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
