From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will@kernel.org>
Cc: He Zhe <zhe.he@windriver.com>,
Catalin Marinas <catalin.marinas@arm.com>,
oleg@redhat.com, linux-kernel@vger.kernel.org,
linux-audit@redhat.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat
Date: Wed, 21 Apr 2021 18:10:05 +0100 [thread overview]
Message-ID: <20210421171005.GA46949@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210419121932.GA30004@willie-the-truck>
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?
> 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.
Thanks,
Mark.
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next prev parent reply other threads:[~2021-04-21 19:39 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 7:55 [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat He Zhe
2021-04-16 7:55 ` [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat He Zhe
2021-04-16 9:43 ` Oleg Nesterov
2021-04-20 8:38 ` He Zhe
2021-04-21 17:41 ` Mark Rutland
2021-04-22 16:55 ` Mark Rutland
2021-04-16 7:55 ` [PATCH 3/3] audit: Use syscall_get_return_value to get syscall return code in audit_syscall_exit He Zhe
2021-04-16 12:33 ` [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat Catalin Marinas
2021-04-16 13:34 ` Mark Rutland
2021-04-17 13:19 ` David Laight
2021-04-19 12:19 ` Will Deacon
2021-04-20 8:54 ` He Zhe
2021-04-21 17:10 ` Mark Rutland [this message]
2021-04-22 16:07 ` Will Deacon
2021-04-22 16:42 ` Mark Rutland
2021-04-22 18:57 ` Dmitry V. Levin
2021-04-20 8:42 ` He Zhe
2021-04-21 17:17 ` Mark Rutland
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