[PATCH 2/2] OvmfPkg: Relax assertion that interrupts do not occur at TPL_HIGH_LEVEL

["Michael Brown" <mcb30@ipxe.org>]

At TPL_HIGH_LEVEL, CPU interrupts are disabled (as per the UEFI
specification) and so we should never encounter a situation in which
an interrupt occurs at TPL_HIGH_LEVEL.  The specification also
restricts usage of TPL_HIGH_LEVEL to the firmware itself.

However, nothing prevents a rogue UEFI application from illegally
calling gBS->RaiseTPL(TPL_HIGH_LEVEL) and then deliberately violating
the invariant by enabling interrupts via the STI or equivalent
instruction.  Some versions of the Microsoft Windows bootloader are
known to do this.

NestedInterruptTplLib maintains the invariant that interrupts are
disabled at TPL_HIGH_LEVEL (even when performing the dark art of
deliberately manipulating the stack so that IRET will return with
interrupts still disabled), but does not itself rely on external code
maintaining this invariant.

Relax the assertion that the interrupted TPL is below TPL_HIGH_LEVEL
to an error message, to allow rogue UEFI applications such as the
Microsoft Windows bootloader to continue to function.

Debugged-by: Gerd Hoffmann <kraxel@redhat.com>
Debugged-by: Laszlo Ersek <lersek@redhat.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136
Signed-off-by: Michael Brown <mcb30@ipxe.org>
---
 OvmfPkg/Library/NestedInterruptTplLib/Tpl.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c b/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
index e921a09c5599..a91f2d3cb8c7 100644
--- a/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
+++ b/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
@@ -34,12 +34,27 @@ NestedInterruptRaiseTPL (
 
   //
   // Raise TPL and assert that we were called from within an interrupt
-  // handler (i.e. with TPL below TPL_HIGH_LEVEL but with interrupts
-  // disabled).
+  // handler (i.e. with interrupts already disabled before raising the
+  // TPL).
   //
   ASSERT (GetInterruptState () == FALSE);
   InterruptedTPL = gBS->RaiseTPL (TPL_HIGH_LEVEL);
-  ASSERT (InterruptedTPL < TPL_HIGH_LEVEL);
+
+  //
+  // At TPL_HIGH_LEVEL, CPU interrupts are disabled (as per the UEFI
+  // specification) and so we should never encounter a situation in
+  // which InterruptedTPL==TPL_HIGH_LEVEL.  The specification also
+  // restricts usage of TPL_HIGH_LEVEL to the firmware itself.
+  //
+  // However, nothing prevents a rogue UEFI application from illegally
+  // calling gBS->RaiseTPL(TPL_HIGH_LEVEL) and then deliberately
+  // violating the invariant by enabling interrupts via the STI or
+  // equivalent instruction.  Some versions of the Microsoft Windows
+  // bootloader are known to do this.
+  //
+  if (InterruptedTPL >= TPL_HIGH_LEVEL) {
+    DEBUG ((DEBUG_ERROR, "Illegal interrupt at TPL_HIGH_LEVEL!\n"));
+  }
 
   return InterruptedTPL;
 }
-- 
2.39.0