linux-kernel - Re: [PATCH 09/14] fs: don't change the address limit for ->write_iter in __kernel

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Message-ID: <20200529133744.GA654@lst.de>
Date:   Fri, 29 May 2020 15:37:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 09/14] fs: don't change the address limit for
 ->write_iter in __kernel_write

On Fri, May 29, 2020 at 07:57:36AM +0200, Christoph Hellwig wrote:
> On Thu, May 28, 2020 at 08:00:52PM +0100, Al Viro wrote:
> > On Thu, May 28, 2020 at 07:40:38AM +0200, Christoph Hellwig wrote:
> > > If we write to a file that implements ->write_iter there is no need
> > > to change the address limit if we send a kvec down.  Implement that
> > > case, and prefer it over using plain ->write with a changed address
> > > limit if available.
> > 
> > Umm...  It needs a comment along the lines of "weird shits like
> > /dev/sg that currently check for uaccess_kernel() will just
> > have to make sure they never switch to ->write_iter()"
> 
> sg and hid has the uaccess_kernel because it accesses userspace memory not
> in the range passed to it.  Something using write_iter/read_iter should
> never access any memory outside the iter passed to.  rdma has it because
> it uses write as a bidirectional interface, which obviously can't work at
> all with an iter.  So I'm not sure what we should comment on, but if
> you have a desire and a proposal for a comment I'll happily add it.

And looking over all three again they actually comment why they
check uaccess_kernel.  More importantly if someone switched them to
the ->write_iter carelessly that means the uaccess outside of the range
would actually aways fail now as we didn't allow access to userspace
memory, so this should show up when testing instantly.