linux-kernel - [PATCH 4.4 35/43] x86/speculation/l1tf: Extend 64bit swap file size limit

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Message-Id: <20180814171519.410521293@linuxfoundation.org>
Date:   Tue, 14 Aug 2018 19:18:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Guenter Roeck <linux@...ck-us.net>
Subject: [PATCH 4.4 35/43] x86/speculation/l1tf: Extend 64bit swap file size limit

4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Vlastimil Babka <vbabka@...e.cz>

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@...e.cz>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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
@@ -778,7 +778,15 @@ 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;
 }