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

[Andrii Nakryiko]

On Sun, Feb 9, 2025 at 2:05 PM Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx>
> Cc: Eyal Birger <eyal.birger@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  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()?

> +               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
>