Commit-ID: a632375764aa25c97b78beb56c71b0ba59d1cf83
Gitweb: http://git.kernel.org/tip/a632375764aa25c97b78beb56c71b0ba59d1cf83
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 26 Jul 2017 07:16:30 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 27 Jul 2017 09:12:57 +0200
x86/ldt/64: Refresh DS and ES when modify_ldt changes an entry
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 <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bpetkov@xxxxxxx>
Cc: Chang Seok <chang.seok.bae@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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 a870910..f0e64db 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. */
--
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