linux-kernel - Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)

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Message-ID: <87a6pgb78r.fsf@oldenburg.str.redhat.com>
Date:   Fri, 30 Apr 2021 08:45:40 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        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>,
        "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: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle
 signals for shadow stack)

* Andy Lutomirski:

> The kernel has:
>
> struct rt_sigframe {
>     char __user *pretcode;
>     struct ucontext uc;
>     struct siginfo info;
>     /* fp state follows here */
> };
>
> This is roughly the actual signal frame.  But userspace does not have
> this struct declared, and user code does not know the sizes of the
> fields.  So it's accessed in a nonsensical way.  The signal handler
> function is passed a pointer to the whole sigframe implicitly in RSP,
> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
> User code can *find* the fp state by following a pointer from
> mcontext, which is, in turn, found via uc:
>
> struct ucontext {
>     unsigned long      uc_flags;
>     struct ucontext  *uc_link;
>     stack_t          uc_stack;
>     struct sigcontext uc_mcontext;  <-- fp pointer is in here
>     sigset_t      uc_sigmask;    /* mask last for extensibility */
> };

I must say that I haven't reviewed this in detail, but for historic
reasons, glibc userspace has a differently-sized sigset_t.  So the
kernel ucontext (used in signals) and user ucontext (used for
swapcontext et al.) are already fully decoupled?

Thanks,
Florian