Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat

[Will Deacon]

Hi Mark,

On Wed, Apr 21, 2021 at 06:10:05PM +0100, Mark Rutland wrote:
> On Mon, Apr 19, 2021 at 01:19:33PM +0100, Will Deacon wrote:
> > On Fri, Apr 16, 2021 at 02:34:41PM +0100, Mark Rutland wrote:
> > > I think this is a problem we created for ourselves back in commit:
> > > 
> > >   15956689a0e60aa0 ("arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return)
> > > 
> > > AFAICT, the perf regs samples are the only place this matters, since for
> > > ptrace the compat regs are implicitly truncated to compat_ulong_t, and
> > > audit expects the non-truncated return value. Other architectures don't
> > > truncate here, so I think we're setting ourselves up for a game of
> > > whack-a-mole to truncate and extend wherever we need to.
> > > 
> > > Given that, I suspect it'd be better to do something like the below.
> > > 
> > > Will, thoughts?
> > 
> > I think perf is one example, but this is also visible to userspace via the
> > native ptrace interface and I distinctly remember needing this for some
> > versions of arm64 strace to work correctly when tracing compat tasks.
> 
> FWIW, you've convinced me on your approach (more on that below), but
> when I went digging here this didn't seem to be exposed via ptrace --
> for any task tracing a compat task, the GPRs are exposed via
> compat_gpr_{get,set}(), which always truncate to compat_ulong_t, giving
> the lower 32 bits. See task_user_regset_view() for where we get the
> regset.
> 
> Am I missing something, or are you thinking of another issue you fixed
> at the same time?

I think it may depend on whether strace pokes at the GPRs or instead issues
a PTRACE_GET_SYSCALL_INFO request but I've forgotten the details,
unfortunately. I do remember seeing an issue though, and it was only last
year.

> > So I do think that clearing the upper bits on the return path is the right
> > approach, but it sounds like we need some more work to handle syscall(-1)
> > and audit (what exactly is the problem here after these patches have been
> > applied?)
> 
> From digging a bit more, I think I agree, and I think these patches are
> sufficient for audit. I have some comments I'll leave separately.
> 
> The remaining issues are wherever we assign a signed value to a compat
> GPR without explicit truncation. That'll leak via perf sampling the user
> regs, but I haven't managed to convince myself whether that causes any
> functional change in behaviour for audit, seccomp, or syscall tracing.
> 
> Since we mostly use compat_ulong_t for intermediate values in compat
> code, it does look like this is only an issue for x0 where we assign an
> error value, e.g. the -ENOSYS case in el0_svc_common. I'll go see if I
> can find any more.
> 
> With those fixed up we can remove the x0 truncation from entry.S,
> which'd be nice too.

If we remove that then we should probably have a (debug?) check on the
return-to-user path just to make sure.

Will