Re: [PATCH 4/9] x86, pkeys: override pkey when moving away from PROT_EXEC

[Thomas Gleixner]

On Wed, 25 Apr 2018, Shakeel Butt wrote:
> On Mon, Mar 26, 2018 at 5:27 PM, Dave Hansen
> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
> >
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >
> > I got a bug report that the following code (roughly) was
> > causing a SIGSEGV:
> >
> >         mprotect(ptr, size, PROT_EXEC);
> >         mprotect(ptr, size, PROT_NONE);
> >         mprotect(ptr, size, PROT_READ);
> >         *ptr = 100;
> >
> > The problem is hit when the mprotect(PROT_EXEC)
> > is implicitly assigned a protection key to the VMA, and made
> > that key ACCESS_DENY|WRITE_DENY.  The PROT_NONE mprotect()
> > failed to remove the protection key, and the PROT_NONE->
> > PROT_READ left the PTE usable, but the pkey still in place
> > and left the memory inaccessible.
> >
> > To fix this, we ensure that we always "override" the pkee
> > at mprotect() if the VMA does not have execute-only
> > permissions, but the VMA has the execute-only pkey.
> >
> > We had a check for PROT_READ/WRITE, but it did not work
> > for PROT_NONE.  This entirely removes the PROT_* checks,
> > which ensures that PROT_NONE now works.
> >
> > Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> >
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Fixes: 62b5f7d013f ("mm/core, x86/mm/pkeys: Add execute-only protection keys support")
> 
> Hi Dave, are you planning to send the next version of this patch or
> going with this one?

Right, some enlightment would be appreciated. I'm lost in the dozen
different threads discussing this back and forth.

Thanks,

	tglx