RE: [PATCH 32/33] sctp: add sctp_sock_get_primary_addr

[David Laight <David.Laight@ACULAB.COM>]

From: David Laight
> Sent: 14 May 2020 13:30
> Subject: RE: [PATCH 32/33] sctp: add sctp_sock_get_primary_addr
> 
> From: David Laight
> > Sent: 14 May 2020 10:51
> > From: Marcelo Ricardo Leitner
> > > Sent: 13 May 2020 19:03
> > >
> > > On Wed, May 13, 2020 at 08:26:47AM +0200, Christoph Hellwig wrote:
> > > > Add a helper to directly get the SCTP_PRIMARY_ADDR sockopt from kernel
> > > > space without going through a fake uaccess.
> > >
> > > Same comment as on the other dlm/sctp patch.
> >
> > Wouldn't it be best to write sctp_[gs]etsockotp() that
> > use a kernel buffer and then implement the user-space
> > calls using a wrapper that does the copies to an on-stack
> > (or malloced if big) buffer.
> 
> Actually looking at __sys_setsockopt() it calls
> BPF_CGROUP_RUN_PROG_SETSOCKOPT() which (by the look of it)
> can copy the user buffer into malloc()ed memory and
> cause set_fs(KERNEL_DS) be called.
> 
> The only way to get rid of that set_fs() is to always
> have the buffer in kernel memory when the underlying
> setsockopt() code is called.

And having started to try coding __sys_setsockopt()
and then found the compat code I suspect that would
be a whole lot more sane if the buffer was in kernel
and it knew that at least (say) 64 bytes were allocated.

The whole compat_alloc_user_space() 'crap' could probably go.

Actually it looks like an application can avoid whatever
checks BPF_CGROUP_RUN_PROG_SETSOCKOPT() is trying to do
by using the 32bit compat ioctls.

	David

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