On Tue, Aug 18, 2020 at 12:58:07PM -0700, Kees Cook wrote:

On Tue, Aug 18, 2020 at 09:54:46PM +0200, Christoph Hellwig wrote:

quoted

On Tue, Aug 18, 2020 at 12:39:34PM -0700, Kees Cook wrote:

quoted

On Mon, Aug 17, 2020 at 09:32:04AM +0200, Christoph Hellwig wrote:

quoted

default_file_splice_write is the last piece of generic code that uses
set_fs to make the uaccess routines operate on kernel pointers.  It
implements a "fallback loop" for splicing from files that do not actually
provide a proper splice_read method.  The usual file systems and other
high bandwith instances all provide a ->splice_read, so this just removes
support for various device drivers and procfs/debugfs files.  If splice
support for any of those turns out to be important it can be added back
by switching them to the iter ops and using generic_file_splice_read.

Signed-off-by: Christoph Hellwig <hch@lst.de>

This seems a bit disruptive? I feel like this is going to make fuzzers
really noisy (e.g. trinity likes to splice random stuff out of /sys and
/proc).

Noisy in the sence of triggering the pr_debug or because they can't
handle -EINVAL?

Well, maybe both? I doubt much _expects_ to be using splice, so I'm fine
with that, but it seems weird not to have a fall-back, especially if
something would like to splice a file out of there. But, I'm not opposed
to the change, it just seems like it might cause pain down the road.

The problem is that without pretending a buffer is in user space when
it actually isn't, we can't have a generic fallback.  So we'll have to
have specific support - I wrote generic support for seq_file, and
willy did for /proc/sys, but at least the first caused a few problems
and a fair amount of churn, so I'd rather see first if we can get
away without it.

-- 
Kees Cook

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