Commit d5c678ae authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Thomas Gleixner
Browse files

x86/debug: Allow a single level of #DB recursion 



Trying to clear DR7 around a #DB from usermode malfunctions if the tasks
schedules when delivering SIGTRAP.

Rather than trying to define a special no-recursion region, just allow a
single level of recursion.  The same mechanism is used for NMI, and it
hasn't caused any problems yet.

Fixes: 9f58fdde ("x86/db: Split out dr6/7 handling")
Reported-by: default avatarKyle Huey <me@kylehuey.com>
Debugged-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/8b9bd05f187231df008d48cf818a6a311cbd5c98.1597882384.git.luto@kernel.org
Link: https://lore.kernel.org/r/20200902133200.726584153@infradead.org
parent 662a0221

arch/x86/kernel/traps.c

+31 −34
Original line number Diff line number Diff line
@@ -729,20 +729,9 @@ static bool is_sysenter_singlestep(struct pt_regs *regs) 
#endif

}



static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)

static __always_inline unsigned long debug_read_clear_dr6(void)

{

	/*

	 * Disable breakpoints during exception handling; recursive exceptions

	 * are exceedingly 'fun'.

	 *

	 * Since this function is NOKPROBE, and that also applies to

	 * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a

	 * HW_BREAKPOINT_W on our stack)

	 *

	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which

	 * includes the entry stack is excluded for everything.

	 */

	*dr7 = local_db_save();

	unsigned long dr6;



	/*

	 * The Intel SDM says:
@@ -755,15 +744,12 @@ static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7) 
	 *

	 * Keep it simple: clear DR6 immediately.

	 */

	get_debugreg(*dr6, 6);

	get_debugreg(dr6, 6);

	set_debugreg(0, 6);

	/* Filter out all the reserved bits which are preset to 1 */

	*dr6 &= ~DR6_RESERVED;

}

	dr6 &= ~DR6_RESERVED;



static __always_inline void debug_exit(unsigned long dr7)

{

	local_db_restore(dr7);

	return dr6;

}



/*
@@ -863,6 +849,18 @@ out: 
static __always_inline void exc_debug_kernel(struct pt_regs *regs,

					     unsigned long dr6)

{

	/*

	 * Disable breakpoints during exception handling; recursive exceptions

	 * are exceedingly 'fun'.

	 *

	 * Since this function is NOKPROBE, and that also applies to

	 * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a

	 * HW_BREAKPOINT_W on our stack)

	 *

	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which

	 * includes the entry stack is excluded for everything.

	 */

	unsigned long dr7 = local_db_save();

	bool irq_state = idtentry_enter_nmi(regs);

	instrumentation_begin();
@@ -883,6 +881,8 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs, 


	instrumentation_end();

	idtentry_exit_nmi(regs, irq_state);



	local_db_restore(dr7);

}



static __always_inline void exc_debug_user(struct pt_regs *regs,
@@ -894,6 +894,15 @@ static __always_inline void exc_debug_user(struct pt_regs *regs, 
	 */

	WARN_ON_ONCE(!user_mode(regs));



	/*

	 * NB: We can't easily clear DR7 here because

	 * idtentry_exit_to_usermode() can invoke ptrace, schedule, access

	 * user memory, etc.  This means that a recursive #DB is possible.  If

	 * this happens, that #DB will hit exc_debug_kernel() and clear DR7.

	 * Since we're not on the IST stack right now, everything will be

	 * fine.

	 */



	irqentry_enter_from_user_mode(regs);

	instrumentation_begin();
@@ -907,36 +916,24 @@ static __always_inline void exc_debug_user(struct pt_regs *regs, 
/* IST stack entry */

DEFINE_IDTENTRY_DEBUG(exc_debug)

{

	unsigned long dr6, dr7;



	debug_enter(&dr6, &dr7);

	exc_debug_kernel(regs, dr6);

	debug_exit(dr7);

	exc_debug_kernel(regs, debug_read_clear_dr6());

}



/* User entry, runs on regular task stack */

DEFINE_IDTENTRY_DEBUG_USER(exc_debug)

{

	unsigned long dr6, dr7;



	debug_enter(&dr6, &dr7);

	exc_debug_user(regs, dr6);

	debug_exit(dr7);

	exc_debug_user(regs, debug_read_clear_dr6());

}

#else

/* 32 bit does not have separate entry points. */

DEFINE_IDTENTRY_RAW(exc_debug)

{

	unsigned long dr6, dr7;



	debug_enter(&dr6, &dr7);

	unsigned long dr6 = debug_read_clear_dr6();



	if (user_mode(regs))

		exc_debug_user(regs, dr6);

	else

		exc_debug_kernel(regs, dr6);



	debug_exit(dr7);

}

#endif

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