linux-kernel - Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

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Date:   Fri, 13 Oct 2017 15:56:15 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

On 10/13/2017 03:40 PM, Mathieu Desnoyers wrote:
> The proposed ABI does not require to store any function pointer. For a given
> rseq_finish() critical section, pointers to specific instructions (within a
> function) are emitted at link-time into a struct rseq_cs:
> 
> struct rseq_cs {
>          RSEQ_FIELD_u32_u64(start_ip);
>          RSEQ_FIELD_u32_u64(post_commit_ip);
>          RSEQ_FIELD_u32_u64(abort_ip);
>          uint32_t flags;
> } __attribute__((aligned(4 * sizeof(uint64_t))));
> 
> Then, at runtime, the fast-path stores the address of that struct rseq_cs
> into the TLS struct rseq "rseq_cs" field.
> 
> So all we store at runtime is a pointer to data, not a pointer to functions.
> 
> But you seem to hint that having a pointer to data containing pointers to code
> may still be making it easier for exploit writers. Can you elaborate on the
> scenario ?

I'm concerned that the exploit writer writes a totally made up struct 
rseq_cs object into writable memory, along with function pointers, and 
puts the address of that in to the rseq_cs field.

This would be comparable to how C++ vtable pointers are targeted 
(including those in the glibc libio implementation of stdio streams).

Does this answer your questions?

Thanks,
Florian