From 8bb5643686d2898bec181c407651cf84e77d413f Mon Sep 17 00:00:00 2001 From: Andy Lutomirski <luto@kernel.org> Date: Wed, 9 Mar 2016 19:00:29 -0800 Subject: [PATCH] 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> --- arch/x86/kernel/traps.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 1af56217fb5dc..7394be8b1a669 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -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; -- 2.30.2