From 9f58fdde95c9509a4ded27a6d0035e79294002b4 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 6 Apr 2020 21:02:56 +0200 Subject: [PATCH] x86/db: Split out dr6/7 handling DR6/7 should be handled before nmi_enter() is invoked and restore after nmi_exit() to minimize the exposure. Split it out into helper inlines and bring it into the correct order. Signed-off-by: Peter Zijlstra Signed-off-by: Thomas Gleixner Reviewed-by: Alexandre Chartre Acked-by: Andy Lutomirski Link: https://lkml.kernel.org/r/20200505135314.808628211@linutronix.de --- arch/x86/kernel/hw_breakpoint.c | 6 +--- arch/x86/kernel/traps.c | 75 ++++++++++++++++++++++++++++++----------- 2 files changed, 57 insertions(+), 24 deletions(-) diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c index d42fc0eaf193..9ddf441ccaa8 100644 --- a/arch/x86/kernel/hw_breakpoint.c +++ b/arch/x86/kernel/hw_breakpoint.c @@ -464,7 +464,7 @@ static int hw_breakpoint_handler(struct die_args *args) { int i, cpu, rc = NOTIFY_STOP; struct perf_event *bp; - unsigned long dr7, dr6; + unsigned long dr6; unsigned long *dr6_p; /* The DR6 value is pointed by args->err */ @@ -479,9 +479,6 @@ static int hw_breakpoint_handler(struct die_args *args) if ((dr6 & DR_TRAP_BITS) == 0) return NOTIFY_DONE; - get_debugreg(dr7, 7); - /* Disable breakpoints during exception handling */ - set_debugreg(0UL, 7); /* * Assert that local interrupts are disabled * Reset the DRn bits in the virtualized register value. @@ -538,7 +535,6 @@ static int hw_breakpoint_handler(struct die_args *args) (dr6 & (~DR_TRAP_BITS))) rc = NOTIFY_DONE; - set_debugreg(dr7, 7); put_cpu(); return rc; diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 21c8cfce24d3..de5120e2fbe1 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -700,6 +700,57 @@ static bool is_sysenter_singlestep(struct pt_regs *regs) #endif } +static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7) +{ + /* + * 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. + */ + get_debugreg(*dr7, 7); + set_debugreg(0, 7); + + /* + * Ensure the compiler doesn't lower the above statements into + * the critical section; disabling breakpoints late would not + * be good. + */ + barrier(); + + /* + * 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. + */ + get_debugreg(*dr6, 6); + set_debugreg(0, 6); + /* Filter out all the reserved bits which are preset to 1 */ + *dr6 &= ~DR6_RESERVED; +} + +static __always_inline void debug_exit(unsigned long dr7) +{ + /* + * Ensure the compiler doesn't raise this statement into + * the critical section; enabling breakpoints early would + * not be good. + */ + barrier(); + set_debugreg(dr7, 7); +} + /* * Our handling of the processor debug registers is non-trivial. * We do not clear them on entry and exit from the kernel. Therefore @@ -727,28 +778,13 @@ static bool is_sysenter_singlestep(struct pt_regs *regs) dotraplinkage void do_debug(struct pt_regs *regs, long error_code) { struct task_struct *tsk = current; + unsigned long dr6, dr7; int user_icebp = 0; - unsigned long dr6; int si_code; - nmi_enter(); - - 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); + debug_enter(&dr6, &dr7); - /* Filter out all the reserved bits which are preset to 1 */ - dr6 &= ~DR6_RESERVED; + nmi_enter(); /* * The SDM says "The processor clears the BTF flag when it @@ -786,7 +822,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code) #endif if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, error_code, - SIGTRAP) == NOTIFY_STOP) + SIGTRAP) == NOTIFY_STOP) goto exit; /* @@ -825,6 +861,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code) exit: nmi_exit(); + debug_exit(dr7); } NOKPROBE_SYMBOL(do_debug); -- 2.11.0