linux-kernel - Re: [RFC PATCH for 4.18 1/2] rseq: use __u64 for rseq_cs fields, validate abort_ip < TASK

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Date:   Mon, 2 Jul 2018 18:16:45 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: use __u64 for rseq_cs fields,
 validate abort_ip < TASK_SIZE

----- On Jul 2, 2018, at 6:08 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Mon, Jul 2, 2018 at 3:03 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>>         if (__get_user(ptr, &t->rseq->rseq_cs))
>>                 return -EINVAL;
>>         if (check_rseq_cs_padding(t))
>>                 return -EINVAL;
> 
> Small nit.
> 
> I think the _actual_ user access faults should return -EFAULT, and
> then the *validation* checks should return -EINVAL.
> 
> So when the "copy_from_user()" fails, that's -EFAULT, but when you
> have (rseq_cs->start_ip >= TASK_SIZE), that's -EINVAL.

Fair enough.

> 
> That said, nothing actually cares or exposes the error number, I
> think. Afaik, all the callers just check "did it work" or not.

Indeed, it's a static function and callers just check for zero/nonzero.

> 
> So this is more a "let's be consistent" than anything that matters.

Allright, here is the function updated accordingly:

static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
{
        struct rseq_cs __user *urseq_cs;
        unsigned long ptr;
        u32 __user *usig;
        u32 sig;
        int ret;

        ret = __get_user(ptr, &t->rseq->rseq_cs);
        if (ret)
                return ret;
        if (check_rseq_cs_padding(t))
                return -EINVAL;
        if (!ptr) {
                memset(rseq_cs, 0, sizeof(*rseq_cs));
                return 0;
        }
        urseq_cs = (struct rseq_cs __user *)ptr;
        if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
                return -EFAULT;

        if (rseq_cs->start_ip >= TASK_SIZE ||
            rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
            rseq_cs->abort_ip >= TASK_SIZE ||
            rseq_cs->version > 0)
                return -EINVAL;
        /* Check for overflow. */
        if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
                return -EINVAL;
        /* Ensure that abort_ip is not in the critical section. */
        if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
                return -EINVAL;

        usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
        ret = get_user(sig, usig);
        if (ret)
                return ret;

        if (current->rseq_sig != sig) {
                printk_ratelimited(KERN_WARNING
                        "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
                        sig, current->rseq_sig, current->pid, usig);
                return -EINVAL;
        }
        return 0;
}

Thanks for the feedback!

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com