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/9bef90327c9cb109d736c40115684fd32f49e6b0.1612546384.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 6 deletions

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 4aedb8455352..88e833986332 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1136,19 +1136,27 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
 	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;
 }