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Re: [PATCH] Arm: avoid .init.data to be marked as executable

Hi Jan,

On 14/06/2021 14:17, Jan Beulich wrote:
On 14.06.2021 12:32, Julien Grall wrote:

On 14/06/2021 12:02, Jan Beulich wrote:
On 14.06.2021 11:41, Julien Grall wrote:
On 11/06/2021 11:39, Jan Beulich wrote:
This confuses disassemblers, at the very least. Move
.altinstr_replacement to .init.text,

The alternative code was borrowed from Linux. The code has now changed
to cater very large kernel. They used to keep the .altinstr_replacement
and altinstructions close to each other (albeit they were both in

I am not entirely why, but I am a bit worry to separate them. What sort
of test did you do?

Well, just build tests, on the assumption that relocation overflows
would be reported by the linker if the sections ended up too far

Hmmm, fair point. They should also not be further than the original
instruction. So there ought to be fine.

dropping the redundant ALIGN().

Also, to have .altinstr_replacement have consistent attributes in the
object files, add "x" to the one instance where it was missing. >
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
I'm uncertain whether having .altinstr_replacement inside or outside the
[_sinittext,_einittext) region is better; I simply followed what we have
on the x86 side right now.

This means the altinstructions will be marked executable in the
page-table. They technically should not be executable, so I would move
them outside _einittext and make sure the section is aligned to a PAGE_SIZE.

Hmm, are you saying you bother getting attributes right for .init.*
in the page tables? I ask because we don't on x86, and because it
would seem wasteful to me to pad to PAGE_SIZE just for this. But
you're the maintainer, i.e. I'm merely double checking ...

So this is a defense in depth. Your assumption is .init.text is going to
disappear after boot. However, if there is a bug that would leave
.init.text present then this may add more attack surface. So I think it
is a good practice to keep the permission correct.

However... looking the alternative code again, there is another reason
to move this change out of the range _sinitext - _einittext. The
function branch_insn_requires_update() will forbid branch target in
another alternative instructions.

This is first checking that the target is part of an active text. With
this change, this will return true because alternative instruction
replacement will be between _sinittext and _einittext.

So .altinstructions_replacement must outside of the region [_stinittext,

I see. But I'm not sure about the defense-in-depth aspect: By putting
it outside [_sinittext,_einittext) it'll get mapped r/w, while I think
you were implying that it would become r/o. Not even .init.rodata gets
mapped r/o.

Yes it is no r/o and that should be fixed at some point. However, I feel that r/w is better than allowing execution because some the instructions can lead to a DoS if executed on platform not supporting them.

But that's a matter of opinion and I think this confused the messaging here.

As a result I'm not convinced yet that you really want me to make the

I wrote "must", so I am not sure what else I could say to convince you that I really want to make this change...

To re-iterate, this code will break runtime check in the alternative patching code. So the .altinstruction_replacement **should** be placed after _einittext.


Julien Grall



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