Re: [PATCH 08/13] s390/pci: Remove races against pte updates

Daniel Vetter
2020-10-08T17:16:33Z

On Thu, Oct 8, 2020 at 6:44 PM Gerald Schaefer
<gerald.schaefer@linux.ibm.com> wrote:
>
> On Wed, 7 Oct 2020 18:44:21 +0200
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> > Way back it was a reasonable assumptions that iomem mappings never
> > change the pfn range they point at. But this has changed:
> >
> > - gpu drivers dynamically manage their memory nowadays, invalidating
> > ptes with unmap_mapping_range when buffers get moved
> >
> > - contiguous dma allocations have moved from dedicated carvetouts to
> > cma regions. This means if we miss the unmap the pfn might contain
> > pagecache or anon memory (well anything allocated with GFP_MOVEABLE)
> >
> > - even /dev/mem now invalidates mappings when the kernel requests that
> > iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
> > ("/dev/mem: Revoke mappings when a driver claims the region")
> >
> > Accessing pfns obtained from ptes without holding all the locks is
> > therefore no longer a good idea. Fix this.
> >
> > Since zpci_memcpy_from|toio seems to not do anything nefarious with
> > locks we just need to open code get_pfn and follow_pfn and make sure
> > we drop the locks only after we've done. The write function also needs
> > the copy_from_user move, since we can't take userspace faults while
> > holding the mmap sem.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Jérôme Glisse <jglisse@redhat.com>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-arm-kernel@lists.infradead.org
> > Cc: linux-samsung-soc@vger.kernel.org
> > Cc: linux-media@vger.kernel.org
> > Cc: Niklas Schnelle <schnelle@linux.ibm.com>
> > Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> > Cc: linux-s390@vger.kernel.org
> > ---
> > arch/s390/pci/pci_mmio.c | 98 +++++++++++++++++++++++-----------------
> > 1 file changed, 57 insertions(+), 41 deletions(-)
>
> Looks good, thanks. Also survived some basic function test. Only some
> minor nitpick, see below.
>
> Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
>
> >
> > diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
> > index 401cf670a243..4d194cb09372 100644
> > --- a/arch/s390/pci/pci_mmio.c
> > +++ b/arch/s390/pci/pci_mmio.c
> > @@ -119,33 +119,15 @@ static inline int __memcpy_toio_inuser(void __iomem *dst,
> > return rc;
> > }
> >
> > -static long get_pfn(unsigned long user_addr, unsigned long access,
> > - unsigned long *pfn)
> > -{
> > - struct vm_area_struct *vma;
> > - long ret;
> > -
> > - mmap_read_lock(current->mm);
> > - ret = -EINVAL;
> > - vma = find_vma(current->mm, user_addr);
> > - if (!vma)
> > - goto out;
> > - ret = -EACCES;
> > - if (!(vma->vm_flags & access))
> > - goto out;
> > - ret = follow_pfn(vma, user_addr, pfn);
> > -out:
> > - mmap_read_unlock(current->mm);
> > - return ret;
> > -}
> > -
> > SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
> > const void __user *, user_buffer, size_t, length)
> > {
> > u8 local_buf[64];
> > void __iomem *io_addr;
> > void *buf;
> > - unsigned long pfn;
> > + struct vm_area_struct *vma;
> > + pte_t *ptep;
> > + spinlock_t *ptl;
> > long ret;
> >
> > if (!zpci_is_enabled())
> > @@ -158,7 +140,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
> > * We only support write access to MIO capable devices if we are on
> > * a MIO enabled system. Otherwise we would have to check for every
> > * address if it is a special ZPCI_ADDR and would have to do
> > - * a get_pfn() which we don't need for MIO capable devices. Currently
> > + * a pfn lookup which we don't need for MIO capable devices. Currently
> > * ISM devices are the only devices without MIO support and there is no
> > * known need for accessing these from userspace.
> > */
> > @@ -176,21 +158,37 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
> > } else
> > buf = local_buf;
> >
> > - ret = get_pfn(mmio_addr, VM_WRITE, &pfn);
> > + ret = -EFAULT;
> > + if (copy_from_user(buf, user_buffer, length))
> > + goto out_free;
> > +
> > + mmap_read_lock(current->mm);
> > + ret = -EINVAL;
> > + vma = find_vma(current->mm, mmio_addr);
> > + if (!vma)
> > + goto out_unlock_mmap;
> > + ret = -EACCES;
> > + if (!(vma->vm_flags & VM_WRITE))
> > + goto out_unlock_mmap;
> > + if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > + goto out_unlock_mmap;
>
> That check for VM_IO | VM_PFNMAP was previously hidden inside follow_pfn(),
> and that would have returned -EINVAL in this case. With your change, we
> now return -EACCES. Not sure how important that is, but it feels wrong.
> Maybe move the VM_IO | VM_PFNMAP check up, before the ret = -EACCES?

I tried to keep the errno unchanged, but fumbled this. Will fix in the
next round, thanks a lot for reviewing and testing.

For merging I think this one here would be best through the s390 tree,
since it can be merged without any of the others in here.

Thanks, Daniel

>
> [...]
> > @@ -306,22 +306,38 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
> > buf = local_buf;
> > }
> >
> > - ret = get_pfn(mmio_addr, VM_READ, &pfn);
> > + mmap_read_lock(current->mm);
> > + ret = -EINVAL;
> > + vma = find_vma(current->mm, mmio_addr);
> > + if (!vma)
> > + goto out_unlock_mmap;
> > + ret = -EACCES;
> > + if (!(vma->vm_flags & VM_WRITE))
> > + goto out_unlock_mmap;
> > + if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > + goto out_unlock_mmap;
>
> Same here with VM_IO | VM_PFNMAP and -EINVAL.

--
Daniel Vetter
Software Engineer, Intel Corporation