[PATCH 3.16 110/131] x86/speculation/l1tf: Extend 64bit swap file size limit

From: Ben Hutchings
Date: Sat Sep 29 2018 - 17:57:44 EST


3.16.59-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Vlastimil Babka <vbabka@xxxxxxx>

commit 1a7ed1ba4bba6c075d5ad61bb75e3fbc870840d6 upstream.

The previous patch has limited swap file size so that large offsets cannot
clear bits above MAX_PA/2 in the pte and interfere with L1TF mitigation.

It assumed that offsets are encoded starting with bit 12, same as pfn. But
on x86_64, offsets are encoded starting with bit 9.

Thus the limit can be raised by 3 bits. That means 16TB with 42bit MAX_PA
and 256TB with 46bit MAX_PA.

Fixes: 377eeaa8e11f ("x86/speculation/l1tf: Limit swap file size to MAX_PA/2")
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
arch/x86/mm/init.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -710,7 +710,15 @@ unsigned long max_swapfile_size(void)

if (boot_cpu_has_bug(X86_BUG_L1TF)) {
/* Limit the swap file size to MAX_PA/2 for L1TF workaround */
- pages = min_t(unsigned long, l1tf_pfn_limit() + 1, pages);
+ unsigned long l1tf_limit = l1tf_pfn_limit() + 1;
+ /*
+ * We encode swap offsets also with 3 bits below those for pfn
+ * which makes the usable limit higher.
+ */
+#ifdef CONFIG_X86_64
+ l1tf_limit <<= PAGE_SHIFT - SWP_OFFSET_FIRST_BIT;
+#endif
+ pages = min_t(unsigned long, l1tf_limit, pages);
}
return pages;
}