From: David Miller

Sent: 24 July 2020 23:44

From: Christoph Hellwig <hch@lst.de>
Date: Thu, 23 Jul 2020 08:08:42 +0200

quoted

setsockopt is the last place in architecture-independ code that still
uses set_fs to force the uaccess routines to operate on kernel pointers.

This series adds a new sockptr_t type that can contained either a kernel
or user pointer, and which has accessors that do the right thing, and
then uses it for setsockopt, starting by refactoring some low-level
helpers and moving them over to it before finally doing the main
setsockopt method.

Note that apparently the eBPF selftests do not even cover this path, so
the series has been tested with a testing patch that always copies the
data first and passes a kernel pointer.  This is something that works for
most common sockopts (and is something that the ePBF support relies on),
but unfortunately in various corner cases we either don't use the passed
in length, or in one case actually copy data back from setsockopt, or in
case of bpfilter straight out do not work with kernel pointers at all.

Against net-next/master.

Changes since v1:
 - check that users don't pass in kernel addresses
 - more bpfilter cleanups
 - cosmetic mptcp tweak

Series applied to net-next, I'm build testing and will push this out when
that is done.

Hmmm... this code does:

int __sys_setsockopt(int fd, int level, int optname, char __user *user_optval,
		int optlen)
{
	sockptr_t optval;
	char *kernel_optval = NULL;
	int err, fput_needed;
	struct socket *sock;

	if (optlen < 0)
		return -EINVAL;

	err = init_user_sockptr(&optval, user_optval);
	if (err)
		return err;

And the called code does:
	if (copy_from_sockptr(&opt, optbuf, sizeof(opt)))
		return -EFAULT;


Which means that only the base of the user's buffer is checked
for being in userspace.

I'm sure there is code that processes options in chunks.
This probably means it is possible to put a chunk boundary
at the end of userspace and continue processing the very start
of kernel memory.

At best this faults on the kernel copy code and crashes the system.

Maybe there wasn't any code that actually incremented the user address.
But it is hardly robust.

	David

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