Re: [PATCH] io_uring: fix compat for IORING_REGISTER_FILES_UPDATE

[Eugene Syromiatnikov <esyr@redhat.com>]

On Wed, Jan 15, 2020 at 09:41:58AM -0700, Jens Axboe wrote:
> On 1/15/20 9:35 AM, Eugene Syromiatnikov wrote:
> > fds field of struct io_uring_files_update is problematic with regards
> > to compat user space, as pointer size is different in 32-bit, 32-on-64-bit,
> > and 64-bit user space.  In order to avoid custom handling of compat in
> > the syscall implementation, make fds __u64 and use u64_to_user_ptr in
> > order to retrieve it.  Also, align the field naturally and check that
> > no garbage is passed there.
> 
> Good point, it's an s32 pointer so won't align nicely. But how about
> just having it be:
> 
> struct io_uring_files_update {
> 	__u32 offset;
> 	__u32 resv;
> 	__s32 *fds;
> };
> 
> which should align nicely on both 32 and 64-bit?

The issue is that 32-bit user space would pass a 12-byte structure with
a 4-byte pointer in it to the 64-bit kernel, that, in turn, would treat it
as a 8-byte value (which might sometimes work on little-endian architectures,
if there are happen to be zeroes after the pointer, but will be always broken
on big-endian ones). __u64 is used in order to avoid special compat wrapper;
see, for example, __u64 usage in btrfs or BPF for similar purposes.

> -- 
> Jens Axboe
>