linux-kernel - Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack

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Message-ID: <cd064202-2c5f-b1d5-2970-9bff0a762a95@intel.com>
Date:   Wed, 21 Jul 2021 11:37:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     John Allen <john.allen@....com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack

On 7/21/21 11:14 AM, John Allen wrote:
>> +int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
>> +			     unsigned long stack_size)
>> +{
>> +	struct thread_shstk *shstk = &tsk->thread.shstk;
>> +	struct cet_user_state *state;
>> +	unsigned long addr;
>> +
>> +	if (!stack_size)
>> +		return -EINVAL;
> I've been doing some light testing on AMD hardware and I've found that
> this version of the patchset doesn't boot for me. It appears that when
> systemd processes start spawning, they hit the above case, return
> -EINVAL, and the fork fails. In these cases, copy_thread has been passed
> 0 for both sp and stack_size.

A few tangential things I noticed:

This hunk is not mentioned in the version changelog at all.  I also
don't see any feedback that might have prompted it.  This is one reason
per-patch changelogs are preferred.

As a general rule, new features should strive to be implemented in a way
that it's *obvious* that they won't break old code.
shstk_alloc_thread_stack() fails that test for me.  If it had:

	if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) // or whatever
		return 0;

in the function, it would be obviously harmless.  Better yet would be
doing the feature check at the shstk_alloc_thread_stack() call site,
that way even the function call can be optimized out.

Further, this confused me because the changelog didn't even mention the
arg -> stack_size rename.  That would have been nice for another patch,
or an extra sentence in the changelog.