[GIT pull] x86/entry for 5.10-rc1

From: Thomas Gleixner
Date: Mon Oct 12 2020 - 13:47:18 EST

Linus,

please pull the latest x86/entry branch from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-entry-2020-10-12

up to: d53d9bc0cf78: x86/debug: Change thread.debugreg6 to thread.virtual_dr6


More consolidation and correctness fixes for the debug exception:

- Ensure BTF synchronization under all circumstances

- Distangle kernel and user mode #DB further

- Get ordering vs. the debug notifier correct to make KGDB work more
reliably.

- Cleanup historical gunk and make the code simpler to understand.

Thanks,

tglx

------------------>
Peter Zijlstra (11):
x86/debug: Sync BTF earlier
x86/debug: Move kprobe_debug_handler() into exc_debug_kernel()
x86/debug: Remove handle_debug(.user) argument
x86/debug: Simplify #DB signal code
x86/debug: Move historical SYSENTER junk into exc_debug_kernel()
x86/debug: Move cond_local_irq_enable() block into exc_debug_user()
x86/debug: Remove the historical junk
x86/debug: Remove aout_dump_debugregs()
x86/debug: Simplify hw_breakpoint_handler()
x86/debug: Support negative polarity DR6 bits
x86/debug: Change thread.debugreg6 to thread.virtual_dr6


arch/x86/include/asm/debugreg.h | 2 -
arch/x86/include/asm/kprobes.h | 4 ++
arch/x86/include/asm/processor.h | 2 +-
arch/x86/kernel/hw_breakpoint.c | 58 ++--------------
arch/x86/kernel/kgdb.c | 5 +-
arch/x86/kernel/ptrace.c | 6 +-
arch/x86/kernel/traps.c | 146 +++++++++++++++++++++------------------
7 files changed, 94 insertions(+), 129 deletions(-)

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index e89558a3fe4a..cfdf307ddc01 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -90,8 +90,6 @@ static __always_inline bool hw_breakpoint_active(void)
return __this_cpu_read(cpu_dr7) & DR_GLOBAL_ENABLE_MASK;
}

-extern void aout_dump_debugregs(struct user *dump);
-
extern void hw_breakpoint_restore(void);

static __always_inline unsigned long local_db_save(void)
diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h
index 143bc9abe99c..991a7ad540c7 100644
--- a/arch/x86/include/asm/kprobes.h
+++ b/arch/x86/include/asm/kprobes.h
@@ -106,5 +106,9 @@ extern int kprobe_exceptions_notify(struct notifier_block *self,
extern int kprobe_int3_handler(struct pt_regs *regs);
extern int kprobe_debug_handler(struct pt_regs *regs);

+#else
+
+static inline int kprobe_debug_handler(struct pt_regs *regs) { return 0; }
+
#endif /* CONFIG_KPROBES */
#endif /* _ASM_X86_KPROBES_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 97143d87994c..d8a82e650810 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -517,7 +517,7 @@ struct thread_struct {
/* Save middle states of ptrace breakpoints */
struct perf_event *ptrace_bps[HBP_NUM];
/* Debug status used for traps, single steps, etc... */
- unsigned long debugreg6;
+ unsigned long virtual_dr6;
/* Keep track of the exact dr7 value set by the user */
unsigned long ptrace_dr7;
/* Fault info: */
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index b98ff620ba77..03aa33b58165 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -441,42 +441,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
return 0;
}

-/*
- * Dump the debug register contents to the user.
- * We can't dump our per cpu values because it
- * may contain cpu wide breakpoint, something that
- * doesn't belong to the current task.
- *
- * TODO: include non-ptrace user breakpoints (perf)
- */
-void aout_dump_debugregs(struct user *dump)
-{
- int i;
- int dr7 = 0;
- struct perf_event *bp;
- struct arch_hw_breakpoint *info;
- struct thread_struct *thread = &current->thread;
-
- for (i = 0; i < HBP_NUM; i++) {
- bp = thread->ptrace_bps[i];
-
- if (bp && !bp->attr.disabled) {
- dump->u_debugreg[i] = bp->attr.bp_addr;
- info = counter_arch_bp(bp);
- dr7 |= encode_dr7(i, info->len, info->type);
- } else {
- dump->u_debugreg[i] = 0;
- }
- }
-
- dump->u_debugreg[4] = 0;
- dump->u_debugreg[5] = 0;
- dump->u_debugreg[6] = current->thread.debugreg6;
-
- dump->u_debugreg[7] = dr7;
-}
-EXPORT_SYMBOL_GPL(aout_dump_debugregs);
-
/*
* Release the user breakpoints used by ptrace
*/
@@ -490,7 +454,7 @@ void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
t->ptrace_bps[i] = NULL;
}

- t->debugreg6 = 0;
+ t->virtual_dr6 = 0;
t->ptrace_dr7 = 0;
}

@@ -500,7 +464,7 @@ void hw_breakpoint_restore(void)
set_debugreg(__this_cpu_read(cpu_debugreg[1]), 1);
set_debugreg(__this_cpu_read(cpu_debugreg[2]), 2);
set_debugreg(__this_cpu_read(cpu_debugreg[3]), 3);
- set_debugreg(current->thread.debugreg6, 6);
+ set_debugreg(DR6_RESERVED, 6);
set_debugreg(__this_cpu_read(cpu_dr7), 7);
}
EXPORT_SYMBOL_GPL(hw_breakpoint_restore);
@@ -523,10 +487,10 @@ EXPORT_SYMBOL_GPL(hw_breakpoint_restore);
*/
static int hw_breakpoint_handler(struct die_args *args)
{
- int i, cpu, rc = NOTIFY_STOP;
+ int i, rc = NOTIFY_STOP;
struct perf_event *bp;
- unsigned long dr6;
unsigned long *dr6_p;
+ unsigned long dr6;

/* The DR6 value is pointed by args->err */
dr6_p = (unsigned long *)ERR_PTR(args->err);
@@ -540,14 +504,6 @@ static int hw_breakpoint_handler(struct die_args *args)
if ((dr6 & DR_TRAP_BITS) == 0)
return NOTIFY_DONE;

- /*
- * Assert that local interrupts are disabled
- * Reset the DRn bits in the virtualized register value.
- * The ptrace trigger routine will add in whatever is needed.
- */
- current->thread.debugreg6 &= ~DR_TRAP_BITS;
- cpu = get_cpu();
-
/* Handle all the breakpoints that were triggered */
for (i = 0; i < HBP_NUM; ++i) {
if (likely(!(dr6 & (DR_TRAP0 << i))))
@@ -561,7 +517,7 @@ static int hw_breakpoint_handler(struct die_args *args)
*/
rcu_read_lock();

- bp = per_cpu(bp_per_reg[i], cpu);
+ bp = this_cpu_read(bp_per_reg[i]);
/*
* Reset the 'i'th TRAP bit in dr6 to denote completion of
* exception handling
@@ -592,12 +548,10 @@ static int hw_breakpoint_handler(struct die_args *args)
* breakpoints (to generate signals) and b) when the system has
* taken exception due to multiple causes
*/
- if ((current->thread.debugreg6 & DR_TRAP_BITS) ||
+ if ((current->thread.virtual_dr6 & DR_TRAP_BITS) ||
(dr6 & (~DR_TRAP_BITS)))
rc = NOTIFY_DONE;

- put_cpu();
-
return rc;
}

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index c2f02f308ecf..ff7878df96b4 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -629,9 +629,10 @@ static void kgdb_hw_overflow_handler(struct perf_event *event,
struct task_struct *tsk = current;
int i;

- for (i = 0; i < 4; i++)
+ for (i = 0; i < 4; i++) {
if (breakinfo[i].enabled)
- tsk->thread.debugreg6 |= (DR_TRAP0 << i);
+ tsk->thread.virtual_dr6 |= (DR_TRAP0 << i);
+ }
}

void kgdb_arch_late(void)
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index e7537c5440bb..bedca011459c 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -465,7 +465,7 @@ static void ptrace_triggered(struct perf_event *bp,
break;
}

- thread->debugreg6 |= (DR_TRAP0 << i);
+ thread->virtual_dr6 |= (DR_TRAP0 << i);
}

/*
@@ -601,7 +601,7 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
if (bp)
val = bp->hw.info.address;
} else if (n == 6) {
- val = thread->debugreg6;
+ val = thread->virtual_dr6 ^ DR6_RESERVED; /* Flip back to arch polarity */
} else if (n == 7) {
val = thread->ptrace_dr7;
}
@@ -657,7 +657,7 @@ static int ptrace_set_debugreg(struct task_struct *tsk, int n,
if (n < HBP_NUM) {
rc = ptrace_set_breakpoint_addr(tsk, n, val);
} else if (n == 6) {
- thread->debugreg6 = val;
+ thread->virtual_dr6 = val ^ DR6_RESERVED; /* Flip to positive polarity */
rc = 0;
} else if (n == 7) {
rc = ptrace_write_dr7(tsk, val);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 81a2fb711091..df9c6554f83e 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -745,9 +745,21 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
* 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;
+ 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;
+
+ /*
+ * 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;
}
@@ -776,74 +788,20 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
*
* May run on IST stack.
*/
-static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
-{
- struct task_struct *tsk = current;
- bool user_icebp;
- int si_code;
-
- /*
- * 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 is zero, no point in trying to handle it. The kernel is
- * not using INT1.
- */
- if (!user && !dr6)
- return;

+static bool notify_debug(struct pt_regs *regs, unsigned long *dr6)
+{
/*
- * 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.
- * User wants a sigtrap for that.
+ * Notifiers will clear bits in @dr6 to indicate the event has been
+ * consumed - hw_breakpoint_handler(), single_stop_cont().
+ *
+ * Notifiers will set bits in @virtual_dr6 to indicate the desire
+ * for signals - ptrace_triggered(), kgdb_hw_overflow_handler().
*/
- user_icebp = user && !dr6;
-
- /* Store the virtualized DR6 value */
- tsk->thread.debugreg6 = dr6;
-
-#ifdef CONFIG_KPROBES
- if (kprobe_debug_handler(regs)) {
- return;
- }
-#endif
-
- if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, 0,
- SIGTRAP) == NOTIFY_STOP) {
- return;
- }
-
- /* It's safe to allow irq's after DR6 has been saved */
- cond_local_irq_enable(regs);
-
- if (v8086_mode(regs)) {
- handle_vm86_trap((struct kernel_vm86_regs *) regs, 0,
- X86_TRAP_DB);
- goto out;
- }
-
- if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) {
- /*
- * Historical junk that used to handle SYSENTER single-stepping.
- * This should be unreachable now. If we survive for a while
- * without anyone hitting this warning, we'll turn this into
- * an oops.
- */
- tsk->thread.debugreg6 &= ~DR_STEP;
- set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
- regs->flags &= ~X86_EFLAGS_TF;
- }
-
- si_code = get_si_code(tsk->thread.debugreg6);
- if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
- send_sigtrap(regs, 0, si_code);
+ if (notify_die(DIE_DEBUG, "debug", regs, (long)dr6, 0, SIGTRAP) == NOTIFY_STOP)
+ return true;

-out:
- cond_local_irq_disable(regs);
+ return false;
}

static __always_inline void exc_debug_kernel(struct pt_regs *regs,
@@ -877,8 +835,32 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
if ((dr6 & DR_STEP) && is_sysenter_singlestep(regs))
dr6 &= ~DR_STEP;

- handle_debug(regs, dr6, false);
+ if (kprobe_debug_handler(regs))
+ goto out;
+
+ /*
+ * The kernel doesn't use INT1
+ */
+ if (!dr6)
+ goto out;

+ if (notify_debug(regs, &dr6))
+ goto out;
+
+ /*
+ * The kernel doesn't use TF single-step outside of:
+ *
+ * - Kprobes, consumed through kprobe_debug_handler()
+ * - KGDB, consumed through notify_debug()
+ *
+ * So if we get here with DR_STEP set, something is wonky.
+ *
+ * A known way to trigger this is through QEMU's GDB stub,
+ * which leaks #DB into the guest and causes IST recursion.
+ */
+ if (WARN_ON_ONCE(dr6 & DR_STEP))
+ regs->flags &= ~X86_EFLAGS_TF;
+out:
instrumentation_end();
idtentry_exit_nmi(regs, irq_state);

@@ -888,6 +870,8 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
static __always_inline void exc_debug_user(struct pt_regs *regs,
unsigned long dr6)
{
+ bool icebp;
+
/*
* If something gets miswired and we end up here for a kernel mode
* #DB, we will malfunction.
@@ -906,8 +890,32 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
irqentry_enter_from_user_mode(regs);
instrumentation_begin();

- handle_debug(regs, dr6, true);
+ /*
+ * 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.
+ * User wants a sigtrap for that.
+ */
+ icebp = !dr6;

+ if (notify_debug(regs, &dr6))
+ goto out;
+
+ /* It's safe to allow irq's after DR6 has been saved */
+ local_irq_enable();
+
+ if (v8086_mode(regs)) {
+ handle_vm86_trap((struct kernel_vm86_regs *)regs, 0, X86_TRAP_DB);
+ goto out_irq;
+ }
+
+ /* Add the virtual_dr6 bits for signals. */
+ dr6 |= current->thread.virtual_dr6;
+ if (dr6 & (DR_STEP | DR_TRAP_BITS) || icebp)
+ send_sigtrap(regs, 0, get_si_code(dr6));
+
+out_irq:
+ local_irq_disable();
+out:
instrumentation_end();
irqentry_exit_to_user_mode(regs);
}