pve-edge-kernel/patches/kernel/0038-x86-ldt-64-Refresh-DS-and-ES-when-modify_ldt-changes.patch

From 34aa933a9bce5fb9c88e6ed98b268cbf058e51eb Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <[email protected]>
Date: Wed, 26 Jul 2017 07:16:30 -0700
Subject: [PATCH 038/242] x86/ldt/64: Refresh DS and ES when modify_ldt changes
 an entry
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

CVE-2017-5754

On x86_32, modify_ldt() implicitly refreshes the cached DS and ES
segments because they are refreshed on return to usermode.

On x86_64, they're not refreshed on return to usermode.  To improve
determinism and match x86_32's behavior, refresh them when we update
the LDT.

This avoids a situation in which the DS points to a descriptor that is
changed but the old cached segment persists until the next reschedule.
If this happens, then the user-visible state will change
nondeterministically some time after modify_ldt() returns, which is
unfortunate.

Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Chang Seok <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
(cherry picked from commit a632375764aa25c97b78beb56c71b0ba59d1cf83)
Signed-off-by: Andy Whitcroft <[email protected]>
Signed-off-by: Kleber Sacilotto de Souza <[email protected]>
(cherry picked from commit 295cb0b06150958ec84ee4b8844ef7e389e22c4e)
Signed-off-by: Fabian Grünbichler <[email protected]>
---
 arch/x86/kernel/ldt.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index a870910c8565..f0e64db18ac8 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -21,6 +21,25 @@
 #include <asm/mmu_context.h>
 #include <asm/syscalls.h>
 
+static void refresh_ldt_segments(void)
+{
+#ifdef CONFIG_X86_64
+	unsigned short sel;
+
+	/*
+	 * Make sure that the cached DS and ES descriptors match the updated
+	 * LDT.
+	 */
+	savesegment(ds, sel);
+	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
+		loadsegment(ds, sel);
+
+	savesegment(es, sel);
+	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
+		loadsegment(es, sel);
+#endif
+}
+
 /* context.lock is held for us, so we don't need any locking. */
 static void flush_ldt(void *__mm)
 {
@@ -32,6 +51,8 @@ static void flush_ldt(void *__mm)
 
 	pc = &mm->context;
 	set_ldt(pc->ldt->entries, pc->ldt->nr_entries);
+
+	refresh_ldt_segments();
 }
 
 /* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
-- 
2.14.2