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

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

On Mon, May 08, 2023 at 09:31:23PM +0000, 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 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 <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

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>

take care,
  Gerd