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Commit 09884964 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

mm: do not grow the stack vma just because of an overrun on preceding vma 



The stack vma is designed to grow automatically (marked with VM_GROWSUP
or VM_GROWSDOWN depending on architecture) when an access is made beyond
the existing boundary.  However, particularly if you have not limited
your stack at all ("ulimit -s unlimited"), this can cause the stack to
grow even if the access was really just one past *another* segment.

And that's wrong, especially since we first grow the segment, but then
immediately later enforce the stack guard page on the last page of the
segment.  So _despite_ first growing the stack segment as a result of
the access, the kernel will then make the access cause a SIGSEGV anyway!

So do the same logic as the guard page check does, and consider an
access to within one page of the next segment to be a bad access, rather
than growing the stack to abut the next segment.

Reported-and-tested-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d895cb1a

mm/mmap.c

+27 −0
Original line number Diff line number Diff line
@@ -2185,9 +2185,28 @@ int expand_downwards(struct vm_area_struct *vma, 
	return error;

}



/*

 * Note how expand_stack() refuses to expand the stack all the way to

 * abut the next virtual mapping, *unless* that mapping itself is also

 * a stack mapping. We want to leave room for a guard page, after all

 * (the guard page itself is not added here, that is done by the

 * actual page faulting logic)

 *

 * This matches the behavior of the guard page logic (see mm/memory.c:

 * check_stack_guard_page()), which only allows the guard page to be

 * removed under these circumstances.

 */

#ifdef CONFIG_STACK_GROWSUP

int expand_stack(struct vm_area_struct *vma, unsigned long address)

{

	struct vm_area_struct *next;



	address &= PAGE_MASK;

	next = vma->vm_next;

	if (next && next->vm_start == address + PAGE_SIZE) {

		if (!(next->vm_flags & VM_GROWSUP))

			return -ENOMEM;

	}

	return expand_upwards(vma, address);

}
@@ -2209,6 +2228,14 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr) 
#else

int expand_stack(struct vm_area_struct *vma, unsigned long address)

{

	struct vm_area_struct *prev;



	address &= PAGE_MASK;

	prev = vma->vm_prev;

	if (prev && prev->vm_end == address) {

		if (!(prev->vm_flags & VM_GROWSDOWN))

			return -ENOMEM;

	}

	return expand_downwards(vma, address);

}