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

Catalin Marinas
2020-11-04T14:41:47Z

On Wed, Nov 04, 2020 at 08:57:05AM +0000, Szabolcs Nagy wrote:
> The 11/03/2020 23:41, Jeremy Linton wrote:
> > 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.
>
> note that changing mprotect into mmap in glibc works
> even if the kernel or systemd decides to do things
> differently: currently the only wart is that on the
> main exe we have to use mprotect and silently ignore
> the failures.

Can the dynamic loader mmap() the main exe again while munmap'ing the
original one? (sorry if it was already discussed)

--
Catalin