mm, x86: fix pte_page() crash in gup_pte_range()

Commit 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings") has
moved up the pte_page(pte) in x86's fast gup_pte_range(), for no
discernible reason: put it back where it belongs, after the pte_flags
check and the pfn_valid cross-check.

That may be the cause of the NULL pointer dereference in
gup_pte_range(), seen when vfio called vaddr_get_pfn() when starting a
qemu-kvm based VM.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Michael Long <Harn-Solo@gmx.de>
Tested-by: Michael Long <Harn-Solo@gmx.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 6d5eb59003721b79213a7661994be1a339bc5590..d8a798d8bf50ac037fa2ac4854085bd78759c249 100644 (file)
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -102,7 +102,6 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
                        return 0;
                }
 
-               page = pte_page(pte);
                if (pte_devmap(pte)) {
                        pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
                        if (unlikely(!pgmap)) {
@@ -115,6 +114,7 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
                        return 0;
                }
                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+               page = pte_page(pte);
                get_page(page);
                put_dev_pagemap(pgmap);
                SetPageReferenced(page);