linux-kernel - Re: [PATCH v2] fs/proc: Expose RSEQ configuration

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Date:   Fri, 15 Jan 2021 10:44:20 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Piotr Figiel <figiel@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michel Lespinasse <walken@...gle.com>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Andrei Vagin <avagin@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Peter Oskolkov <posk@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Chris Kennelly <ckennelly@...gle.com>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] fs/proc: Expose RSEQ configuration

----- On Jan 14, 2021, at 1:54 PM, Piotr Figiel figiel@...gle.com wrote:

Added PeterZ, Paul and Boqun to CC. They are also listed as maintainers of rseq.
Please CC them in your next round of patches.

> For userspace checkpoint and restore (C/R) some way of getting process
> state containing RSEQ configuration is needed.
> 
> There are two ways this information is going to be used:
> - to re-enable RSEQ for threads which had it enabled before C/R
> - to detect if a thread was in a critical section during C/R
> 
> Since C/R preserves TLS memory and addresses RSEQ ABI will be restored
> using the address registered before C/R.

Indeed, if the process goes through a checkpoint/restore while within a
rseq c.s., that critical section should abort. Given that it's only the
restored process which resumes user-space execution, there should be some
way to ensure that the rseq tls pointer is restored before that thread goes
back to user-space, or some way to ensure the rseq TLS is registered
before that thread returns to the saved instruction pointer.

How do you plan to re-register the rseq TLS for each thread upon restore ?

I suspect you move the return IP to the abort either at checkpoint or restore
if you detect that the thread is running in a rseq critical section.

> 
> Detection whether the thread is in a critical section during C/R is
> needed to enforce behavior of RSEQ abort during C/R. Attaching with
> ptrace() before registers are dumped itself doesn't cause RSEQ abort.

Right, because the RSEQ abort is only done when going back to user-space,
and AFAIU the checkpointed process will cease to exist, and won't go back
to user-space, therefore bypassing any RSEQ abort.

> Restoring the instruction pointer within the critical section is
> problematic because rseq_cs may get cleared before the control is
> passed to the migrated application code leading to RSEQ invariants not
> being preserved.

The commit message should state that both the per-thread rseq TLS area address
and the signature are dumped within this new proc file.

> 
> Signed-off-by: Piotr Figiel <figiel@...gle.com>
> 
> ---
> 
> v2:
> - fixed string formatting for 32-bit architectures
> 
> v1:
> - https://lkml.kernel.org/r/20210113174127.2500051-1-figiel@google.com
> 
> ---
> fs/proc/base.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index b3422cda2a91..7cc36a224b8b 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -662,6 +662,21 @@ static int proc_pid_syscall(struct seq_file *m, struct
> pid_namespace *ns,
> 
> 	return 0;
> }
> +
> +#ifdef CONFIG_RSEQ
> +static int proc_pid_rseq(struct seq_file *m, struct pid_namespace *ns,
> +				struct pid *pid, struct task_struct *task)
> +{
> +	int res = lock_trace(task);

AFAIU lock_trace prevents concurrent exec() from modifying the task's content.
What prevents a concurrent rseq register/unregister to be executed concurrently
with proc_pid_rseq ?

> +
> +	if (res)
> +		return res;
> +	seq_printf(m, "%tx %08x\n", (ptrdiff_t)((uintptr_t)task->rseq),

I wonder if all those parentheses are needed. Wouldn't it be enough to have:

  (ptrdiff_t)(uintptr_t)task->rseq

?

Thanks,

Mathieu

> +		   task->rseq_sig);
> +	unlock_trace(task);
> +	return 0;
> +}
> +#endif /* CONFIG_RSEQ */
> #endif /* CONFIG_HAVE_ARCH_TRACEHOOK */
> 
> /************************************************************************/
> @@ -3182,6 +3197,9 @@ static const struct pid_entry tgid_base_stuff[] = {
> 	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
> #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
> 	ONE("syscall",    S_IRUSR, proc_pid_syscall),
> +#ifdef CONFIG_RSEQ
> +	ONE("rseq",       S_IRUSR, proc_pid_rseq),
> +#endif
> #endif
> 	REG("cmdline",    S_IRUGO, proc_pid_cmdline_ops),
> 	ONE("stat",       S_IRUGO, proc_tgid_stat),
> @@ -3522,6 +3540,9 @@ static const struct pid_entry tid_base_stuff[] = {
> 			 &proc_pid_set_comm_operations, {}),
> #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
> 	ONE("syscall",   S_IRUSR, proc_pid_syscall),
> +#ifdef CONFIG_RSEQ
> +	ONE("rseq",      S_IRUSR, proc_pid_rseq),
> +#endif
> #endif
> 	REG("cmdline",   S_IRUGO, proc_pid_cmdline_ops),
> 	ONE("stat",      S_IRUGO, proc_tid_stat),
> --
> 2.30.0.284.gd98b1dd5eaa7-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com