[PATCH 1/2] x86, mm: fix gup_pte_range() vs DAX mappings

[Dan Williams]

gup_pte_range() fails to check pte_allows_gup() before translating a DAX
pte entry, pte_devmap(), to a page. This allows writes to read-only
mappings, and bypasses the DAX cacheline dirty tracking due to missed
'mkwrite' faults. The gup_huge_pmd() path and the gup_huge_pud() path
correctly check pte_allows_gup() before checking for _devmap() entries.

Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Cc: <x86@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Xiong Zhou <xzhou@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reported-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/mm/gup.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 0d4fb3ebbbac..1680768d392c 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -120,6 +120,11 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 			return 0;
 		}
 
+		if (!pte_allows_gup(pte_val(pte), write)) {
+			pte_unmap(ptep);
+			return 0;
+		}
+
 		if (pte_devmap(pte)) {
 			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 			if (unlikely(!pgmap)) {
@@ -127,8 +132,7 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 				pte_unmap(ptep);
 				return 0;
 			}
-		} else if (!pte_allows_gup(pte_val(pte), write) ||
-			   pte_special(pte)) {
+		} else if (pte_special(pte)) {
 			pte_unmap(ptep);
 			return 0;
 		}