mm: fix ia64 crash when gcore reads gate area

Debian's ia64 autobuilders have been seeing kernel freeze or reboot
when running the gdb testsuite (Debian bug 588574): dannf bisected to
2.6.32 62eede62dafb4a6633eae7ffbeb34c60dba5e7b1 "mm: ZERO_PAGE without
PTE_SPECIAL"; and reproduced it with gdb's gcore on a simple target.

I'd missed updating the gate_vma handling in __get_user_pages(): that
happens to use vm_normal_page() (nowadays failing on the zero page),
yet reported success even when it failed to get a page - boom when
access_process_vm() tried to copy that to its intermediate buffer.

Fix this, resisting cleanups: in particular, leave it for now reporting
success when not asked to get any pages - very probably safe to change,
but let's not risk it without testing exposure.

Why did ia64 crash with 16kB pages, but succeed with 64kB pages?
Because setup_gate() pads each 64kB of its gate area with zero pages.

Reported-by: Andreas Barth <aba@not.so.argh.org>
Bisected-by: dann frazier <dannf@debian.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Tested-by: dann frazier <dannf@dannf.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 119b7ccdf39b..bde42c6d3633 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1394,10 +1394,20 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                 return i ? : -EFAULT;
                         }
                         if (pages) {
-                                struct page *page = vm_normal_page(gate_vma, start, *pte);
+                                struct page *page;
+
+                                page = vm_normal_page(gate_vma, start, *pte);
+                                if (!page) {
+                                        if (!(gup_flags & FOLL_DUMP) &&
+                                             is_zero_pfn(pte_pfn(*pte)))
+                                                page = pte_page(*pte);
+                                        else {
+                                                pte_unmap(pte);
+                                                return i ? : -EFAULT;
+                                        }
+                                }
                                 pages[i] = page;
-                                if (page)
-                                        get_page(page);
+                                get_page(page);
                         }
                         pte_unmap(pte);
                         if (vmas)