[5/6] mm: fix page table unmap for stack guard page properly

[Greg KH]

2.6.27-stable review patch.  If anyone has any objections, please let us know.

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

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

commit 11ac552477e32835cb6970bf0a70c210807f5673 upstream.

We do in fact need to unmap the page table _before_ doing the whole
stack guard page logic, because if it is needed (mainly 32-bit x86 with
PAE and CONFIG_HIGHPTE, but other architectures may use it too) then it
will do a kmap_atomic/kunmap_atomic.

And those kmaps will create an atomic region that we cannot do
allocations in.  However, the whole stack expand code will need to do
anon_vma_prepare() and vma_lock_anon_vma() and they cannot do that in an
atomic region.

Now, a better model might actually be to do the anon_vma_prepare() when
_creating_ a VM_GROWSDOWN segment, and not have to worry about any of
this at page fault time.  But in the meantime, this is the
straightforward fix for the issue.

See https://bugzilla.kernel.org/show_bug.cgi?id=16588 for details.

Reported-by: Wylda <wylda@xxxxxxxx>
Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Reported-by: Mike Pagano <mpagano@xxxxxxxxxx>
Reported-by: François Valenduc <francois.valenduc@xxxxxxxxxxxxx>
Tested-by: Ed Tomlinson <edt@xxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 mm/memory.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2428,14 +2428,13 @@ static int do_anonymous_page(struct mm_s
 	spinlock_t *ptl;
 	pte_t entry;
 
-	if (check_stack_guard_page(vma, address) < 0) {
-		pte_unmap(page_table);
+	pte_unmap(page_table);
+
+	/* Check if we need to add a guard page to the stack */
+	if (check_stack_guard_page(vma, address) < 0)
 		return VM_FAULT_SIGBUS;
-	}
 
 	/* Allocate our own private page. */
-	pte_unmap(page_table);
-
 	if (unlikely(anon_vma_prepare(vma)))
 		goto oom;
 	page = alloc_zeroed_user_highpage_movable(vma, address);


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