From: David Laight <David.Laight@ACULAB.COM>
To: dccp@vger.kernel.org
Subject: RE: get rid of the address_space override in setsockopt v2
Date: Mon, 27 Jul 2020 14:09:09 +0000 [thread overview]
Message-ID: <5d958e937db54849b4ef9046e7e12277@AcuMS.aculab.com> (raw)
In-Reply-To: <20200720124737.118617-1-hch@lst.de>
From: Al Viro
> Sent: 27 July 2020 14:48
>
> On Mon, Jul 27, 2020 at 09:51:45AM +0000, David Laight wrote:
>
> > 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.
>
> Really? Care to provide some details, or is it another of your "I can't
> be possibly arsed to check what I'm saying, but it stands for reason
> that..." specials?
I did more 'homework' than sometimes :-)
Slightly difficult without a searchable net-next tree.
However, as has been pointed out is a different thread
this code is used to update IPv6 flow labels:
> > - if (copy_from_user(fl->opt+1, optval+CMSG_ALIGN(sizeof(*freq)), olen))
> > + sockptr_advance(optval, CMSG_ALIGN(sizeof(*freq)));
> > + if (copy_from_sockptr(fl->opt + 1, optval, olen))
> > goto done;
and doesn't work because the advances are no longer cumulative.
Now access_ok() has to take the base address and length to stop
'running into' kernel space, but the code above can advance from
a valid user pointer (which won't fault) to a kernel address.
If there were always an unmapped 'guard' page in the user address
space the access_ok() check prior to copy_to/from_user() wouldn't
need the length.
So I surmise that no such guard page exists and so the above
can advance from user addresses into kernel ones.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
prev parent reply other threads:[~2020-07-27 14:09 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-20 12:47 get rid of the address_space override in setsockopt Christoph Hellwig
2020-07-20 16:38 ` Eric Biggers
2020-07-20 17:43 ` Christoph Hellwig
2020-07-20 20:47 ` Alexei Starovoitov
2020-07-21 9:38 ` David Laight
2020-07-21 10:26 ` David Laight
2020-07-22 7:56 ` Christoph Hellwig
2020-07-22 8:06 ` 'Christoph Hellwig'
2020-07-22 8:07 ` 'Christoph Hellwig'
2020-07-22 8:21 ` David Laight
2020-07-22 17:09 ` Alexei Starovoitov
2020-07-23 6:08 ` get rid of the address_space override in setsockopt v2 Christoph Hellwig
2020-07-24 22:43 ` David Miller
2020-07-26 7:03 ` Christoph Hellwig
2020-07-26 7:08 ` Andreas Schwab
2020-07-26 7:46 ` David Miller
2020-07-27 9:51 ` David Laight
2020-07-27 13:48 ` Al Viro
2020-07-27 14:09 ` David Laight [this message]
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