x86/sev: Make sure IRQs are disabled while GHCB is active

The #VC handler only cares about IRQs being disabled while the GHCB is
active, as it must not be interrupted by something which could cause
another #VC while it holds the GHCB (NMI is the exception for which the
backup GHCB exits).

Make sure nothing interrupts the code path while the GHCB is active
by making sure that callers of __sev_{get,put}_ghcb() have disabled
interrupts upfront.

 [ bp: Massage commit message. ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210618115409.22735-2-joro@8bytes.org

arch/x86/kernel/sev.c

@@ -12,7 +12,6 @@
 #include <linux/sched/debug.h>	/* For show_regs() */
 #include <linux/percpu-defs.h>
 #include <linux/mem_encrypt.h>
-#include <linux/lockdep.h>
 #include <linux/printk.h>
 #include <linux/mm_types.h>
 #include <linux/set_memory.h>
@@ -192,11 +191,19 @@ void noinstr __sev_es_ist_exit(void)
 	this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], *(unsigned long *)ist);
 }
 
-static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
+/*
+ * Nothing shall interrupt this code path while holding the per-CPU
+ * GHCB. The backup GHCB is only for NMIs interrupting this path.
+ *
+ * Callers must disable local interrupts around it.
+ */
+static noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
 {
 	struct sev_es_runtime_data *data;
 	struct ghcb *ghcb;
 
+	WARN_ON(!irqs_disabled());
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
@@ -213,7 +220,9 @@ static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
 			data->ghcb_active        = false;
 			data->backup_ghcb_active = false;
 
+			instrumentation_begin();
 			panic("Unable to handle #VC exception! GHCB and Backup GHCB are already in use");
+			instrumentation_end();
 		}
 
 		/* Mark backup_ghcb active before writing to it */
@@ -486,11 +495,13 @@ static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt
 /* Include code shared with pre-decompression boot stage */
 #include "sev-shared.c"
 
-static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
+static noinstr void __sev_put_ghcb(struct ghcb_state *state)
 {
 	struct sev_es_runtime_data *data;
 	struct ghcb *ghcb;
 
+	WARN_ON(!irqs_disabled());
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
@@ -514,7 +525,7 @@ void noinstr __sev_es_nmi_complete(void)
 	struct ghcb_state state;
 	struct ghcb *ghcb;
 
-	ghcb = sev_es_get_ghcb(&state);
+	ghcb = __sev_get_ghcb(&state);
 
 	vc_ghcb_invalidate(ghcb);
 	ghcb_set_sw_exit_code(ghcb, SVM_VMGEXIT_NMI_COMPLETE);
@@ -524,7 +535,7 @@ void noinstr __sev_es_nmi_complete(void)
 	sev_es_wr_ghcb_msr(__pa_nodebug(ghcb));
 	VMGEXIT();
 
-	sev_es_put_ghcb(&state);
+	__sev_put_ghcb(&state);
 }
 
 static u64 get_jump_table_addr(void)
@@ -536,7 +547,7 @@ static u64 get_jump_table_addr(void)
 
 	local_irq_save(flags);
 
-	ghcb = sev_es_get_ghcb(&state);
+	ghcb = __sev_get_ghcb(&state);
 
 	vc_ghcb_invalidate(ghcb);
 	ghcb_set_sw_exit_code(ghcb, SVM_VMGEXIT_AP_JUMP_TABLE);
@@ -550,7 +561,7 @@ static u64 get_jump_table_addr(void)
 	    ghcb_sw_exit_info_2_is_valid(ghcb))
 		ret = ghcb->save.sw_exit_info_2;
 
-	sev_es_put_ghcb(&state);
+	__sev_put_ghcb(&state);
 
 	local_irq_restore(flags);
 
@@ -675,7 +686,7 @@ static void sev_es_ap_hlt_loop(void)
 	struct ghcb_state state;
 	struct ghcb *ghcb;
 
-	ghcb = sev_es_get_ghcb(&state);
+	ghcb = __sev_get_ghcb(&state);
 
 	while (true) {
 		vc_ghcb_invalidate(ghcb);
@@ -692,7 +703,7 @@ static void sev_es_ap_hlt_loop(void)
 			break;
 	}
 
-	sev_es_put_ghcb(&state);
+	__sev_put_ghcb(&state);
 }
 
 /*
@@ -1351,7 +1362,6 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 	}
 
 	irq_state = irqentry_nmi_enter(regs);
-	lockdep_assert_irqs_disabled();
 	instrumentation_begin();
 
 	/*
@@ -1360,7 +1370,7 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 	 * keep the IRQs disabled to protect us against concurrent TLB flushes.
 	 */
 
-	ghcb = sev_es_get_ghcb(&state);
+	ghcb = __sev_get_ghcb(&state);
 
 	vc_ghcb_invalidate(ghcb);
 	result = vc_init_em_ctxt(&ctxt, regs, error_code);
@@ -1368,7 +1378,7 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 	if (result == ES_OK)
 		result = vc_handle_exitcode(&ctxt, ghcb, error_code);
 
-	sev_es_put_ghcb(&state);
+	__sev_put_ghcb(&state);
 
 	/* Done - now check the result */
 	switch (result) {