On Mon, Jul 14, 2025 at 07:19:35PM +0900, Masami Hiramatsu wrote:
> On Mon, 14 Jul 2025 11:39:03 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Jul 14, 2025 at 05:39:15PM +0900, Masami Hiramatsu wrote:
> >
> > > > + /*
> > > > + * Some of the uprobe consumers has changed sp, we can do nothing,
> > > > + * just return via iret.
> > > > + */
> > >
> > > Do we allow consumers to change the `sp`? It seems dangerous
> > > because consumer needs to know whether it is called from
> > > breakpoint or syscall. Note that it has to set up ax, r11
> > > and cx on the stack correctly only if it is called from syscall,
> > > that is not compatible with breakpoint mode.
> > >
> > > > + if (regs->sp != sp)
> > > > + return regs->ax;
> > >
> > > Shouldn't we recover regs->ip? Or in this case does consumer has
> > > to change ip (== return address from trampline) too?
> > >
> > > IMHO, it should not allow to change the `sp` and `ip` directly
> > > in syscall mode. In case of kprobes, kprobe jump optimization
> > > must be disabled explicitly (e.g. setting dummy post_handler)
> > > if the handler changes `ip`.
> > >
> > > Or, even if allowing to modify `sp` and `ip`, it should be helped
> > > by this function, e.g. stack up the dummy regs->ax/r11/cx on the
> > > new stack at the new `regs->sp`. This will allow modifying those
> > > registries transparently as same as breakpoint mode.
> > > In this case, I think we just need to remove above 2 lines.
> >
> > There are two syscall return paths; the 'normal' is sysret and for that
> > you need to undo all things just right.
> >
> > The other is IRET. At which point we can have whatever state we want,
> > including modified SP.
> >
> > See arch/x86/entry/syscall_64.c:do_syscall_64() and
> > arch/x86/entry/entry_64.S:entry_SYSCALL_64
> >
> > The IRET path should return pt_regs as is from an interrupt/exception
> > very much like INT3.
>
> OK, so SYSRET case, we need to follow;
>
> sys_uprobe -> do_syscall_64 -> entry_SYSCALL_64 -> trampoline -> retaddr
>
> But using IRET to return, we can skip returning to trampoline,
>
> sys_uprobe -> do_syscall_64 -> entry_SYSCALL_64 -> regs->ip
the handler gets the original breakpoint address, it's set in:
regs->ip = ax_r11_cx_ip[3] - 5;
and at the point we do:
/*
* Some of the uprobe consumers has changed sp, we can do nothing,
* just return via iret.
*/
if (regs->sp != sp)
return regs->ax;
.. regs->ip value wasn't restored for the trampoline's return address,
so iret will skip the trampoline
but perhaps we could do the extra check below to land on the next instruction?
jirka
---
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 043d826295a3..4318517aa852 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -817,8 +817,12 @@ SYSCALL_DEFINE0(uprobe)
* Some of the uprobe consumers has changed sp, we can do nothing,
* just return via iret.
*/
- if (regs->sp != sp)
+ if (regs->sp != sp) {
+ /* skip the trampoline call */
+ if (ax_r11_cx_ip[3] - 5 == regs->ip)
+ regs->ip += 5;
return regs->ax;
+ }
regs->sp -= sizeof(ax_r11_cx_ip);
