Re: [PATCH RFC bpf-next 4/3] uprobe: ensure sys_uretprobe uses sysret

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Mar 20, 2024 at 10:44:30AM -0700, Andrii Nakryiko wrote:
> On Wed, Mar 20, 2024 at 8:30 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > On 03/20, Oleg Nesterov wrote:
> > >
> > > On 03/20, Jiri Olsa wrote:
> > > >
> > > > are you ok if I squash the patches together
> > >
> > > Yes, thanks, I am fine.
> > >
> > > But lets discuss this change a bit more. So, with this poc we have the
> > > (intentionally) oversimplified
> > >
> > >       SYSCALL_DEFINE0(uretprobe)
> > >       {
> > >               struct pt_regs *regs = task_pt_regs(current);
> > >               unsigned long __user *ax_and_ret = (unsigned long __user *)regs->sp + 2;
> > >               unsigned long ip, err;
> > >
> > >               ip = regs->ip;
> > >               regs->orig_ax = -1;
> > >               err = get_user(regs->ax, ax_and_ret);
> > >               WARN_ON_ONCE(err);
> > >
> > >               uprobe_handle_trampoline(regs);
> > >
> > >               err = put_user(regs->ip, ax_and_ret);
> > >               WARN_ON_ONCE(err);
> > >               regs->ip = ip;
> > >
> > >               return regs->ax;
> > >       }
> > >
> > > I have no idea what uprobe consumers / bpf programs can do, so let me ask:
> > >
> > >       - uprobe_consumer's will see the "wrong" values of regs->cx/r11/sp
> > >         Is it OK? If not - easy to fix.
> > >
> > >       - can uprobe_consumer change regs->cx/r11 ? If yes - easy to fix.
> > >
> > >       - can uprobe_consumer change regs->sp ? If yes - easy to fix too,
> > >         but needs a separate check/code.
> >
> > IOW. If answer is "yes" to all the questions above, then we probably need
> > something like
> 
> yes to first, so ideally we fix registers to "correct" values
> (especially sp), but no to the last two (at least as far as BPF is
> concerned)

I think we should keep the same behaviour as it was for the trap,
so I think we should restore all registers and allow consumer to change it

jirka

> 
> >
> >         SYSCALL_DEFINE0(uretprobe)
> >         {
> >                 struct pt_regs *regs = task_pt_regs(current);
> >                 unsigned long err, ip, sp, r11_cx_ax[3];
> >
> >                 err = copy_from_user(r11_cx_ax, (void __user*)regs->sp, sizeof(r11_cx_ax));
> >                 WARN_ON_ONCE(err);
> >
> >                 // Q1: apart from ax, do we really care?
> >                 // expose the "right" values of r11/cx/ax/sp to uprobe_consumer's
> >                 regs->r11 = r11_cx_ax[0];
> >                 regs->cx  = r11_cx_ax[1];
> >                 regs->ax  = r11_cx_ax[2];
> >                 regs->sp += sizeof(r11_cx_ax);
> >                 regs->orig_ax = -1;
> >
> >                 ip = regs->ip;
> >                 sp = regs->sp;
> >
> >                 uprobe_handle_trampoline(regs);
> >
> >                 // Q2: is it possible? do we care?
> >                 // uprobe_consumer has changed sp, we can do nothing,
> >                 // just return via iret.
> >                 if (regs->sp != sp)
> >                         return regs->ax;
> >                 regs->sp -= sizeof(r11_cx_ax);
> >
> >                 // Q3: is it possible? do we care?
> >                 // for the case uprobe_consumer has changed r11/cx
> >                 r11_cx_ax[0] = regs->r11;
> >                 r11_cx_ax[1] = regs->cx;
> >
> >                 // comment to explain this hack
> >                 r11_cx_ax[2] = regs->ip;
> >                 regs->ip = ip;
> >
> >                 err = copy_to_user((void __user*)regs->sp, r11_cx_ax, sizeof(r11_cx_ax));
> >                 WARN_ON_ONCE(err);
> >
> >                 // ensure sysret, see do_syscall_64()
> >                 regs->r11 = regs->flags;
> >                 regs->cx  = regs->ip;
> >
> >                 return regs->ax;
> >         }
> >
> > Oleg.
> >