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

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

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?

[...]
> @@ -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.