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. Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136 Cc: Laszlo Ersek <ler...@redhat.com> Cc: Gerd Hoffmann <kra...@redhat.com> Cc: Oliver Steffen <ostef...@redhat.com> Cc: Pawel Polawski <ppola...@redhat.com> Cc: Jiewen Yao <jiewen....@intel.com> Cc: Ard Biesheuvel <ardb+tianoc...@kernel.org> Cc: Jordan Justen <jordan.l.jus...@intel.com> Michael Brown (2): OvmfPkg: Clarify invariants for NestedInterruptTplLib OvmfPkg: Relax assertion that interrupts do not occur at TPL_HIGH_LEVEL OvmfPkg/Library/NestedInterruptTplLib/Tpl.c | 31 +++++++++++++++++---- 1 file changed, 26 insertions(+), 5 deletions(-) -- 2.39.0 -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#104290): https://edk2.groups.io/g/devel/message/104290 Mute This Topic: https://groups.io/mt/98771393/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
