Re: [PATCH] staging: android: ashmem: Fix mmap size validation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jun 19, 2018 at 03:24:44PM -0700, Alistair Strachan wrote:
> The ashmem driver did not check that the size/offset of the vma passed
> to its .mmap() function was not larger than the ashmem object being
> mapped. This could cause mmap() to succeed, even though accessing parts
> of the mapping would later fail with a segmentation fault.
> 
> Ensure an error is returned by the ashmem_mmap() function if the vma
> size is larger than the ashmem object size. This enables safer handling
> of the problem in userspace.

Is this going to break current users of this api as their original call
was succeeding?

> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arve Hjønnevåg <arve@android.com>
> Cc: Todd Kjos <tkjos@android.com>
> Cc: Martijn Coenen <maco@android.com>
> Cc: devel@driverdev.osuosl.org
> Cc: kernel-team@android.com
> Signed-off-by: Alistair Strachan <astrachan@google.com>
> ---
>  drivers/staging/android/ashmem.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index a1a0025b59e0..1eeedb529a10 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -366,6 +366,12 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
>  		goto out;
>  	}
>  
> +	/* requested mapping size larger than object size */
> +	if (unlikely(vma->vm_end - vma->vm_start > PAGE_ALIGN(asma->size))) {
> +		ret = -EINVAL;
> +		goto out;
> +	}

Unless you can measure the speed difference, never use likely/unlikely.
The CPU and compiler almost always knows how to do this better than we
do.  I know there are other checks like this in this function, those
should also be fixed as well.

thanks,

greg k-h