[PATCH] mm: fix zero page check in vm_normal_page

[Vinayak Menon <vinayakm.list@gmail.com>]

An issue was observed when a userspace task exits.
The page which hits error here is the zero page.
In zap_pte_range, vm_normal_page gets called, and it
returns a page address and not NULL, even though the
pte corresponds to zero pfn. In this case,
HAVE_PTE_SPECIAL is not set, and VM_MIXEDMAP is set
in vm_flags. In the case of VM_MIXEDMAP , only pfn_valid
is checked, and not is_zero_pfn. This results in
zero page being returned instead of NULL.

BUG: Bad page map in process mediaserver  pte:9dff379f pmd:9bfbd831
page:c0ed8e60 count:1 mapcount:-1 mapping:  (null) index:0x0
page flags: 0x404(referenced|reserved)
addr:40c3f000 vm_flags:10220051 anon_vma:  (null) mapping:d9fe0764 index:fd
vma->vm_ops->fault:   (null)
vma->vm_file->f_op->mmap: binder_mmap+0x0/0x274
CPU: 0 PID: 1463 Comm: mediaserver Tainted: G        W    3.10.17+ #1
[<c001549c>] (unwind_backtrace+0x0/0x11c) from [<c001200c>] (show_stack+0x10/0x14)
[<c001200c>] (show_stack+0x10/0x14) from [<c0103d78>] (print_bad_pte+0x158/0x190)
[<c0103d78>] (print_bad_pte+0x158/0x190) from [<c01055f0>] (unmap_single_vma+0x2e4/0x598)
[<c01055f0>] (unmap_single_vma+0x2e4/0x598) from [<c010618c>] (unmap_vmas+0x34/0x50)
[<c010618c>] (unmap_vmas+0x34/0x50) from [<c010a9e4>] (exit_mmap+0xc8/0x1e8)
[<c010a9e4>] (exit_mmap+0xc8/0x1e8) from [<c00520f0>] (mmput+0x54/0xd0)
[<c00520f0>] (mmput+0x54/0xd0) from [<c005972c>] (do_exit+0x360/0x990)
[<c005972c>] (do_exit+0x360/0x990) from [<c0059ef0>] (do_group_exit+0x84/0xc0)
[<c0059ef0>] (do_group_exit+0x84/0xc0) from [<c0066de0>] (get_signal_to_deliver+0x4d4/0x548)
[<c0066de0>] (get_signal_to_deliver+0x4d4/0x548) from [<c0011500>] (do_signal+0xa8/0x3b8)

Signed-off-by: Vinayak Menon <vinayakm.list@gmail.com>
---
 mm/memory.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 037b812..c9a5027 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -771,6 +771,8 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 		if (vma->vm_flags & VM_MIXEDMAP) {
 			if (!pfn_valid(pfn))
 				return NULL;
+			if (is_zero_pfn(pfn))
+				return NULL;
 			goto out;
 		} else {
 			unsigned long off;
-- 
1.7.6

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