[PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border

[Kirill A. Shutemov]

In case of 5-level paging, we don't put any mapping above 47-bit, unless
userspace explicitly asked for it.

Userspace can ask for allocation from full address space by specifying
hint address above 47-bit.

Nicholas noticed that current implementation violates this interface:
we can get vma partly in high addresses if we ask for a mapping at very
end of 47-bit address space.

Let's make sure that, when consider hint address for non-MAP_FIXED
mapping, start and end of resulting vma are on the same side of 47-bit
border.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
 arch/x86/kernel/sys_x86_64.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index a63fe77b3217..64b1a0d22247 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -198,11 +198,19 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
 	/* requesting a specific address */
 	if (addr) {
 		addr = PAGE_ALIGN(addr);
+		if (TASK_SIZE - len < addr)
+			goto get_unmapped_area;
+
+		/* The mapping shouldn't cross DEFAULT_MAP_WINDOW border */
+		if ((addr > DEFAULT_MAP_WINDOW) !=
+				(addr + len > DEFAULT_MAP_WINDOW))
+			goto get_unmapped_area;
+
 		vma = find_vma(mm, addr);
-		if (TASK_SIZE - len >= addr &&
-				(!vma || addr + len <= vm_start_gap(vma)))
+		if (!vma || addr + len <= vm_start_gap(vma))
 			return addr;
 	}
+get_unmapped_area:
 
 	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
 	info.length = len;
-- 
2.14.2