From 7272c2fbe66941f0785be7ec437ed79ab9e35b80 Mon Sep 17 00:00:00 2001 From: Michael Brown Date: Tue, 9 May 2023 12:09:33 +0000 Subject: [PATCH 17/18] OvmfPkg: Relax assertion that interrupts do not occur at 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 an interrupt occurs at TPL_HIGH_LEVEL. The specification also restricts usage of TPL_HIGH_LEVEL to the firmware itself. However, nothing actually prevents a UEFI application from calling gBS->RaiseTPL(TPL_HIGH_LEVEL) and then 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 UEFI applications such as these versions of the Microsoft Windows bootloader to continue to function. Debugged-by: Gerd Hoffmann Debugged-by: Laszlo Ersek Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136 Signed-off-by: Michael Brown Acked-by: Laszlo Ersek Reviewed-by: Gerd Hoffmann (cherry picked from commit bee67e0c142af6599a85aa7640094816b8a24c4f) --- 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..d56c12a44529 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 actually prevents a UEFI application from + // invalidly calling gBS->RaiseTPL(TPL_HIGH_LEVEL) and then + // 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, "ERROR: Interrupts enabled at TPL_HIGH_LEVEL!\n")); + } return InterruptedTPL; } -- 2.40.1