[PATCH v4 1/2] powerpc/mm: Support execute-only memory on the Radix MMU

[Russell Currey]

On Wed, 2023-03-08 at 16:27 +0100, Michal Suchánek wrote:
> Hello,
> 
> On Wed, Aug 31, 2022 at 11:13:59PM +1000, Michael Ellerman wrote:
> > On Wed, 17 Aug 2022 15:06:39 +1000, Russell Currey wrote:
> > > Add support for execute-only memory (XOM) for the Radix MMU by
> > > using an
> > > execute-only mapping, as opposed to the RX mapping used by
> > > powerpc's
> > > other MMUs.
> > > 
> > > The Hash MMU already supports XOM through the execute-only pkey,
> > > which is a separate mechanism shared with x86.  A PROT_EXEC-only
> > > mapping
> > > will map to RX, and then the pkey will be applied on top of it.
> > > 
> > > [...]
> > 
> > Applied to powerpc/next.
> > 
> > [1/2] powerpc/mm: Support execute-only memory on the Radix MMU
> >      
> > https://git.kernel.org/powerpc/c/395cac7752b905318ae454a8b859d4c190485510
> 
> This breaks libaio tests (on POWER9 hash PowerVM):
> https://pagure.io/libaio/blob/master/f/harness/cases/5.t#_43
> 
> cases/5.p
> expect   512: (w), res =   512 [Success]
> expect   512: (r), res =   512 [Success]
> expect   512: (r), res =   512 [Success]
> expect   512: (w), res =   512 [Success]
> expect   512: (w), res =   512 [Success]
> expect   -14: (r), res =   -14 [Bad address]
> expect   512: (r), res =   512 [Success]
> expect   512: (w), res =   512 [Success]
> test cases/5.t completed PASSED.
> 
> cases/5.p
> expect   512: (w), res =   512 [Success]
> expect   512: (r), res =   512 [Success]
> expect   512: (r), res =   512 [Success]
> expect   512: (w), res =   512 [Success]
> expect   512: (w), res =   512 [Success]
> expect   -14: (r), res =   -14 [Bad address]
> expect   512: (r), res =   512 [Success]
> expect   -14: (w), res =   512 [Success] -- FAILED
> test cases/5.t completed FAILED.
> 
> Can you have a look if that test assumption is OK?

Hi Michal, thanks for the report.

This wasn't an intended behaviour change, so it is a bug.  I have no
idea why we hit the fault in write() but not in io_submit(), though. 
The same issue applies under Radix.

What's happening here is that we're taking a page fault and calling
into access_error() and returning true when we shouldn't.  Previously
we didn't check for read faults and only checked for PROT_NONE.  My
patch checks the vma flags to see if they lack VM_READ after we check
for exec and write, which ignores that VM_WRITE implies read 

This means we're mishandling faults for write-only mappings by assuming
that the lack of VM_READ means we're faulting from read, when that
should only be possible under a PROT_EXEC-only mapping.

I think the correct behaviour is

	if (unlikely(!(vma->vm_flags & (VM_READ | VM_WRITE))))

in access_error().

Will do some more testing and send a patch soon.  I also need to verify
that write implying read is true for all powerpc platforms.

- Russell

> 
> Thanks
> 
> Michal