[PATCH -tip] x86/mm/32: Sync only to LDT_BASE_ADDR in vmalloc_sync_all()

Joerg Roedel
2019-11-26T10:10:10Z

From: Joerg Roedel <jroedel@suse.de>

When vmalloc_sync_all() iterates over the address space until
FIX_ADDR_TOP it will sync the whole kernel address space starting from
VMALLOC_START.

This is not a problem when the kernel address range is identical in
all page-tables, but this is no longer the case when PTI is enabled on
x86-32. In that case the per-process LDT is mapped in the kernel
address range and vmalloc_sync_all() clears the LDT mapping for all
processes.

To make LDT working again vmalloc_sync_all() must only iterate over
the volatile parts of the kernel address range that are identical
between all processes. This includes the VMALLOC and the PKMAP areas
on x86-32.

The order of the ranges in the address space is:

VMALLOC -> PKMAP -> LDT -> CPU_ENTRY_AREA -> FIX_ADDR

So the right check in vmalloc_sync_all() is "address < LDT_BASE_ADDR"
to make sure the VMALLOC and PKMAP areas are synchronized and the LDT
mapping is not falsely overwritten. the CPU_ENTRY_AREA and
the FIXMAP area are no longer synced as well, but these
ranges are synchronized on page-table creation time and do
not change during runtime.

This change fixes the ldt_gdt selftest in my setup.

Fixes: 7757d607c6b3 ("x86/pti: AllowCONFIG_PAGE_TABLE_ISOLATION for x86_32")
Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/mm/fault.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 9ceacd1156db..144329c44436 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -197,7 +197,7 @@ void vmalloc_sync_all(void)
return;

for (address = VMALLOC_START & PMD_MASK;
- address >= TASK_SIZE_MAX && address < FIXADDR_TOP;
+ address >= TASK_SIZE_MAX && address < LDT_BASE_ADDR;
address += PMD_SIZE) {
struct page *page;

--
2.16.4