[PATCH] RLIMIT_AS checking fix

Address bug #4508: there's potential for wraparound in the various places
where we perform RLIMIT_AS checking.

(I'm a bit worried about acct_stack_growth().  Are we sure that vma->vm_mm is
always equal to current->mm?  If not, then we're comparing some other
process's total_vm with the calling process's rlimits).
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mm.h

@@ -726,6 +726,7 @@ extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
 extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
 	unsigned long addr, unsigned long len, pgoff_t pgoff);
 extern void exit_mmap(struct mm_struct *);
+extern int may_expand_vm(struct mm_struct *mm, unsigned long npages);
 
 extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
 

mm/mmap.c

@@ -1009,8 +1009,7 @@ unsigned long do_mmap_pgoff(struct file * file, unsigned long addr,
 	}
 
 	/* Check against address space limit. */
-	if ((mm->total_vm << PAGE_SHIFT) + len
-	    > current->signal->rlim[RLIMIT_AS].rlim_cur)
+	if (!may_expand_vm(mm, len >> PAGE_SHIFT))
 		return -ENOMEM;
 
 	if (accountable && (!(flags & MAP_NORESERVE) ||
@@ -1421,7 +1420,7 @@ static int acct_stack_growth(struct vm_area_struct * vma, unsigned long size, un
 	struct rlimit *rlim = current->signal->rlim;
 
 	/* address space limit tests */
-	if (mm->total_vm + grow > rlim[RLIMIT_AS].rlim_cur >> PAGE_SHIFT)
+	if (!may_expand_vm(mm, grow))
 		return -ENOMEM;
 
 	/* Stack limit test */
@@ -1848,8 +1847,7 @@ unsigned long do_brk(unsigned long addr, unsigned long len)
 	}
 
 	/* Check against address space limits *after* clearing old maps... */
-	if ((mm->total_vm << PAGE_SHIFT) + len
-	    > current->signal->rlim[RLIMIT_AS].rlim_cur)
+	if (!may_expand_vm(mm, len >> PAGE_SHIFT))
 		return -ENOMEM;
 
 	if (mm->map_count > sysctl_max_map_count)
@@ -2019,3 +2017,19 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	}
 	return new_vma;
 }
+
+/*
+ * Return true if the calling process may expand its vm space by the passed
+ * number of pages
+ */
+int may_expand_vm(struct mm_struct *mm, unsigned long npages)
+{
+	unsigned long cur = mm->total_vm;	/* pages */
+	unsigned long lim;
+
+	lim = current->signal->rlim[RLIMIT_AS].rlim_cur >> PAGE_SHIFT;
+
+	if (cur + npages > lim)
+		return 0;
+	return 1;
+}

mm/mremap.c

@@ -347,10 +347,10 @@ unsigned long do_mremap(unsigned long addr,
 		if (locked > lock_limit && !capable(CAP_IPC_LOCK))
 			goto out;
 	}
-	ret = -ENOMEM;
-	if ((current->mm->total_vm << PAGE_SHIFT) + (new_len - old_len)
-	    > current->signal->rlim[RLIMIT_AS].rlim_cur)
+	if (!may_expand_vm(current->mm, (new_len - old_len) >> PAGE_SHIFT)) {
+		ret = -ENOMEM;
 		goto out;
+	}
 
 	if (vma->vm_flags & VM_ACCOUNT) {
 		charged = (new_len - old_len) >> PAGE_SHIFT;