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Subject: [Bug 191359] [memguard] [panic] Memory modified after free
 w/MEMGUARD build
Date: Wed, 07 Jan 2015 13:44:34 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191359

luke.tw@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |luke.tw@gmail.com

--- Comment #1 from luke.tw@gmail.com ---
Dear Peter, 

I managed to find the root cause.
The bug can be reproduced by setting "sysctl vm.memguard.options=2" and ssh
activity
   1. memguard.options = 2 enable memguard to protect all allocations that are
bigger than PAGE_SIZE.
   2. ssh activity allocates mbuf that uses zone with UMA_ZONE_REFCNT flag. The
zone is protected by memguard.

However, these two features save values in the same union plinks in vm_page
   1. memguard save allocation size in vm_page->plinks.memguard.v
   2. UMA_ZONE_REFCNT save refcount in vm_page->plinks.s.pv

The following patch can work around this bug.

Index: sys/vm/memguard.c
===================================================================
--- sys/vm/memguard.c   (revision 276729)
+++ sys/vm/memguard.c   (working copy)
@@ -506,6 +506,9 @@
            zone->uz_flags & UMA_ZONE_NOFREE)
                return (0);

+       if (zone->uz_flags & UMA_ZONE_REFCNT)
+               return (0);
+
        if (memguard_cmp(zone->uz_size))
                return (1);

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