Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm_struct entires

[Serge Hallyn]

On 11/29/2011 02:29 PM, Cyrill Gorcunov wrote:
> On Tue, Nov 29, 2011 at 12:19:38PM -0800, Kees Cook wrote:
> > On Tue, Nov 29, 2011 at 11:12:55PM +0400, Cyrill Gorcunov wrote:
> > > At restore time we need a mechanism to restore those values
> > > back and for this sake PR_SET_MM prctl code is introduced.
> > >
> > > Note at moment this inteface is allowed for CAP_SYS_ADMIN
> > > only.
> >
> > NAK from me; this needs more bounds checking. Though, yes, it absolutely
> > must be a privileged action since this is potentially very dangerous. Can
> > we invent something stronger than CAP_SYS_ADMIN? ;)
>
> Heh.
>
> > > @@ -1841,6 +1841,58 @@ SYSCALL_DEFINE5(prctl, int, option, unsi
> > >   			else
> > >   				error = PR_MCE_KILL_DEFAULT;
> > >   			break;
> > > +		case PR_SET_MM: {
> > > +			struct mm_struct *mm;
> > > +			struct vm_area_struct *vma;
> > > +
> > > +			if (arg4 | arg5)
> > > +				return -EINVAL;
> > > +
> > > +			if (!capable(CAP_SYS_ADMIN))
> > > +				return -EPERM;
> > > +
> > > +			error = -ENOENT;
> > > +			mm = get_task_mm(current);
> > > +			if (!mm)
> > > +				return error;
> > > +
> > > +			down_read(&mm->mmap_sem);
> > > +			vma = find_vma(mm, arg3);
> > > +			if (!vma)
> > > +				goto out;
> >
> > arg3 needs to be significantly more carefully validated. find_vma() doesn't
> > say that vm_start<= addr, only that vm_end>  addr. This effectively
> > bypasses all the vma checks (mmap_min_addr, max process size, etc), with
> > some pretty crazy side-effects, I think.
>
> Yes, I know it needs some more testing, but apart from vma bounds (yup,
> good point with find_vma, I'll fix) I thought about what else should be
> checked? I think VMA prototype should be checked to fit "code", "data"
> templates, ie code should be at least readable and execytable, but what
> about data and stack and brk, should stack be executable? That is the
> point where I've got a bit confused and though putting RFC out might be
> a good idea to collect opinions.

My memory is a bit hazy here, but cryo 
(http://git.sr71.net/?p=cryo-forhallyn.git;a=summary) did also do this 
from userspace.  As I recall the one problem we had was ... that we 
couldn't lower the mm_start of the first segment?  I think.  But I bring 
it up only because the advantage of doing it this way was that all of 
the ptrace protections automatically applied.

-serge
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