bugAVR C Runtime Library - Bugs: bug #53284, realloc() does not respect...

 
 

bug #53284: realloc() does not respect __freelist size for small allocations

Submitted by:  David Glaze <djglaze>
Submitted on:  Sun 04 Mar 2018 10:19:44 PM UTC  
 
Category:  Library Severity:  3 - Normal
Priority:  5 - Normal Item Group:  libc code
Status:  None Percent Complete:  0%
Assigned to:  None Open/Closed:  Open
Release:  2.0.0 Fixed Release:  None

Add a New Comment (Rich Markup)
   

You are not logged in

Please log in, so followups can be emailed to you.

 

Sun 04 Mar 2018 10:19:44 PM UTC, original submission:  

The realloc() function does not respect the __freelist size when resizing an allocation down to 0 or 1 bytes.  If this allocation is then deallocated with free(), a __freelist entry is placed in its slot that can partially overwrite an adjacent allocation, leading to irrecoverable memory corruption.  The following code can reproduce the corruption:

// Relevant bytes in heap allocation shown in comments (User storage denoted with 0xFF)
char * resizedVar = (char*)malloc(6);       // 0x06 0x00 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
resizedVar = (char*)realloc(resizedVar, 1); // 0x01 0x00 0xFF
char * fixedVar = (char*)malloc(4);         // 0x01 0x00 0xFF 0x04 0x00 0xFF 0xFF 0xFF 0xFF
free(resizedVar);                           // 0x01 0x00 0x00 0x00 0x00 0xFF 0xFF 0xFF 0xFF
                                            //                  ^ Size of fixedVar overwritten

malloc() handles this issue with a minimum allocation size, so that requested allocations of 0, 1, or 2 bytes all take the same storage.  The same technique can be used here to prevent problems, as shown in the attached patch.

David Glaze <djglaze>

 

(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attach Files:
   
   
Comment:
   

Attached Files
file #43463:  realloc_small_size.patch added by djglaze (614B - text/x-patch)

 

Depends on the following items: None found

Items that depend on this one: None found

 

Carbon-Copy List
  • -email is unavailable- added by djglaze (Submitted the item)
  •  

     

     

    Follows 1 latest change.

    Date Changed by Updated Field Previous Value => Replaced by
    2018-03-04 djglaze Attached File- => Added realloc_small_size.patch, #43463

    Back to the top


    Powered by Savane 3.5