efi: Implement mandatory locking for UEFI Runtime Services

According to section 7.1 of the UEFI spec, Runtime Services are not fully
reentrant, and there are particular combinations of calls that need to be
serialized. Use a spinlock to serialize all Runtime Services with respect
to all others, even if this is more than strictly needed.

We've managed to get away without requiring a runtime services lock
until now because most of the interactions with EFI involve EFI
variables, and those operations are already serialised with
__efivars->lock.

Some of the assumptions underlying the decision whether locks are
needed or not (e.g., SetVariable() against ResetSystem()) may not
apply universally to all [new] architectures that implement UEFI.
Rather than try to reason our way out of this, let's just implement at
least what the spec requires in terms of locking.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 144 insertions, 10 deletions

diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c
index 10daa4bbb258..9694cba665c4 100644
--- a/drivers/firmware/efi/runtime-wrappers.c
+++ b/drivers/firmware/efi/runtime-wrappers.c
@@ -14,11 +14,83 @@
  * This file is released under the GPLv2.
  */
 
+#include <linux/bug.h>
 #include <linux/efi.h>
-#include <linux/spinlock.h>             /* spinlock_t */
+#include <linux/mutex.h>
+#include <linux/spinlock.h>
 #include <asm/efi.h>
 
 /*
+ * According to section 7.1 of the UEFI spec, Runtime Services are not fully
+ * reentrant, and there are particular combinations of calls that need to be
+ * serialized. (source: UEFI Specification v2.4A)
+ *
+ * Table 31. Rules for Reentry Into Runtime Services
+ * +------------------------------------+-------------------------------+
+ * | If previous call is busy in        | Forbidden to call             |
+ * +------------------------------------+-------------------------------+
+ * | Any                                | SetVirtualAddressMap()        |
+ * +------------------------------------+-------------------------------+
+ * | ConvertPointer()                   | ConvertPointer()              |
+ * +------------------------------------+-------------------------------+
+ * | SetVariable()                      | ResetSystem()                 |
+ * | UpdateCapsule()                    |                               |
+ * | SetTime()                          |                               |
+ * | SetWakeupTime()                    |                               |
+ * | GetNextHighMonotonicCount()        |                               |
+ * +------------------------------------+-------------------------------+
+ * | GetVariable()                      | GetVariable()                 |
+ * | GetNextVariableName()              | GetNextVariableName()         |
+ * | SetVariable()                      | SetVariable()                 |
+ * | QueryVariableInfo()                | QueryVariableInfo()           |
+ * | UpdateCapsule()                    | UpdateCapsule()               |
+ * | QueryCapsuleCapabilities()         | QueryCapsuleCapabilities()    |
+ * | GetNextHighMonotonicCount()        | GetNextHighMonotonicCount()   |
+ * +------------------------------------+-------------------------------+
+ * | GetTime()                          | GetTime()                     |
+ * | SetTime()                          | SetTime()                     |
+ * | GetWakeupTime()                    | GetWakeupTime()               |
+ * | SetWakeupTime()                    | SetWakeupTime()               |
+ * +------------------------------------+-------------------------------+
+ *
+ * Due to the fact that the EFI pstore may write to the variable store in
+ * interrupt context, we need to use a spinlock for at least the groups that
+ * contain SetVariable() and QueryVariableInfo(). That leaves little else, as
+ * none of the remaining functions are actually ever called at runtime.
+ * So let's just use a single spinlock to serialize all Runtime Services calls.
+ */
+static DEFINE_SPINLOCK(efi_runtime_lock);
+
+/*
+ * Some runtime services calls can be reentrant under NMI, even if the table
+ * above says they are not. (source: UEFI Specification v2.4A)
+ *
+ * Table 32. Functions that may be called after Machine Check, INIT and NMI
+ * +----------------------------+------------------------------------------+
+ * | Function                   | Called after Machine Check, INIT and NMI |
+ * +----------------------------+------------------------------------------+
+ * | GetTime()                  | Yes, even if previously busy.            |
+ * | GetVariable()              | Yes, even if previously busy             |
+ * | GetNextVariableName()      | Yes, even if previously busy             |
+ * | QueryVariableInfo()        | Yes, even if previously busy             |
+ * | SetVariable()              | Yes, even if previously busy             |
+ * | UpdateCapsule()            | Yes, even if previously busy             |
+ * | QueryCapsuleCapabilities() | Yes, even if previously busy             |
+ * | ResetSystem()              | Yes, even if previously busy             |
+ * +----------------------------+------------------------------------------+
+ *
+ * In order to prevent deadlocks under NMI, the wrappers for these functions
+ * may only grab the efi_runtime_lock or rtc_lock spinlocks if !efi_in_nmi().
+ * However, not all of the services listed are reachable through NMI code paths,
+ * so the the special handling as suggested by the UEFI spec is only implemented
+ * for QueryVariableInfo() and SetVariable(), as these can be reached in NMI
+ * context through efi_pstore_write().
+ */
+#ifndef efi_in_nmi
+#define efi_in_nmi()    (0)
+#endif
+
+/*
  * As per commit ef68c8f87ed1 ("x86: Serialize EFI time accesses on rtc_lock"),
  * the EFI specification requires that callers of the time related runtime
  * functions serialize with other CMOS accesses in the kernel, as the EFI time
@@ -32,7 +104,9 @@ static efi_status_t virt_efi_get_time(efi_time_t *tm, efi_time_cap_t *tc)
         efi_status_t status;
 
         spin_lock_irqsave(&rtc_lock, flags);
+        spin_lock(&efi_runtime_lock);
         status = efi_call_virt(get_time, tm, tc);
+        spin_unlock(&efi_runtime_lock);
         spin_unlock_irqrestore(&rtc_lock, flags);
         return status;
 }
@@ -43,7 +117,9 @@ static efi_status_t virt_efi_set_time(efi_time_t *tm)
         efi_status_t status;
 
         spin_lock_irqsave(&rtc_lock, flags);
+        spin_lock(&efi_runtime_lock);
         status = efi_call_virt(set_time, tm);
+        spin_unlock(&efi_runtime_lock);
         spin_unlock_irqrestore(&rtc_lock, flags);
         return status;
 }
@@ -56,7 +132,9 @@ static efi_status_t virt_efi_get_wakeup_time(efi_bool_t *enabled,
         efi_status_t status;
 
         spin_lock_irqsave(&rtc_lock, flags);
+        spin_lock(&efi_runtime_lock);
         status = efi_call_virt(get_wakeup_time, enabled, pending, tm);
+        spin_unlock(&efi_runtime_lock);
         spin_unlock_irqrestore(&rtc_lock, flags);
         return status;
 }
@@ -67,7 +145,9 @@ static efi_status_t virt_efi_set_wakeup_time(efi_bool_t enabled, efi_time_t *tm)
         efi_status_t status;
 
         spin_lock_irqsave(&rtc_lock, flags);
+        spin_lock(&efi_runtime_lock);
         status = efi_call_virt(set_wakeup_time, enabled, tm);
+        spin_unlock(&efi_runtime_lock);
         spin_unlock_irqrestore(&rtc_lock, flags);
         return status;
 }
@@ -78,14 +158,27 @@ static efi_status_t virt_efi_get_variable(efi_char16_t *name,
                                           unsigned long *data_size,
                                           void *data)
 {
-        return efi_call_virt(get_variable, name, vendor, attr, data_size, data);
+        unsigned long flags;
+        efi_status_t status;
+
+        spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(get_variable, name, vendor, attr, data_size,
+                               data);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static efi_status_t virt_efi_get_next_variable(unsigned long *name_size,
                                                efi_char16_t *name,
                                                efi_guid_t *vendor)
 {
-        return efi_call_virt(get_next_variable, name_size, name, vendor);
+        unsigned long flags;
+        efi_status_t status;
+
+        spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(get_next_variable, name_size, name, vendor);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static efi_status_t virt_efi_set_variable(efi_char16_t *name,
@@ -94,7 +187,17 @@ static efi_status_t virt_efi_set_variable(efi_char16_t *name,
                                           unsigned long data_size,
                                           void *data)
 {
-        return efi_call_virt(set_variable, name, vendor, attr, data_size, data);
+        unsigned long flags;
+        efi_status_t status;
+        bool __in_nmi = efi_in_nmi();
+
+        if (!__in_nmi)
+                spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(set_variable, name, vendor, attr, data_size,
+                               data);
+        if (!__in_nmi)
+                spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static efi_status_t virt_efi_query_variable_info(u32 attr,
@@ -102,16 +205,31 @@ static efi_status_t virt_efi_query_variable_info(u32 attr,
                                                  u64 *remaining_space,
                                                  u64 *max_variable_size)
 {
+        unsigned long flags;
+        efi_status_t status;
+        bool __in_nmi = efi_in_nmi();
+
         if (efi.runtime_version < EFI_2_00_SYSTEM_TABLE_REVISION)
                 return EFI_UNSUPPORTED;
 
-        return efi_call_virt(query_variable_info, attr, storage_space,
-                             remaining_space, max_variable_size);
+        if (!__in_nmi)
+                spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(query_variable_info, attr, storage_space,
+                               remaining_space, max_variable_size);
+        if (!__in_nmi)
+                spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static efi_status_t virt_efi_get_next_high_mono_count(u32 *count)
 {
-        return efi_call_virt(get_next_high_mono_count, count);
+        unsigned long flags;
+        efi_status_t status;
+
+        spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(get_next_high_mono_count, count);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static void virt_efi_reset_system(int reset_type,
@@ -119,17 +237,27 @@ static void virt_efi_reset_system(int reset_type,
                                   unsigned long data_size,
                                   efi_char16_t *data)
 {
+        unsigned long flags;
+
+        spin_lock_irqsave(&efi_runtime_lock, flags);
         __efi_call_virt(reset_system, reset_type, status, data_size, data);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
 }
 
 static efi_status_t virt_efi_update_capsule(efi_capsule_header_t **capsules,
                                             unsigned long count,
                                             unsigned long sg_list)
 {
+        unsigned long flags;
+        efi_status_t status;
+
         if (efi.runtime_version < EFI_2_00_SYSTEM_TABLE_REVISION)
                 return EFI_UNSUPPORTED;
 
-        return efi_call_virt(update_capsule, capsules, count, sg_list);
+        spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(update_capsule, capsules, count, sg_list);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 static efi_status_t virt_efi_query_capsule_caps(efi_capsule_header_t **capsules,
@@ -137,11 +265,17 @@ static efi_status_t virt_efi_query_capsule_caps(efi_capsule_header_t **capsules,
                                                 u64 *max_size,
                                                 int *reset_type)
 {
+        unsigned long flags;
+        efi_status_t status;
+
         if (efi.runtime_version < EFI_2_00_SYSTEM_TABLE_REVISION)
                 return EFI_UNSUPPORTED;
 
-        return efi_call_virt(query_capsule_caps, capsules, count, max_size,
-                             reset_type);
+        spin_lock_irqsave(&efi_runtime_lock, flags);
+        status = efi_call_virt(query_capsule_caps, capsules, count, max_size,
+                               reset_type);
+        spin_unlock_irqrestore(&efi_runtime_lock, flags);
+        return status;
 }
 
 void efi_native_runtime_setup(void)