linux-kernel - Re: [PATCH v7 32/41] x86/shstk: Handle signals for shadow stack

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Message-ID: <ZAoRI8J9MjOf58+r@zn.tnic>
Date:   Thu, 9 Mar 2023 18:02:27 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...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>,
        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>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        david@...hat.com, debug@...osinc.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v7 32/41] x86/shstk: Handle signals for shadow stack

On Mon, Feb 27, 2023 at 02:29:48PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> When a signal is handled normally the context is pushed to the stack

s/normally //

> before handling it. For shadow stacks, since the shadow stack only track's

"tracks"

> return addresses, there isn't any state that needs to be pushed. However,
> there are still a few things that need to be done. These things are
> userspace visible and which will be kernel ABI for shadow stacks.

"visible to userspace"

s/which //

> One is to make sure the restorer address is written to shadow stack, since
> the signal handler (if not changing ucontext) returns to the restorer, and
> the restorer calls sigreturn. So add the restorer on the shadow stack
> before handling the signal, so there is not a conflict when the signal
> handler returns to the restorer.
> 
> The other thing to do is to place some type of checkable token on the
> thread's shadow stack before handling the signal and check it during
> sigreturn. This is an extra layer of protection to hamper attackers
> calling sigreturn manually as in SROP-like attacks.
> 
> For this token we can use the shadow stack data format defined earlier.
		^^^

Please use passive voice in your commit message: no "we" or "I", etc.

> Have the data pushed be the previous SSP. In the future the sigreturn
> might want to return back to a different stack. Storing the SSP (instead
> of a restore offset or something) allows for future functionality that
> may want to restore to a different stack.
> 
> So, when handling a signal push
>  - the SSP pointing in the shadow stack data format
>  - the restorer address below the restore token.
> 
> In sigreturn, verify SSP is stored in the data format and pop the shadow
> stack.

...

> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 13c02747386f..40f0a55762a9 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -232,6 +232,104 @@ static int get_shstk_data(unsigned long *data, unsigned long __user *addr)
>  	return 0;
>  }
>  
> +static int shstk_push_sigframe(unsigned long *ssp)
> +{
> +	unsigned long target_ssp = *ssp;
> +
> +	/* Token must be aligned */
> +	if (!IS_ALIGNED(*ssp, 8))
> +		return -EINVAL;
> +
> +	if (!IS_ALIGNED(target_ssp, 8))
> +		return -EINVAL;

Those two statements are identical AFAICT.

> +	*ssp -= SS_FRAME_SIZE;
> +	if (put_shstk_data((void *__user)*ssp, target_ssp))
> +		return -EFAULT;
> +
> +	return 0;
> +}

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette