Re: [edk2-devel] [PATCH v2 0/2] OvmfPkg: Relax assertion that interrupts do not occur at TPL_HIGH_LEVEL

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

On 09/05/2023 14:31, Laszlo Ersek wrote:
> On 5/9/23 14:09, Michael Brown wrote:
>> 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.
>>
>> Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Cc: Gerd Hoffmann <kraxel@redhat.com>
>> Cc: Oliver Steffen <osteffen@redhat.com>
>> Cc: Pawel Polawski <ppolawsk@redhat.com>
>> Cc: Jiewen Yao <jiewen.yao@intel.com>
>> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
>> Cc: Jordan Justen <jordan.l.justen@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(-)
>>
> 
> series
> Acked-by: Laszlo Ersek <lersek@redhat.com>

Thank you!

Gerd: are you happy for your Reviewed-by to stand, since the only 
changes since v1 were to comment wording?  (My apologies for forgetting 
to include a v2 description in the cover letter.)

Thanks,

Michael