libpayload: malloc: Fix realloc for overlapping buffers

The current realloc() works by freeing the origin buffer, allocating a
new one, and copying the data over. It's true that free() won't touch
the actual memory. However, the alloc() following it will potentially
modify the memory that belongs to the old buffer in order to create a
new free block (right after the newly allocated block). This causes 8
bytes (HDRSIZE) to be overwritten before being copied to the new buffer.

To fix the problem, we must create the header of the new free block
after the data is copied. In this patch, the content of alloc() is split
into two functions:

1. find_free_block(): Find a free block with large enough size, without
   touching the memory
2. use_block(): Update the header of the newly allocated block, and
   create the header of the new free block right after it

Then, inside realloc(), call memmove() call right after
find_free_block() while before use_block().

BUG=b:165439970
TEST=emerge-puff libpayload
TEST=Puff boots
TEST=Verified realloc() correctly copied data when buffers overlapped

Change-Id: I9418320a26820909144890300ddfb09ec2570f43
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45284
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/libc/malloc.c

@@ -123,7 +123,8 @@ int dma_coherent(void *ptr)
 	return !dma_initialized() || (dma->start <= ptr && dma->end > ptr);
 }
 
-static void *alloc(int len, struct memory_type *type)
+/* Find free block of size >= len */
+static hdrtype_t volatile *find_free_block(int len, struct memory_type *type)
 {
 	hdrtype_t header;
 	hdrtype_t volatile *ptr = (hdrtype_t volatile *)type->start;
@@ -156,37 +157,53 @@ static void *alloc(int len, struct memory_type *type)
 			halt();
 		}
 
-		if (header & FLAG_FREE) {
-			if (len <= size) {
-				hdrtype_t volatile *nptr = (hdrtype_t volatile *)((uintptr_t)ptr + HDRSIZE + len);
-				int nsize = size - (HDRSIZE + len);
-
-				/* If there is still room in this block,
-				 * then mark it as such otherwise account
-				 * the whole space for that block.
-				 */
-
-				if (nsize > 0) {
-					/* Mark the block as used. */
-					*ptr = USED_BLOCK(len);
-
-					/* Create a new free block. */
-					*nptr = FREE_BLOCK(nsize);
-				} else {
-					/* Mark the block as used. */
-					*ptr = USED_BLOCK(size);
-				}
-
-				return (void *)((uintptr_t)ptr + HDRSIZE);
-			}
-		}
+		if ((header & FLAG_FREE) && len <= size)
+			return ptr;
 
 		ptr = (hdrtype_t volatile *)((uintptr_t)ptr + HDRSIZE + size);
 
 	} while (ptr < (hdrtype_t *) type->end);
 
 	/* Nothing available. */
-	return (void *)NULL;
+	return NULL;
+}
+
+/* Mark the block with length 'len' as used */
+static void use_block(hdrtype_t volatile *ptr, int len)
+{
+	/* Align the size. */
+	len = ALIGN_UP(len, HDRSIZE);
+
+	hdrtype_t volatile *nptr = (hdrtype_t volatile *)
+		((uintptr_t)ptr + HDRSIZE + len);
+	int size = SIZE(*ptr);
+	int nsize = size - (HDRSIZE + len);
+
+	/*
+	 * If there is still room in this block, then mark it as such otherwise
+	 * account the whole space for that block.
+	 */
+	if (nsize > 0) {
+		/* Mark the block as used. */
+		*ptr = USED_BLOCK(len);
+
+		/* Create a new free block. */
+		*nptr = FREE_BLOCK(nsize);
+	} else {
+		/* Mark the block as used. */
+		*ptr = USED_BLOCK(size);
+	}
+}
+
+static void *alloc(int len, struct memory_type *type)
+{
+	hdrtype_t volatile *ptr = find_free_block(len, type);
+
+	if (ptr == NULL)
+		return NULL;
+
+	use_block(ptr, len);
+	return (void *)((uintptr_t)ptr + HDRSIZE);
 }
 
 static void _consolidate(struct memory_type *type)
@@ -277,6 +294,7 @@ void *calloc(size_t nmemb, size_t size)
 void *realloc(void *ptr, size_t size)
 {
 	void *ret, *pptr;
+	hdrtype_t volatile *block;
 	unsigned int osize;
 	struct memory_type *type = heap;
 
@@ -300,18 +318,23 @@ void *realloc(void *ptr, size_t size)
 	 * reallocated the new space.
 	 */
 	free(ptr);
-	ret = alloc(size, type);
+
+	block = find_free_block(size, type);
+	if (block == NULL)
+		return NULL;
+
+	ret = (void *)((uintptr_t)block + HDRSIZE);
 
 	/*
-	 * if ret == NULL, then doh - failure.
-	 * if ret == ptr then woo-hoo! no copy needed.
+	 * If ret == ptr, then no copy is needed. Otherwise, move the memory to
+	 * the new location, which might be before the old one and overlap since
+	 * the free() above includes a _consolidate().
 	 */
-	if (ret == NULL || ret == ptr)
-		return ret;
+	if (ret != ptr)
+		memmove(ret, ptr, osize > size ? size : osize);
 
-	/* Move the memory to the new location. Might be before the old location
-	   and overlap since the free() above includes a _consolidate(). */
-	memmove(ret, ptr, osize > size ? size : osize);
+	/* Mark the block as used. */
+	use_block(block, size);
 
 	return ret;
 }