[PATCH 1/3] x86/hw_breakpoints: Make DR7 updates NMI safe
From: Jinchao Wang
Date: Sun Jul 26 2026 - 08:24:14 EST
Hardware breakpoint installation and removal run with IRQs disabled, but
an NMI can still enter the same code through KGDB. The interrupted
operation and the NMI can consequently claim the same slot or overwrite
each other's DR7 state.
Claim and release per-CPU slots with cmpxchg. Update cpu_dr7 with
single-instruction per-CPU operations, and preserve hardware-first
disable and hardware-last enable ordering. Add a per-CPU sequence number
so interrupted DR7 writers and restore paths detect an NMI update and
retry from the latest shadow state.
Signed-off-by: Jinchao Wang <wangjinchao600@xxxxxxxxx>
---
arch/x86/include/asm/debugreg.h | 36 ++++++++----
arch/x86/kernel/cpu/mce/core.c | 20 ++++---
arch/x86/kernel/hw_breakpoint.c | 98 ++++++++++++++++-----------------
arch/x86/kernel/nmi.c | 10 +++-
arch/x86/kernel/traps.c | 10 +++-
5 files changed, 101 insertions(+), 73 deletions(-)
diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index a2c1f2d24b64..b1fe1c47978d 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -18,6 +18,7 @@
#define DR7_FIXED_1 0x00000400
DECLARE_PER_CPU(unsigned long, cpu_dr7);
+DECLARE_PER_CPU(unsigned int, cpu_dr7_seq);
#ifndef CONFIG_PARAVIRT_XXL
/*
@@ -125,18 +126,20 @@ static __always_inline bool hw_breakpoint_active(void)
extern void hw_breakpoint_restore(void);
-static __always_inline unsigned long local_db_save(void)
+static __always_inline void local_db_save(unsigned long *dr7,
+ unsigned int *dr7_seq)
{
- unsigned long dr7;
+ *dr7 = 0;
+ *dr7_seq = this_cpu_read(cpu_dr7_seq);
if (static_cpu_has(X86_FEATURE_HYPERVISOR) && !hw_breakpoint_active())
- return 0;
+ return;
- get_debugreg(dr7, 7);
+ get_debugreg(*dr7, 7);
/* Architecturally set bit */
- dr7 &= ~DR7_FIXED_1;
- if (dr7)
+ *dr7 &= ~DR7_FIXED_1;
+ if (*dr7)
set_debugreg(DR7_FIXED_1, 7);
/*
@@ -145,20 +148,33 @@ static __always_inline unsigned long local_db_save(void)
* be good.
*/
barrier();
-
- return dr7;
}
-static __always_inline void local_db_restore(unsigned long dr7)
+static __always_inline void local_db_restore(unsigned long dr7,
+ unsigned int dr7_seq)
{
+ unsigned int seq;
+
/*
* Ensure the compiler doesn't raise this statement into
* the critical section; enabling breakpoints early would
* not be good.
*/
barrier();
- if (dr7)
+
+ do {
+ seq = this_cpu_read(cpu_dr7_seq);
+ if (seq == dr7_seq) {
+ if (!dr7)
+ return;
+ } else {
+ dr7 = this_cpu_read(cpu_dr7);
+ if (!dr7)
+ dr7 = DR7_FIXED_1;
+ }
+
set_debugreg(dr7, 7);
+ } while (unlikely(seq != this_cpu_read(cpu_dr7_seq)));
}
#ifdef CONFIG_CPU_SUP_AMD
diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 9bba1e2f03af..8dba9cd04bfa 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -2139,20 +2139,22 @@ static __always_inline void exc_machine_check_user(struct pt_regs *regs)
DEFINE_IDTENTRY_MCE(exc_machine_check)
{
unsigned long dr7;
+ unsigned int dr7_seq;
- dr7 = local_db_save();
+ local_db_save(&dr7, &dr7_seq);
exc_machine_check_kernel(regs);
- local_db_restore(dr7);
+ local_db_restore(dr7, dr7_seq);
}
/* The user mode variant. */
DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
{
unsigned long dr7;
+ unsigned int dr7_seq;
- dr7 = local_db_save();
+ local_db_save(&dr7, &dr7_seq);
exc_machine_check_user(regs);
- local_db_restore(dr7);
+ local_db_restore(dr7, dr7_seq);
}
#ifdef CONFIG_X86_FRED
@@ -2170,13 +2172,14 @@ DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
DEFINE_FREDENTRY_MCE(exc_machine_check)
{
unsigned long dr7;
+ unsigned int dr7_seq;
- dr7 = local_db_save();
+ local_db_save(&dr7, &dr7_seq);
if (user_mode(regs))
exc_machine_check_user(regs);
else
exc_machine_check_kernel(regs);
- local_db_restore(dr7);
+ local_db_restore(dr7, dr7_seq);
}
#endif
#else
@@ -2184,13 +2187,14 @@ DEFINE_FREDENTRY_MCE(exc_machine_check)
DEFINE_IDTENTRY_RAW(exc_machine_check)
{
unsigned long dr7;
+ unsigned int dr7_seq;
- dr7 = local_db_save();
+ local_db_save(&dr7, &dr7_seq);
if (user_mode(regs))
exc_machine_check_user(regs);
else
exc_machine_check_kernel(regs);
- local_db_restore(dr7);
+ local_db_restore(dr7, dr7_seq);
}
#endif
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index f846c15f21ca..9ef24b55737f 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -40,6 +40,9 @@
DEFINE_PER_CPU(unsigned long, cpu_dr7);
EXPORT_PER_CPU_SYMBOL(cpu_dr7);
+/* Sequence number of the per-CPU DR7 state. */
+DEFINE_PER_CPU(unsigned int, cpu_dr7_seq);
+
/* Per cpu debug address registers values */
static DEFINE_PER_CPU(unsigned long, cpu_debugreg[HBP_NUM]);
@@ -97,38 +100,30 @@ int decode_dr7(unsigned long dr7, int bpnum, unsigned *len, unsigned *type)
int arch_install_hw_breakpoint(struct perf_event *bp)
{
struct arch_hw_breakpoint *info = counter_arch_bp(bp);
- unsigned long *dr7;
+ unsigned int seq;
int i;
lockdep_assert_irqs_disabled();
for (i = 0; i < HBP_NUM; i++) {
- struct perf_event **slot = this_cpu_ptr(&bp_per_reg[i]);
-
- if (!*slot) {
- *slot = bp;
+ if (!this_cpu_cmpxchg(bp_per_reg[i], NULL, bp))
break;
- }
}
if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
return -EBUSY;
- set_debugreg(info->address, i);
- __this_cpu_write(cpu_debugreg[i], info->address);
-
- dr7 = this_cpu_ptr(&cpu_dr7);
- *dr7 |= encode_dr7(i, info->len, info->type);
-
- /*
- * Ensure we first write cpu_dr7 before we set the DR7 register.
- * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
- */
- barrier();
-
- set_debugreg(*dr7, 7);
- if (info->mask)
- amd_set_dr_addr_mask(info->mask, i);
+ do {
+ seq = this_cpu_inc_return(cpu_dr7_seq);
+ this_cpu_write(cpu_debugreg[i], info->address);
+ barrier();
+ set_debugreg(info->address, i);
+ if (info->mask)
+ amd_set_dr_addr_mask(info->mask, i);
+ this_cpu_or(cpu_dr7, encode_dr7(i, info->len, info->type));
+ barrier();
+ set_debugreg(this_cpu_read(cpu_dr7), 7);
+ } while (seq != this_cpu_read(cpu_dr7_seq));
return 0;
}
@@ -146,36 +141,33 @@ void arch_uninstall_hw_breakpoint(struct perf_event *bp)
{
struct arch_hw_breakpoint *info = counter_arch_bp(bp);
unsigned long dr7;
+ unsigned int seq;
int i;
lockdep_assert_irqs_disabled();
for (i = 0; i < HBP_NUM; i++) {
- struct perf_event **slot = this_cpu_ptr(&bp_per_reg[i]);
-
- if (*slot == bp) {
- *slot = NULL;
+ if (this_cpu_read(bp_per_reg[i]) == bp)
break;
- }
}
if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
return;
- dr7 = this_cpu_read(cpu_dr7);
- dr7 &= ~__encode_dr7(i, info->len, info->type);
-
- set_debugreg(dr7, 7);
- if (info->mask)
- amd_set_dr_addr_mask(0, i);
-
- /*
- * Ensure the write to cpu_dr7 is after we've set the DR7 register.
- * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
- */
- barrier();
-
- this_cpu_write(cpu_dr7, dr7);
+ do {
+ seq = this_cpu_inc_return(cpu_dr7_seq);
+ dr7 = this_cpu_read(cpu_dr7);
+ dr7 &= ~__encode_dr7(i, info->len, info->type);
+ set_debugreg(dr7, 7);
+ if (info->mask)
+ amd_set_dr_addr_mask(0, i);
+ barrier();
+ this_cpu_and(cpu_dr7,
+ ~__encode_dr7(i, info->len, info->type));
+ } while (seq != this_cpu_read(cpu_dr7_seq));
+
+ WARN_ONCE(this_cpu_cmpxchg(bp_per_reg[i], bp, NULL) != bp,
+ "Can't release breakpoint slot");
}
static int arch_bp_generic_len(int x86_len)
@@ -309,13 +301,14 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
sizeof(struct tlb_state)))
return true;
- /*
- * When in guest (X86_FEATURE_HYPERVISOR), local_db_save()
- * will read per-cpu cpu_dr7 before clear dr7 register.
- */
+ /* local_db_save() reads this state before clearing DR7. */
if (within_area(addr, end, (unsigned long)&per_cpu(cpu_dr7, cpu),
sizeof(cpu_dr7)))
return true;
+ if (within_area(addr, end,
+ (unsigned long)&per_cpu(cpu_dr7_seq, cpu),
+ sizeof(cpu_dr7_seq)))
+ return true;
}
return false;
@@ -483,12 +476,17 @@ void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
void hw_breakpoint_restore(void)
{
- set_debugreg(__this_cpu_read(cpu_debugreg[0]), 0);
- 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(DR6_RESERVED, 6);
- set_debugreg(__this_cpu_read(cpu_dr7), 7);
+ unsigned int seq;
+
+ do {
+ seq = this_cpu_inc_return(cpu_dr7_seq);
+ set_debugreg(this_cpu_read(cpu_debugreg[0]), 0);
+ 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(DR6_RESERVED, 6);
+ set_debugreg(this_cpu_read(cpu_dr7), 7);
+ } while (seq != this_cpu_read(cpu_dr7_seq));
}
EXPORT_SYMBOL_FOR_KVM(hw_breakpoint_restore);
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 3c9f60d6ca5a..f55a0cbd5927 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -532,10 +532,13 @@ enum nmi_states {
static DEFINE_PER_CPU(enum nmi_states, nmi_state);
static DEFINE_PER_CPU(unsigned long, nmi_cr2);
static DEFINE_PER_CPU(unsigned long, nmi_dr7);
+static DEFINE_PER_CPU(unsigned int, nmi_dr7_seq);
DEFINE_IDTENTRY_RAW(exc_nmi)
{
irqentry_state_t irq_state;
+ unsigned long dr7;
+ unsigned int dr7_seq;
struct nmi_stats *nsp = this_cpu_ptr(&nmi_stats);
/*
@@ -572,7 +575,9 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
*/
sev_es_ist_enter(regs);
- this_cpu_write(nmi_dr7, local_db_save());
+ local_db_save(&dr7, &dr7_seq);
+ this_cpu_write(nmi_dr7, dr7);
+ this_cpu_write(nmi_dr7_seq, dr7_seq);
irq_state = irqentry_nmi_enter(regs);
@@ -594,7 +599,8 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
irqentry_nmi_exit(regs, irq_state);
- local_db_restore(this_cpu_read(nmi_dr7));
+ local_db_restore(this_cpu_read(nmi_dr7),
+ this_cpu_read(nmi_dr7_seq));
sev_es_ist_exit();
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 30aa8369957e..018abe736285 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1231,8 +1231,12 @@ static noinstr void exc_debug_kernel(struct pt_regs *regs, unsigned long dr6)
* it results in an endless recursion and stack overflow. Thus we stay
* with the IDT approach, i.e., save DR7 and disable #DB.
*/
- unsigned long dr7 = local_db_save();
- irqentry_state_t irq_state = irqentry_nmi_enter(regs);
+ unsigned long dr7;
+ unsigned int dr7_seq;
+ irqentry_state_t irq_state;
+
+ local_db_save(&dr7, &dr7_seq);
+ irq_state = irqentry_nmi_enter(regs);
instrumentation_begin();
/*
@@ -1289,7 +1293,7 @@ static noinstr void exc_debug_kernel(struct pt_regs *regs, unsigned long dr6)
instrumentation_end();
irqentry_nmi_exit(regs, irq_state);
- local_db_restore(dr7);
+ local_db_restore(dr7, dr7_seq);
}
static noinstr void exc_debug_user(struct pt_regs *regs, unsigned long dr6)
--
2.53.0