Short description: Don't corrupt heap if top chunk is MINSIZE.
Author(s): Mel Gorman <mgorman@suse.de>
Origin: git://sourceware.org/git/glibc.git
Bug-RHEL: N/A
Bug-Fedora: N/A
Bug-Upstream: #18502
Upstream status: committed

commit f8ef472c0ff4644445ec716036d31430b4fa4bab
Author: Mel Gorman <mgorman@suse.de>
Date:   Mon Jun 8 13:36:13 2015 +0100

    malloc: Do not corrupt the top of a threaded heap if top chunk is MINSIZE [BZ #18502]
    
    mksquashfs was reported in openSUSE to be causing segmentation faults when
    creating installation images. Testing showed that mksquashfs sometimes
    failed and could be reproduced within 10 attempts. The core dump looked
    like the heap top was corrupted and was pointing to an unmapped area. In
    other cases, this has been due to an application corrupting glibc structures
    but mksquashfs appears to be fine in this regard.
    
    The problem is that heap_trim is "growing" the top into unmapped space.
    If the top chunk == MINSIZE then top_area is -1 and this check does not
    behave as expected due to a signed/unsigned comparison
    
      if (top_area <= pad)
        return 0;
    
    The next calculation extra = ALIGN_DOWN(top_area - pad, pagesz) calculates
    extra as a negative number which also is unnoticed due to a signed/unsigned
    comparison. We then call shrink_heap(heap, negative_number) which crashes
    later. This patch adds a simple check against MINSIZE to make sure extra
    does not become negative. It adds a cast to hint to the reader that this
    is a signed vs unsigned issue.
    
    Without the patch, mksquash fails within 10 attempts. With it applied, it
    completed 1000 times without error. The standard test suite "make check"
    showed no changes in the summary of test results.

Index: glibc-2.17-c758a686/malloc/arena.c
===================================================================
--- glibc-2.17-c758a686.orig/malloc/arena.c
+++ glibc-2.17-c758a686/malloc/arena.c
@@ -705,7 +705,7 @@ heap_trim(heap_info *heap, size_t pad)
     return 0;
 
   top_area = top_size - MINSIZE - 1;
-  if (top_area <= pad)
+  if (top_area < 0 || (size_t) top_area <= pad)
     return 0;
 
   /* Release in pagesize units and round down to the nearest page.  */