Re: [PATCH bpf-next] uprobes: Harden uretprobe syscall trampoline check

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Feb 10, 2025 at 09:26:53AM -0800, Andrii Nakryiko wrote:
> On Sun, Feb 9, 2025 at 2:05 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Jann reported [1] possible issue when trampoline_check_ip returns
> > address near the bottom of the address space that is allowed to
> > call into the syscall if uretprobes are not set up.
> >
> > Though the mmap minimum address restrictions will typically prevent
> > creating mappings there, let's make sure uretprobe syscall checks
> > for that.
> >
> > [1] https://lore.kernel.org/bpf/202502081235.5A6F352985@keescook/T/#m9d416df341b8fbc11737dacbcd29f0054413cbbf
> > Cc: Kees Cook <kees@kernel.org>
> > Cc: Eyal Birger <eyal.birger@gmail.com>
> > Cc: stable@vger.kernel.org
> > Reported-by: Jann Horn <jannh@google.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  arch/x86/kernel/uprobes.c | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> > index 5a952c5ea66b..109d6641a1b3 100644
> > --- a/arch/x86/kernel/uprobes.c
> > +++ b/arch/x86/kernel/uprobes.c
> > @@ -357,19 +357,23 @@ void *arch_uprobe_trampoline(unsigned long *psize)
> >         return &insn;
> >  }
> >
> > -static unsigned long trampoline_check_ip(void)
> > +static unsigned long trampoline_check_ip(unsigned long tramp)
> >  {
> > -       unsigned long tramp = uprobe_get_trampoline_vaddr();
> > -
> >         return tramp + (uretprobe_syscall_check - uretprobe_trampoline_entry);
> >  }
> >
> >  SYSCALL_DEFINE0(uretprobe)
> >  {
> >         struct pt_regs *regs = task_pt_regs(current);
> > -       unsigned long err, ip, sp, r11_cx_ax[3];
> > +       unsigned long err, ip, sp, r11_cx_ax[3], tramp;
> > +
> > +       /* If there's no trampoline, we are called from wrong place. */
> > +       tramp = uprobe_get_trampoline_vaddr();
> > +       if (tramp == -1)
> 
> slight nit: mixing -1 and unsigned long looks sloppy. Maybe let's add
> something like
> 
> #define UPROBE_NO_TRAMPOLINE_VADDR ((unsigned long)-1)
> 
> and return that from uprobe_get_trampoline_vaddr()?

ok, will add that

thanks,
jirka

> 
> > +               goto sigill;
> >
> > -       if (regs->ip != trampoline_check_ip())
> > +       /* Make sure the ip matches the only allowed sys_uretprobe caller. */
> > +       if (regs->ip != trampoline_check_ip(tramp))
> >                 goto sigill;
> >
> >         err = copy_from_user(r11_cx_ax, (void __user *)regs->sp, sizeof(r11_cx_ax));
> > --
> > 2.48.1
> >