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Date:   Wed, 15 Jan 2020 17:50:17 +0100
From:   Eugene Syromiatnikov <esyr@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>
Subject: Re: [PATCH] io_uring: fix compat for IORING_REGISTER_FILES_UPDATE
Message-ID: <20200115165017.GI1333@asgard.redhat.com>
References: <20200115163538.GA13732@asgard.redhat.com>
 <cce5ac48-641d-3051-d22c-dab7aaa5704c@kernel.dk>
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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
>