Commit a6323757 authored Jul 26, 2017 by Andy Lutomirski Committed by Ingo Molnar Jul 27, 2017

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@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Chang Seok <chang.seok.bae@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 81d38719

arch/x86/kernel/ldt.c

+21 −0

Original line number	Diff line number	Diff line
		@@ -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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