linux-kernel - RE: [PATCH v6 1/3] x86/speculation: apply IBPB more strictly to avoid cross-process data leak

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Date:   Thu, 13 Sep 2018 00:04:49 +0000
From:   "Schaufler, Casey" <casey.schaufler@...el.com>
To:     Jiri Kosina <jikos@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Schaufler, Casey" <casey.schaufler@...el.com>
Subject: RE: [PATCH v6 1/3] x86/speculation: apply IBPB more strictly to
 avoid cross-process data leak

> -----Original Message-----
> From: Jiri Kosina [mailto:jikos@...nel.org]
> 
> 

> @@ -325,10 +326,13 @@ static int __ptrace_may_access(struct task_struct
> *task, unsigned int mode)
>  	mm = task->mm;
>  	if (mm &&
>  	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
> -	     !ptrace_has_cap(mm->user_ns, mode)))
> +	     ((mode & PTRACE_MODE_NOACCESS_CHK) ||
> +	       !ptrace_has_cap(mm->user_ns, mode))))
>  	    return -EPERM;
> 
> -	return security_ptrace_access_check(task, mode);
> +	if (!(mode & PTRACE_MODE_NOACCESS_CHK))
> +		return security_ptrace_access_check(task, mode);
> +	return 0;

Because PTRACE_MODE_IBPB includes PTRACE_MODE_NOAUDIT you
shouldn't need this change. Do you have a good way to exercise this code
path? I'm having trouble getting to the check, and have yet to get a case
where PTRACE_MODE_NOACCESS_CHK is set.

>  }
> 
>  bool ptrace_may_access(struct task_struct *task, unsigned int mode)
> 
> --
> Jiri Kosina
> SUSE Labs