linux-kernel - [PATCH v1 1/1] : mm : mmap.c Arithmetic overflow in may_expand

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Message-ID: <4AD88804.4090205@agilent.com>
Date:	Fri, 16 Oct 2009 07:49:40 -0700
From:	Earl Chew <earl_chew@...lent.com>
To:	linux-kernel@...r.kernel.org
Subject: [PATCH v1 1/1] : mm : mmap.c Arithmetic overflow in may_expand_vm()

The function may_expand_vm() may return a false positive if the proposed
increment (npages) is sufficient large to overflow the expression:

	cur + npages

Assuming that cur < lim is an invariant, the proposed patch re-arranges
the expression to avoid unsigned arithmetic overflow.

More robustly:

	if (cur > lim || npages > lim - cur)
		return 0;

might be preferred if it cannot be guaranteed that cur < lim.



--- linux-2.6.21_mvlcge500/mm/mmap.c.orig	2008-06-30 22:43:38.000000000 
-0700
+++ linux-2.6.21_mvlcge500/mm/mmap.c	2009-10-16 07:42:23.000000000 -0700
@@ -2303,7 +2303,7 @@ int may_expand_vm(struct mm_struct *mm,

  	lim = current->signal->rlim[RLIMIT_AS].rlim_cur >> PAGE_SHIFT;

-	if (cur + npages > lim)
+	if (npages > lim - cur)
  		return 0;
  	return 1;
  }



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