mm/slab_common.c  |   20 ++++++++++++++------
 2 files changed, 24 insertions(+), 8 deletions(-)

Patch

--- a/mm/kasan/common.c~kasan-mm-optimize-krealloc-poisoning
+++ a/mm/kasan/common.c
@@ -476,7 +476,7 @@  static void *____kasan_kmalloc(struct km
 
 	/*
 	 * The object has already been unpoisoned by kasan_slab_alloc() for
-	 * kmalloc() or by ksize() for krealloc().
+	 * kmalloc() or by kasan_krealloc() for krealloc().
 	 */
 
 	/*
@@ -526,7 +526,7 @@  void * __must_check __kasan_kmalloc_larg
 
 	/*
 	 * The object has already been unpoisoned by kasan_alloc_pages() for
-	 * alloc_pages() or by ksize() for krealloc().
+	 * alloc_pages() or by kasan_krealloc() for krealloc().
 	 */
 
 	/*
@@ -554,8 +554,16 @@  void * __must_check __kasan_krealloc(con
 	if (unlikely(object == ZERO_SIZE_PTR))
 		return (void *)object;
 
+	/*
+	 * Unpoison the object's data.
+	 * Part of it might already have been unpoisoned, but it's unknown
+	 * how big that part is.
+	 */
+	kasan_unpoison(object, size);
+
 	page = virt_to_head_page(object);
 
+	/* Piggy-back on kmalloc() instrumentation to poison the redzone. */
 	if (unlikely(!PageSlab(page)))
 		return __kasan_kmalloc_large(object, size, flags);
 	else
--- a/mm/slab_common.c~kasan-mm-optimize-krealloc-poisoning
+++ a/mm/slab_common.c
@@ -1136,19 +1136,27 @@  static __always_inline void *__do_kreall
 	void *ret;
 	size_t ks;
 
-	if (likely(!ZERO_OR_NULL_PTR(p)) && !kasan_check_byte(p))
-		return NULL;
-
-	ks = ksize(p);
+	/* Don't use instrumented ksize to allow precise KASAN poisoning. */
+	if (likely(!ZERO_OR_NULL_PTR(p))) {
+		if (!kasan_check_byte(p))
+			return NULL;
+		ks = kfence_ksize(p) ?: __ksize(p);
+	} else
+		ks = 0;
 
+	/* If the object still fits, repoison it precisely. */
 	if (ks >= new_size) {
 		p = kasan_krealloc((void *)p, new_size, flags);
 		return (void *)p;
 	}
 
 	ret = kmalloc_track_caller(new_size, flags);
-	if (ret && p)
-		memcpy(ret, p, ks);
+	if (ret && p) {
+		/* Disable KASAN checks as the object's redzone is accessed. */
+		kasan_disable_current();
+		memcpy(ret, kasan_reset_tag(p), ks);
+		kasan_enable_current();
+	}
 
 	return ret;
 }