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2024-02-20 ea08eeccae9297f7aabd2ef7f0c2517ac4549acc

 kernel/arch/x86/include/asm/debugreg.h
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@@ -8,7 +8,7 @@
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 DECLARE_PER_CPU(unsigned long, cpu_dr7);
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-#ifndef CONFIG_PARAVIRT
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+#ifndef CONFIG_PARAVIRT_XXL
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 /*
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  * These special macros can be used to get or set a debugging register
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  */
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@@ -18,7 +18,7 @@
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 	native_set_debugreg(register, value)
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 #endif
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-static inline unsigned long native_get_debugreg(int regno)
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+static __always_inline unsigned long native_get_debugreg(int regno)
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 {
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 	unsigned long val = 0;	/* Damn you, gcc! */
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@@ -39,7 +39,20 @@
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 		asm("mov %%db6, %0" :"=r" (val));
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 		break;
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 	case 7:
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-		asm("mov %%db7, %0" :"=r" (val));
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+		/*
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+		 * Apply __FORCE_ORDER to DR7 reads to forbid re-ordering them
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+		 * with other code.
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+		 *
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+		 * This is needed because a DR7 access can cause a #VC exception
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+		 * when running under SEV-ES. Taking a #VC exception is not a
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+		 * safe thing to do just anywhere in the entry code and
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+		 * re-ordering might place the access into an unsafe location.
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+		 *
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+		 * This happened in the NMI handler, where the DR7 read was
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+		 * re-ordered to happen before the call to sev_es_ist_enter(),
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+		 * causing stack recursion.
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+		 */
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+		asm volatile("mov %%db7, %0" : "=r" (val) : __FORCE_ORDER);
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 		break;
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 	default:
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 		BUG();
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@@ -47,7 +60,7 @@
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 	return val;
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 }
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-static inline void native_set_debugreg(int regno, unsigned long value)
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+static __always_inline void native_set_debugreg(int regno, unsigned long value)
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 {
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 	switch (regno) {
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 	case 0:
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@@ -66,7 +79,16 @@
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 		asm("mov %0, %%db6"	::"r" (value));
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 		break;
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 	case 7:
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-		asm("mov %0, %%db7"	::"r" (value));
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+		/*
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+		 * Apply __FORCE_ORDER to DR7 writes to forbid re-ordering them
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+		 * with other code.
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+		 *
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+		 * While is didn't happen with a DR7 write (see the DR7 read
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+		 * comment above which explains where it happened), add the
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+		 * __FORCE_ORDER here too to avoid similar problems in the
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+		 * future.
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+		 */
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+		asm volatile("mov %0, %%db7"	::"r" (value), __FORCE_ORDER);
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 		break;
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 	default:
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 		BUG();
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@@ -85,35 +107,45 @@
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 	set_debugreg(0UL, 3);
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 }
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-static inline int hw_breakpoint_active(void)
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+static __always_inline bool hw_breakpoint_active(void)
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 {
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 	return __this_cpu_read(cpu_dr7) & DR_GLOBAL_ENABLE_MASK;
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 }
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-extern void aout_dump_debugregs(struct user *dump);
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-
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 extern void hw_breakpoint_restore(void);
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-#ifdef CONFIG_X86_64
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-DECLARE_PER_CPU(int, debug_stack_usage);
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-static inline void debug_stack_usage_inc(void)
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+static __always_inline unsigned long local_db_save(void)
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 {
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-	__this_cpu_inc(debug_stack_usage);
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+	unsigned long dr7;
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+
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+	if (static_cpu_has(X86_FEATURE_HYPERVISOR) && !hw_breakpoint_active())
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+		return 0;
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+
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+	get_debugreg(dr7, 7);
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+	dr7 &= ~0x400; /* architecturally set bit */
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+	if (dr7)
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+		set_debugreg(0, 7);
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+	/*
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+	 * Ensure the compiler doesn't lower the above statements into
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+	 * the critical section; disabling breakpoints late would not
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+	 * be good.
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+	 */
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+	barrier();
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+
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+	return dr7;
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 }
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-static inline void debug_stack_usage_dec(void)
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+
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+static __always_inline void local_db_restore(unsigned long dr7)
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 {
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-	__this_cpu_dec(debug_stack_usage);
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+	/*
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+	 * Ensure the compiler doesn't raise this statement into
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+	 * the critical section; enabling breakpoints early would
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+	 * not be good.
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+	 */
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+	barrier();
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+	if (dr7)
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+		set_debugreg(dr7, 7);
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 }
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-int is_debug_stack(unsigned long addr);
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-void debug_stack_set_zero(void);
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-void debug_stack_reset(void);
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-#else /* !X86_64 */
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-static inline int is_debug_stack(unsigned long addr) { return 0; }
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-static inline void debug_stack_set_zero(void) { }
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-static inline void debug_stack_reset(void) { }
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-static inline void debug_stack_usage_inc(void) { }
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-static inline void debug_stack_usage_dec(void) { }
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-#endif /* X86_64 */
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 #ifdef CONFIG_CPU_SUP_AMD
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 extern void set_dr_addr_mask(unsigned long mask, int dr);