Hi Okash,

Introduced build option [1] limits to MTE1 that supports MTE related instructions, but does not offer any functional usage in terms of detecting tag check faults. That's why memory is not configured as normal tagged (which is an MTE2 feature) in BL31, and support for MTE2 for BL31 is a topic for technical debt.

[1]: https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmware-a/+...

Thanks, Manish Badarkhe

From: Okash Khawaja okash@google.com Date: Tuesday, 5 April 2022 at 16:13 To: Manish Badarkhe Manish.Badarkhe@arm.com Cc: tf-a@lists.trustedfirmware.org tf-a@lists.trustedfirmware.org Subject: Re: [TF-A] Status of MTE for bl31 Hi Manish,

That's another question I wanted to follow up with. Is there any real benefit of MTE for BL31 given that it doesn't allocate memory dynamically. I guess stack tagging is the only use of MTE in BL31?

The concern in the original email is whether the support is incomplete. Can you confirm?

Thanks, Okash

On Tue, Apr 5, 2022 at 3:27 PM Manish Badarkhe <Manish.Badarkhe@arm.commailto:Manish.Badarkhe@arm.com> wrote: Hi Okash,

Could you please let us know the real use case/requirement where you want MTE to be available at EL3?

Thanks, Manish Badarkhe

From: Okash Khawaja via TF-A <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Date: Friday, 1 April 2022 at 19:51 To: tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Subject: [TF-A] Status of MTE for bl31 Hi,

I wanted to check the status of MTE support for bl31 itself. It seems like the support was added [1] for clang and armclang but I couldn't find the memory attribute to map pages as tag checked [2]. Is there something I missed?

Thanks, Okash

[1] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmware-a/+...

[2] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmware-a/+... -- TF-A mailing list -- tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.orgmailto:tf-a-leave@lists.trustedfirmware.org

Hi Okash My 2 cents on this.. Static analyzers can flag use-after-free stack object issues although it is not guaranteed to catch all issues.

Another factor to consider is that BL31 typically runs out of SRAM and it does seem unlikely the hardware implements tag memory for SRAM. So it could be that MTE2 for BL31 would remain an FVP only feature or only for those platforms which allows BL31 to run out of DDR.

So, given the above situation, value in enabling MTE2 for BL31 seems limited.

Best Regards Soby Mathew

-----Original Message----- From: Okash Khawaja via TF-A tf-a@lists.trustedfirmware.org Sent: 07 April 2022 11:41 To: Manish Badarkhe Manish.Badarkhe@arm.com Cc: tf-a@lists.trustedfirmware.org Subject: [TF-A] Re: Status of MTE for bl31

Hi Manish,

Thanks for confirming. Regarding benefit of MTE in BL31, it will basically add guards against stack overflows and use-after-free bugs related to objects allocated on stack. Regarding stack overflow, TF-A already has stack canaries. MTE protection will be stronger than canaries. So MTE adds a marginal benefit here. Regarding use-after-free of stack objects, I'm not sure if TF-A has adequate protections. On the down side, MTE adds performance overhead and increases code size. We probably need some numbers to see the overhead of MTE.

It will be good to know what the community thinks are the benefits of MTE are on balance.

Thanks, Okash

On Thu, Apr 7, 2022 at 9:21 AM Manish Badarkhe Manish.Badarkhe@arm.com wrote:

...

Hi Okash,

Introduced build option [1] limits to MTE1 that supports MTE related instructions, but does not offer any functional usage in terms of detecting tag check faults. That's why memory is not configured as normal tagged (which is an MTE2 feature) in BL31, and support for MTE2 for BL31 is a topic for technical debt.

re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185

Thanks, Manish Badarkhe

From: Okash Khawaja okash@google.com Date: Tuesday, 5 April 2022 at 16:13 To: Manish Badarkhe Manish.Badarkhe@arm.com Cc: tf-a@lists.trustedfirmware.org tf-a@lists.trustedfirmware.org Subject: Re: [TF-A] Status of MTE for bl31

Hi Manish,

That's another question I wanted to follow up with. Is there any real benefit

of MTE for BL31 given that it doesn't allocate memory dynamically. I guess stack tagging is the only use of MTE in BL31?

...

The concern in the original email is whether the support is incomplete. Can

you confirm?

...

Thanks,

Okash

On Tue, Apr 5, 2022 at 3:27 PM Manish Badarkhe

Manish.Badarkhe@arm.com wrote:

...

Hi Okash,

Could you please let us know the real use case/requirement where you want

MTE to be available at EL3?

...

Thanks, Manish Badarkhe

From: Okash Khawaja via TF-A tf-a@lists.trustedfirmware.org Date: Friday, 1 April 2022 at 19:51 To: tf-a@lists.trustedfirmware.org tf-a@lists.trustedfirmware.org Subject: [TF-A] Status of MTE for bl31

Hi,

I wanted to check the status of MTE support for bl31 itself. It seems like the support was added [1] for clang and armclang but I couldn't find the memory attribute to map pages as tag checked [2]. Is there something I missed?

Thanks, Okash

[1] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185

[2] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa re-a/+/refs/tags/v2.6/lib/xlat_tables_v2/aarch64/xlat_tables_arch.c#25 4 -- TF-A mailing list -- tf-a@lists.trustedfirmware.org To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.org

-- TF-A mailing list -- tf-a@lists.trustedfirmware.org To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.org

Hi Okash, There is some benefit in enabling MTE2 in BL31 as it is one more tool to detect code issues. This will probably remain as a development tool and not see any deployment on any real hardware platform for EL3 firmware I think. So, this would be nice to have but not a high priority task and can possibly be only enabled on FVP (or a platform which can run BL31 from DDR).

Best Regards Soby Mathew

From: Okash Khawaja okash@google.com Sent: 14 April 2022 16:46 To: Soby Mathew Soby.Mathew@arm.com Cc: Manish Badarkhe Manish.Badarkhe@arm.com; tf-a@lists.trustedfirmware.org Subject: Re: [TF-A] Re: Status of MTE for bl31

Thanks Soby. I agree, SRAM-based BL31 won't benefit from MTE.

So, given the above situation, value in enabling MTE2 for BL31 seems limited.

Sure. Do we still want to keep MTE1 around? Is there a real benefit of MTE1 without MTE2?

On Wed, Apr 13, 2022 at 4:59 PM Soby Mathew <Soby.Mathew@arm.commailto:Soby.Mathew@arm.com> wrote: Hi Okash My 2 cents on this.. Static analyzers can flag use-after-free stack object issues although it is not guaranteed to catch all issues.

Another factor to consider is that BL31 typically runs out of SRAM and it does seem unlikely the hardware implements tag memory for SRAM. So it could be that MTE2 for BL31 would remain an FVP only feature or only for those platforms which allows BL31 to run out of DDR.

So, given the above situation, value in enabling MTE2 for BL31 seems limited.

Best Regards Soby Mathew

-----Original Message----- From: Okash Khawaja via TF-A <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Sent: 07 April 2022 11:41 To: Manish Badarkhe <Manish.Badarkhe@arm.commailto:Manish.Badarkhe@arm.com> Cc: tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org Subject: [TF-A] Re: Status of MTE for bl31

Hi Manish,

Thanks for confirming. Regarding benefit of MTE in BL31, it will basically add guards against stack overflows and use-after-free bugs related to objects allocated on stack. Regarding stack overflow, TF-A already has stack canaries. MTE protection will be stronger than canaries. So MTE adds a marginal benefit here. Regarding use-after-free of stack objects, I'm not sure if TF-A has adequate protections. On the down side, MTE adds performance overhead and increases code size. We probably need some numbers to see the overhead of MTE.

It will be good to know what the community thinks are the benefits of MTE are on balance.

Thanks, Okash

On Thu, Apr 7, 2022 at 9:21 AM Manish Badarkhe <Manish.Badarkhe@arm.commailto:Manish.Badarkhe@arm.com> wrote:

...

Hi Okash,

Introduced build option [1] limits to MTE1 that supports MTE related instructions, but does not offer any functional usage in terms of detecting tag check faults. That's why memory is not configured as normal tagged (which is an MTE2 feature) in BL31, and support for MTE2 for BL31 is a topic for technical debt.

re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185

Thanks, Manish Badarkhe

From: Okash Khawaja <okash@google.commailto:okash@google.com> Date: Tuesday, 5 April 2022 at 16:13 To: Manish Badarkhe <Manish.Badarkhe@arm.commailto:Manish.Badarkhe@arm.com> Cc: tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Subject: Re: [TF-A] Status of MTE for bl31

Hi Manish,

That's another question I wanted to follow up with. Is there any real benefit

of MTE for BL31 given that it doesn't allocate memory dynamically. I guess stack tagging is the only use of MTE in BL31?

...

The concern in the original email is whether the support is incomplete. Can

you confirm?

...

Thanks,

Okash

On Tue, Apr 5, 2022 at 3:27 PM Manish Badarkhe

<Manish.Badarkhe@arm.commailto:Manish.Badarkhe@arm.com> wrote:

...

Hi Okash,

Could you please let us know the real use case/requirement where you want

MTE to be available at EL3?

...

Thanks, Manish Badarkhe

From: Okash Khawaja via TF-A <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Date: Friday, 1 April 2022 at 19:51 To: tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org> Subject: [TF-A] Status of MTE for bl31

Hi,

I wanted to check the status of MTE support for bl31 itself. It seems like the support was added [1] for clang and armclang but I couldn't find the memory attribute to map pages as tag checked [2]. Is there something I missed?

Thanks, Okash

[1] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185

[2] https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa re-a/+/refs/tags/v2.6/lib/xlat_tables_v2/aarch64/xlat_tables_arch.c#25 4 -- TF-A mailing list -- tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.orgmailto:tf-a-leave@lists.trustedfirmware.org

-- TF-A mailing list -- tf-a@lists.trustedfirmware.orgmailto:tf-a@lists.trustedfirmware.org To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.orgmailto:tf-a-leave@lists.trustedfirmware.org