Commit 2a9baf5a authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
Browse files

x86/debug: Fix BTF handling 



The SDM states that #DB clears DEBUGCTLMSR_BTF, this means that when the
bit is set for userspace (TIF_BLOCKSTEP) and a kernel #DB happens first,
the BTF bit meant for userspace execution is lost.

Have the kernel #DB handler restore the BTF bit when it was requested
for userspace.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarKyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/20201027093607.956147736@infradead.org
parent ed8780e3

arch/x86/kernel/traps.c

+21 −7
Original line number Diff line number Diff line
@@ -799,13 +799,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void) 
	 */

	current->thread.virtual_dr6 = 0;



	/*

	 * The SDM says "The processor clears the BTF flag when it

	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep

	 * TIF_BLOCKSTEP in sync with the hardware BTF flag.

	 */

	clear_thread_flag(TIF_BLOCKSTEP);



	return dr6;

}
@@ -873,6 +866,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs, 
	 */

	WARN_ON_ONCE(user_mode(regs));



	if (test_thread_flag(TIF_BLOCKSTEP)) {

		/*

		 * The SDM says "The processor clears the BTF flag when it

		 * generates a debug exception." but PTRACE_BLOCKSTEP requested

		 * it for userspace, but we just took a kernel #DB, so re-set

		 * BTF.

		 */

		unsigned long debugctl;



		rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);

		debugctl |= DEBUGCTLMSR_BTF;

		wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);

	}



	/*

	 * Catch SYSENTER with TF set and clear DR_STEP. If this hit a

	 * watchpoint at the same time then that will still be handled.
@@ -935,6 +942,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs, 
	irqentry_enter_from_user_mode(regs);

	instrumentation_begin();



	/*

	 * The SDM says "The processor clears the BTF flag when it

	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep

	 * TIF_BLOCKSTEP in sync with the hardware BTF flag.

	 */

	clear_thread_flag(TIF_BLOCKSTEP);



	/*

	 * If dr6 has no reason to give us about the origin of this trap,

	 * then it's very likely the result of an icebp/int01 trap.

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