x86/debug: Only clear/set ->virtual_dr6 for userspace #DB (a195f3d4) · Commits · 戴 / test

The ->virtual_dr6 is the value used by ptrace_{get,set}_debugreg(6). A kernel #DB clearing it could mean spurious malfunction of ptrace() expectations. Signed-off-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by:

Thomas Gleixner <tglx@linutronix.de> Tested-by:

Kyle Huey <me@kylehuey.com> Link: https://lore.kernel.org/r/20201027093608.028952500@infradead.org

arch/x86/kernel/traps.c

+6 −6

Original line number	Diff line number	Diff line
		@@ -793,12 +793,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
		set_debugreg(DR6_RESERVED, 6);
		dr6 ^= DR6_RESERVED; /* Flip to positive polarity */

		/*
		* Clear the virtual DR6 value, ptrace routines will set bits here for
		* things we want signals for.
		*/
		current->thread.virtual_dr6 = 0;

		return dr6;
		}

		@@ -942,6 +936,12 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
		irqentry_enter_from_user_mode(regs);
		instrumentation_begin();

		/*
		* Clear the virtual DR6 value, ptrace() routines will set bits here
		* for things it wants signals for.
		*/
		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

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