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

[Topi Miettinen]

On 3.11.2020 19.34, 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?

Filtering mprotect() and mmap() with seccomp also protects BTI, since 
without it the attacker could remove PROT_BTI from existing pages, or 
map new pages without BTI. This would be possible even with SARA or 
SELinux execmem protections enabled, since they don't care about PROT_BTI.

-Topi