Re: [PATCH 3/9] proc: use open()-time creds for ptrace checks

[Jann Horn <jann@thejh.net>]

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On Mon, Sep 19, 2016 at 09:01:19AM -0400, Stephen Smalley wrote:
> On 09/18/2016 11:05 AM, Jann Horn wrote:
> > This adds a new ptrace_may_access_noncurrent() method that
> > uses the supplied credentials instead of those of the
> > current task. (However, the current task may still be
> > inspected for auditing purposes, e.g. by the Smack LSM.)
> > 
> > procfs used the caller's creds for a few ptrace_may_access()
> > checks at read() time, which made a confused deputy attack
> > by passing an FD to a procfs file to a setuid program
> > possible. Therefore, the following was a local userspace
> > ASLR bypass:
[...]
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 13185a6..f9a0be7 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -2133,10 +2133,10 @@ static int selinux_binder_transfer_file(struct task_struct *from,
> >  }
> >  
> >  static int selinux_ptrace_access_check(struct task_struct *child,
> > -				     unsigned int mode)
> > +				     unsigned int mode, const struct cred *cred)
> >  {
> >  	if (mode & PTRACE_MODE_READ) {
> > -		u32 sid = current_sid();
> > +		u32 sid = cred_sid(cred);
> >  		u32 csid = task_sid(child);
> >  		return avc_has_perm(sid, csid, SECCLASS_FILE, FILE__READ, NULL);
> >  	}
> 
> For consistency, don't you also need to change the next line of code to
> use cred_has_perm() rather than current_has_perm()?

Oh, right.

How about something like this?

  u32 sid = cred_sid(cred);
  u32 csid = task_sid(child);
  if (mode & PTRACE_MODE_READ)
    return avc_has_perm(sid, csid, SECCLASS_FILE, FILE__READ, NULL);
  else
    return avc_has_perm(sid, csid, SECCLASS_PROCESS, PROCESS__PTRACE, NULL);

The current code looks as if the PTRACE_MODE_READ and PTRACE_MODE_ATTACH cases
behave very differently, which they actually don't. And to be able to use
cred_has_perm() here, it would be necessary to explicitly grab and release
an RCU read lock or so (to prevent the child creds from going away).

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