mm: fix page table unmap for stack guard page properly

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@volny.cz>
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Reported-by: Mike Pagano <mpagano@gentoo.org>
Reported-by: François Valenduc <francois.valenduc@tvcablenet.be>
Tested-by: Ed Tomlinson <edt@aei.ca>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Greg KH <gregkh@suse.de>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 7 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 9b3b73f4ae9c..b6e5fd23cc5a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2792,24 +2792,23 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
         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;
-        }
 
+        /* Use the zero-page for reads */
         if (!(flags & FAULT_FLAG_WRITE)) {
                 entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
                                                 vma->vm_page_prot));
-                ptl = pte_lockptr(mm, pmd);
-                spin_lock(ptl);
+                page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
                 if (!pte_none(*page_table))
                         goto unlock;
                 goto setpte;
         }
 
         /* 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);