RE: [PATCH bpf] bpf: verifier: prevent userspace memory access

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

From: Alexei Starovoitov
> Sent: 24 March 2024 20:43
> 
> On Sun, Mar 24, 2024 at 1:05 PM David Laight <David.Laight@aculab.com> wrote:
> >
> > From: Alexei Starovoitov
> > > Sent: 21 March 2024 06:08
> > >
> > > On Wed, Mar 20, 2024 at 3:55 AM Puranjay Mohan <puranjay12@gmail.com> wrote:
> > > >
> > > > The JITs need to implement bpf_arch_uaddress_limit() to define where
> > > > the userspace addresses end for that architecture or TASK_SIZE is taken
> > > > as default.
> > > >
> > > > The implementation is as follows:
> > > >
> > > > REG_AX =  SRC_REG
> > > > if(offset)
> > > >         REG_AX += offset;
> > > > REG_AX >>= 32;
> > > > if (REG_AX <= (uaddress_limit >> 32))
> > > >         DST_REG = 0;
> > > > else
> > > >         DST_REG = *(size *)(SRC_REG + offset);
> > >
> > > The patch looks good, but it seems to be causing s390 CI failures.
> >
> > I'm confused by the need for this check (and, IIRC, some other bpf
> > code that does kernel copies that can fault - and return an error).
> >
> > I though that the entire point of bpf was that is sanitised and
> > verified everything to limit what the 'program' could do in order
> > to stop it overwriting (or even reading) kernel structures that
> > is wasn't supposed to access.
> >
> > So it just shouldn't have a address that might be (in any way)
> > invalid.
> 
> bpf tracing progs can call bpf_probe_read_kernel() which
> can read any kernel memory.
> This is nothing but an inlined version of it.

It was the getsockopt() code were I saw the copy_nocheck() calls.
Those have to be broken.
Although the way some of the options use the ptr:len supplied by
the application you stand no chance of do an in-kernel call
without a proper buffer descriptor argument (with separate optlen
and bufferlen fields.)

> 
> > The only possible address verify is access_ok() to ensure that
> > a uses address really is a user address.
> 
> access_ok() considerations don't apply.
> We're not dealing with user memory access.

If you do need a check for 'not a user address' don't you want to just
require access_ok() fail?
That would be architecture independent.

	David

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