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Date:   Thu, 12 Jul 2018 17:00:31 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Howells <dhowells@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]
Message-ID: <20180712160030.GV30522@ZenIV.linux.org.uk>
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On Thu, Jul 12, 2018 at 08:50:46AM -0700, Linus Torvalds wrote:

> But "write()" simply is *NOT* a good "command" interface. If you want
> to send a command, use an ioctl or a system call.
> 
> Because it's not just about credentials. It's not just about fooling a
> suid app into writing an error message to a descriptor you wrote. It's
> also about things like "splice()", which can write to your target
> using a kernel buffer, and thus trick you into doing a command while
> we have the context set to kernel addresses.

Wait a sec - that's only a problem if your command contains pointer-chasing
et.al.  Which is why e.g. /dev/sg is fucked in head.  But for something that
is plain text, what's the problem with splice/write/sendmsg/whatever?

I'm not talking about this particular interface, but "write is bad for
commands" as general policy looks missing the point.  If anything, it's
pointer-chasing crap that should be banned everywhere.  Just look at SG_IO -
it's a ioctl, and it's absolute garbage...