[PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: Race condition between APHandler's release of Busy spinlock and user-triggered SmmStartupThisAP's

[Damian Nikodem <damian.nikodem@intel.com>]

Race condition between APHandler's release of Busy spinlock and
user-triggered SmmStartupThisAP's acquisition attempt of the Busy spinlock (non-blocking mode).

UserProc is the user's procedure to execute on an AP.
UserProcCompletion is the user procedure's completion spinlock.
All other variables are from EDK2.

BSP                                          AP
=====================================================================================
											 APHandler ()
											   WaitForSemaphore (Run)

								   << initial state >>

AcquireSpinLock (UserProcCompletion)
SmmStartupThisAp (Procedure)
  AcquireSpinLockOrFail (Busy)
  ReleaseSemaphore (Run)
											   UserProc ()
DoStuff()                                        DoSomeOtherStuff ()

AcquireSpinLockOrFail (UserProcCompletion) AcquireSpinLockOrFail (UserProcCompletion)

^^ waiting in a loop for user procedure's
   completion == these fail
ReleaseSpinLock (UserProcCompletion)       AcquireSpinLockOrFail (UserProcCompletion)

^^ this succeeds

ReleaseSpinLock (UserProcCompletion)

<< return control to the caller and
   reenter the flow >>>

AcquireSpinLock (UserProcCompletion)
SmmStartupThisAp (Procedure)
  AcquireSpinLockOrFail (Busy)
  ^^ this wins the race with AP's
	 ReleaseSpinLock and fails;
											   ReleaseSpinLock (Busy)
	return EFI_INVALID_PARAMETER;

To remedy, if AcquireSpinLockOrFail (of the Busy spinlock) fails, perform regular AcquireSpinLock -- this eliminates the race condition.

Signed-off-by: Damian Nikodem <damian.nikodem@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Benjamin You <benjamin.you@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Krzysztof Rusocki <krzysztof.rusocki@intel.com>

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
index d8d2b6f444..206e196a76 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
@@ -1239,8 +1239,16 @@ InternalSmmStartupThisAp (
     AcquireSpinLock (mSmmMpSyncData->CpuData[CpuIndex].Busy);
   } else {
     if (!AcquireSpinLockOrFail (mSmmMpSyncData->CpuData[CpuIndex].Busy)) {
-      DEBUG((DEBUG_ERROR, "Can't acquire mSmmMpSyncData->CpuData[%d].Busy\n", CpuIndex));
-      return EFI_NOT_READY;
+      DEBUG ((DEBUG_INFO, "BSP[%d] finds AP[%d] busy at proc 0x%llX (param 0x%llX), ",
+        mSmmMpSyncData->BspIndex,
+        CpuIndex,
+        *mSmmMpSyncData->CpuData[CpuIndex].Procedure,
+        (VOID*)mSmmMpSyncData->CpuData[CpuIndex].Parameter));
+      DEBUG ((DEBUG_INFO, "new proc 0x%llX (param 0x%llX). Waiting for the previous AP procedure to complete...\n",
+        Procedure,
+        ProcArguments));
+
+      AcquireSpinLock (mSmmMpSyncData->CpuData[CpuIndex].Busy);
     }
 
     *Token = (MM_COMPLETION) CreateToken ();
--------------------------------------------------------------------

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