[PATCH v1 1/1] : mm : mmap.c Arithmetic overflow in may_expand_vm()

[Earl Chew]

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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