Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]

Jeremy Linton
2020-11-04T11:14:00Z

Hi,

On 11/3/20 11:34 AM, Mark Brown wrote:
> On Tue, Nov 03, 2020 at 10:25:37AM +0000, Szabolcs Nagy wrote:
>
>> Re-mmap executable segments instead of mprotecting them in
>> case mprotect is seccomp filtered.
>
>> For the kernel mapped main executable we don't have the fd
>> for re-mmap so linux needs to be updated to add BTI. (In the
>> presence of seccomp filters for mprotect(PROT_EXEC) the libc
>> cannot change BTI protection at runtime based on user space
>> policy so it is better if the kernel maps BTI compatible
>> binaries with PROT_BTI by default.)
>
> Given that there were still some ongoing discussions on a more robust
> kernel interface here and there seem to be a few concerns with this
> series should we perhaps just take a step back and disable this seccomp
> filter in systemd on arm64, at least for the time being? That seems
> safer than rolling out things that set ABI quickly, a big part of the

So, that's a bigger hammer than I think is needed and punishes !BTI
machines. I'm going to suggest that if we need to carry a temp patch its
more like the glibc patch I mentioned in the Fedora defect. That patch
simply logs a message, on the mprotect failures rather than aborting.
Its fairly non-intrusive.

That leaves seccomp functional, and BTI generally functional except when
seccomp is restricting it. I've also been asked that if a patch like
that is needed, its (temporary?) merged to the glibc trunk, rather than
just being carried by the distro's.

Thanks,

> reason we went with having the dynamic linker enable PROT_BTI in the
> first place was to give us more flexibility to handle any unforseen
> consequences of enabling BTI that we run into. We are going to have
> similar issues with other features like MTE so we need to make sure that
> whatever we're doing works with them too.
>
> Also updated to Will's current e-mail address - Will, do you have
> thoughts on what we should do here?
>