Date:   Thu, 9 Mar 2023 18:02:27 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
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        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        John Allen <john.allen@amd.com>, kcc@google.com,
        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
        dethoma@microsoft.com, akpm@linux-foundation.org,
        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com,
        david@redhat.com, debug@rivosinc.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v7 32/41] x86/shstk: Handle signals for shadow stack
Message-ID: <ZAoRI8J9MjOf58+r@zn.tnic>
References: <20230227222957.24501-1-rick.p.edgecombe@intel.com>
 <20230227222957.24501-33-rick.p.edgecombe@intel.com>
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On Mon, Feb 27, 2023 at 02:29:48PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.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