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Subject[PATCH 3.5 16/64] mm: do not grow the stack vma just because of an overrun on preceding vma
Date
3.5.7.24 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 09884964335e85e897876d17783c2ad33cf8a2e0 upstream.

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: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
mm/mmap.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 74e0d0d..7e24763 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1865,9 +1865,28 @@ 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);
}

@@ -1890,6 +1909,14 @@ 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);
}

--
1.8.3.2


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