RE: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

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

From: Michael S. Tsirkin
> Sent: 02 June 2020 21:33
> On Tue, Jun 02, 2020 at 10:18:09AM -0700, Linus Torvalds wrote:
> > On Tue, Jun 2, 2020 at 9:33 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > >
> > > > It's not clear whether we need a new API, I think __uaccess_being() has the
> > > > assumption that the address has been validated by access_ok().
> > >
> > > __uaccess_begin() is a stopgap, not a public API.
> >
> > Correct. It's just an x86 implementation detail.
> >
> > > The problem is real, but "let's add a public API that would do user_access_begin()
> > > with access_ok() already done" is no-go.
> >
> > Yeah, it's completely pointless.
> >
> > The solution to this is easy: remove the incorrect and useless early
> > "access_ok()". Boom, done.
> 
> Hmm are you sure we can drop it? access_ok is done in the context
> of the process. Access itself in the context of a kernel thread
> that borrows the same mm. IIUC if the process can be 32 bit
> while the kernel is 64 bit, access_ok in the context of the
> kernel thread will not DTRT.

In which case you need a 'user_access_begin' that takes the mm
as an additional parameter.

I found an 'interesting' acccess_ok() call in the code that copies
iov[] into kernel (eg for readv()).

a) It is a long way from any copies.
b) It can be conditionally ignored - and is so for one call.
   The oddball is code that reads from a different process.
   I didn't spot an equivalent check, but it all worked by
   mapping in the required page - so I'm not sure what happens.

Are there really just 2 limits for access_ok().
One for 64bit programs and one for 32bit?
With the limit being just below the 'dso' page??
So checking the current processes limit is never going
to restrict access.

	David

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