Why does the firmware memory region have no permissions?

[Chang, Abner (HPS SW/FW Technologist)]

> -----Original Message-----
> From: Anup Patel [mailto:Anup.Patel at wdc.com]
> Sent: Friday, May 14, 2021 7:58 PM
> To: Daniel Schaefer <daniel at danielschaefer.me>
> Cc: opensbi at lists.infradead.org; Chang, Abner (HPS SW/FW Technologist)
> <abner.chang at hpe.com>
> Subject: RE: Why does the firmware memory region have no permissions?
> 
> 
> 
> > -----Original Message-----
> > From: Anup Patel
> > Sent: 14 May 2021 17:22
> > To: Daniel Schaefer <daniel at danielschaefer.me>
> > Cc: opensbi at lists.infradead.org; Chang, Abner (HPS SW/FW Technologist)
> > <abner.chang at hpe.com>
> > Subject: RE: Why does the firmware memory region have no permissions?
> >
> >
> >
> > > -----Original Message-----
> > > From: opensbi <opensbi-bounces at lists.infradead.org> On Behalf Of
> > > Daniel Schaefer
> > > Sent: 13 May 2021 10:26
> > > To: Anup Patel <Anup.Patel at wdc.com>
> > > Cc: opensbi at lists.infradead.org; Chang, Abner (HPS SW/FW Technologist)
> > > <abner.chang at hpe.com>
> > > Subject: Why does the firmware memory region have no permissions?
> > >
> > > Whoops, put CC as subject...
> > >
> > > On 5/12/21 6:20 PM, Daniel Schaefer wrote:
> > > > Hi Anup,
> > > >
> > > > I'm in the process of upgrading EDKII to OpenSBI 0.9 and using the
> > > > Generic
> > > Platform.
> > > >
> > > > Previously we were doing sbi_init with M-Mode, adding our SBI
> > > > extension and then calling sbi_switch_mode to switch to S-Mode. Now
> > > > sbi_init disallows initializing to M-Mode, so I'm directly switching
> > > > to S-Mode. It seems that even from S-Mode I can register our SBI
> > > extension with sbi_ecall_register_extension.
> > > > Is that correct?
> >
> > The OpenSBI sources are meant to run only from M-mode so we cannot
> > register SBI extension using sbi_ecall_register_extension().
> >
> > The  sbi_switch_mode() is not stricter due to OpenSBI domain support.
> 
> The sbi_hart_switch_mode() is fine. It's the sbi_domain_init() which is
> enforcing next booting stage to be at lower privilege for root domain.
> 
> For time being, you can try removing checks on "dom->next_mode"
> in sanitize_domain()

Hi Anup, I think we currently skip that check for moving on the edk2 boot process. So do you have plan to remove this check? Or any alternative?
I think it is unnecessary having this check on the next privilege mode. That should be at OEM discretion of which privilege mode to run their next firmware stage based on the platform design?

Thanks and regards,
Abner
> 
> >
> > Next booting stage has to run from lower privilege mode (S-mode or U-
> > mode) otherwise OpenSBI cannot protect itself from next booting stage if it
> > starts in M-mode.
> >
> > > >
> > > > However, sbi_init when directly initializing to S-Mode checks that
> > > > the
> > > start_address is executable.
> > > > So I'm wondering why the FW region isn't set as executable in OpenSBI?
> > > > How do other FWs like U-Boot get around this?
> > > >
> > > >
> > >
> > https://github.com/riscv/opensbi/commit/b1678af210dc4b4e6d586d6d96617
> > > e
> > > > 9641618994#diff-
> > > 6e8e352a8a90ba5a7adbb58a806ed9b6404c2c67db416332f9c05a
> > > > 6b322eecd6R346
> > > >
> > > > If I try to set my own regions by adding .domains_root_regions I get
> > > > another error because OpenSBI checks that I have a region that is
> > > > the same as the FW region added by OpenSBI. If I duplicate the FW
> > > > region and mark the first one as executable I can pass the
> > > > executable check and also the check that there's an FW region.
> > > > Additionally we have to manually call pmp_set in our custom platform
> > > > to make the FW region RWX.
> > > >
> > > > That seems like a workaround, however. Do you have any suggestion to
> > > properly fix it?
> > > > I'm sure we're misunderstand something.
> >
> > I suggest two things:
> > 1) Register your custom SBI extension from M-mode only before switching to
> > S-mode
> > 2) Make sure that fw_start and fw_size set in the sbi_scratch for each HART
> > only point to the M-mode code and data. Preferably have S-mode code and
> > data not linked in the same binary as M-mode code and data.
> >
> > > >
> > > > For context: We're writing the start addr and size of our FW image
> > > > into the scratch space before OpenSBI is initialized. Therefore
> > > > we're expecting it to set the PMP settings correctly.
> > > >
> > > > Please check out my workaround commit:
> > > > https://github.com/riscv/riscv-edk2-platforms/commit/a5ac63096ca5da7
> > > > 95
> > > > 042baf650170643fe219cab
> > > >
> > > > Thanks,
> > > > Daniel
> >
> > Regards
> > Anup
> 
> Regards,
> Anup