[RFC][PATCH 6/7] malloc: Add more integrity checks to mremap_chunk.

[Istvan Kurucsai]

Similarly to the ones in munmap_chunk, ensure that the mapped region begins at
a page boundary, that the size is page-aligned and that the offset of the
chunk into its page is a power of 2.

    * malloc/malloc.c (mremap_chunk): Additional checks.
---
 malloc/malloc.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index f0c54fa..503d061 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2843,16 +2843,26 @@ mremap_chunk (mchunkptr p, size_t new_size)
   char *cp;
 
   assert (chunk_is_mmapped (p));
-  assert (((size + offset) & (GLRO (dl_pagesize) - 1)) == 0);
+
+  uintptr_t block = (uintptr_t) p - offset;
+  uintptr_t mem = (uintptr_t) chunk2mem(p);
+  size_t total_size = offset + size;
+  if (__glibc_unlikely ((block | total_size) & (pagesize - 1)) != 0
+      || __glibc_unlikely (!powerof2 (mem & (pagesize - 1))))
+    {
+      malloc_printerr (check_action, "mremap_chunk(): invalid pointer",
+                       chunk2mem (p), NULL);
+      return 0;
+    }
 
   /* Note the extra SIZE_SZ overhead as in mmap_chunk(). */
   new_size = ALIGN_UP (new_size + offset + SIZE_SZ, pagesize);
 
   /* No need to remap if the number of pages does not change.  */
-  if (size + offset == new_size)
+  if (total_size == new_size)
     return p;
 
-  cp = (char *) __mremap ((char *) p - offset, size + offset, new_size,
+  cp = (char *) __mremap ((char *) block, total_size, new_size,
                           MREMAP_MAYMOVE);
 
   if (cp == MAP_FAILED)
-- 
2.7.4