Commit 8bb56436 authored Mar 09, 2016 by Andy Lutomirski Committed by Ingo Molnar Mar 10, 2016

x86/entry/traps: Clear DR6 early in do_debug() and improve the comment

Leaving any bits set in DR6 on return from a debug exception is
asking for trouble.  Prevent it by writing zero right away and
clarify the comment.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3857676e1be8fb27db4b89bbb1e2052b7f435ff4.1457578375.git.luto@kernel.org

Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 81edd9f6

arch/x86/kernel/traps.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -593,6 +593,18 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
		ist_enter(regs);

		get_debugreg(dr6, 6);
		/*
		* The Intel SDM says:
		*
		* Certain debug exceptions may clear bits 0-3. The remaining
		* contents of the DR6 register are never cleared by the
		* processor. To avoid confusion in identifying debug
		* exceptions, debug handlers should clear the register before
		* returning to the interrupted task.
		*
		* Keep it simple: clear DR6 immediately.
		*/
		set_debugreg(0, 6);

		/* Filter out all the reserved bits which are preset to 1 */
		dr6 &= ~DR6_RESERVED;
		@@ -616,9 +628,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
		if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
		goto exit;

		/* DR6 may or may not be cleared by the CPU */
		set_debugreg(0, 6);

		/* Store the virtualized DR6 value */
		tsk->thread.debugreg6 = dr6;