[edk2-devel] [PATCH v2 1/1] OvmfPkg/IoMmuDxe: add locking to IoMmuAllocateBounceBuffer

["Gerd Hoffmann" <kraxel@redhat.com>]

Searching for an unused bounce buffer in mReservedMemBitmap and
reserving the buffer by flipping the bit is a critical section
which must not be interrupted.  Raise the TPL level to ensure
that.

Without this fix it can happen that IoMmuDxe hands out the same
bounce buffer twice, causing trouble down the road.  Seen happening
in practice with VirtioNetDxe setting up the network interface (and
calling into IoMmuDxe from a polling timer callback) in parallel with
Boot Manager doing some disk I/O.  An ASSERT() in VirtioNet caught
the buffer inconsistency.

Full story with lots of details and discussions is available here:
https://bugzilla.redhat.com/show_bug.cgi?id=2211060

v2:
 - add locking to IoMmuFreeBounceBuffer too, clearing bits in
   mReservedMemBitmap is not guaranteed to be atomic (Michael Brown).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 OvmfPkg/IoMmuDxe/IoMmuBuffer.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/OvmfPkg/IoMmuDxe/IoMmuBuffer.c b/OvmfPkg/IoMmuDxe/IoMmuBuffer.c
index c8f6cf4818e8..103003cae376 100644
--- a/OvmfPkg/IoMmuDxe/IoMmuBuffer.c
+++ b/OvmfPkg/IoMmuDxe/IoMmuBuffer.c
@@ -367,7 +367,9 @@ IoMmuAllocateBounceBuffer (
 {
   EFI_STATUS  Status;
   UINT32      ReservedMemBitmap;
+  EFI_TPL     OldTpl;
 
+  OldTpl            = gBS->RaiseTPL (TPL_NOTIFY);
   ReservedMemBitmap = 0;
   Status            = InternalAllocateBuffer (
                         Type,
@@ -378,6 +380,7 @@ IoMmuAllocateBounceBuffer (
                         );
   MapInfo->ReservedMemBitmap = ReservedMemBitmap;
   mReservedMemBitmap        |= ReservedMemBitmap;
+  gBS->RestoreTPL (OldTpl);
 
   ASSERT (Status == EFI_SUCCESS);
 
@@ -395,6 +398,8 @@ IoMmuFreeBounceBuffer (
   IN OUT     MAP_INFO  *MapInfo
   )
 {
+  EFI_TPL  OldTpl;
+
   if (MapInfo->ReservedMemBitmap == 0) {
     gBS->FreePages (MapInfo->PlainTextAddress, MapInfo->NumberOfPages);
   } else {
@@ -407,9 +412,11 @@ IoMmuFreeBounceBuffer (
       mReservedMemBitmap,
       mReservedMemBitmap & ((UINT32)(~MapInfo->ReservedMemBitmap))
       ));
+    OldTpl                     = gBS->RaiseTPL (TPL_NOTIFY);
     MapInfo->PlainTextAddress  = 0;
     mReservedMemBitmap        &= (UINT32)(~MapInfo->ReservedMemBitmap);
     MapInfo->ReservedMemBitmap = 0;
+    gBS->RestoreTPL (OldTpl);
   }
 
   return EFI_SUCCESS;
-- 
2.41.0



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