Re: [PATCH bpf v2] bpf: Fix prog_array UAF in __uprobe_perf_func()

[Jann Horn]

On Fri, Dec 6, 2024 at 11:15 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
> On Fri, Dec 6, 2024 at 12:45 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> >
> > Currently, the pointer stored in call->prog_array is loaded in
> > __uprobe_perf_func(), with no RCU annotation and no RCU protection, so the
> > loaded pointer can immediately be dangling. Later,
> > bpf_prog_run_array_uprobe() starts a RCU-trace read-side critical section,
> > but this is too late. It then uses rcu_dereference_check(), but this use of
> > rcu_dereference_check() does not actually dereference anything.
> >
> > It looks like the intention was to pass a pointer to the member
> > call->prog_array into bpf_prog_run_array_uprobe() and actually dereference
> > the pointer in there. Fix the issue by actually doing that.
> >
> > Fixes: 8c7dcb84e3b7 ("bpf: implement sleepable uprobes by chaining gps")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > ---
> > To reproduce, in include/linux/bpf.h, patch in a mdelay(10000) directly
> > before the might_fault() in bpf_prog_run_array_uprobe() and add an
> > include of linux/delay.h.
> >
> > Build this userspace program:
> >
> > ```
> > $ cat dummy.c
> > #include <stdio.h>
> > int main(void) {
> >   printf("hello world\n");
> > }
> > $ gcc -o dummy dummy.c
> > ```
> >
> > Then build this BPF program and load it (change the path to point to
> > the "dummy" binary you built):
> >
> > ```
> > $ cat bpf-uprobe-kern.c
> > #include <linux/bpf.h>
> > #include <bpf/bpf_helpers.h>
> > #include <bpf/bpf_tracing.h>
> > char _license[] SEC("license") = "GPL";
> >
> > SEC("uprobe//home/user/bpf-uprobe-uaf/dummy:main")
> > int BPF_UPROBE(main_uprobe) {
> >   bpf_printk("main uprobe triggered\n");
> >   return 0;
> > }
> > $ clang -O2 -g -target bpf -c -o bpf-uprobe-kern.o bpf-uprobe-kern.c
> > $ sudo bpftool prog loadall bpf-uprobe-kern.o uprobe-test autoattach
> > ```
> >
> > Then run ./dummy in one terminal, and after launching it, run
> > `sudo umount uprobe-test` in another terminal. Once the 10-second
> > mdelay() is over, a use-after-free should occur, which may or may
> > not crash your kernel at the `prog->sleepable` check in
> > bpf_prog_run_array_uprobe() depending on your luck.
> > ---
> > Changes in v2:
> > - remove diff chunk in patch notes that confuses git
> > - Link to v1: https://lore.kernel.org/r/20241206-bpf-fix-uprobe-uaf-v1-1-6869c8a17258@xxxxxxxxxx
> > ---
> >  include/linux/bpf.h         | 4 ++--
> >  kernel/trace/trace_uprobe.c | 2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
>
> Looking at how similar in spirit bpf_prog_run_array() is meant to be
> used, it seems like it is the caller's responsibility to
> RCU-dereference array and keep RCU critical section before calling
> into bpf_prog_run_array(). So I wonder if it's best to do this instead
> (Gmail will butcher the diff, but it's about the idea):

Yeah, that's the other option I was considering. That would be more
consistent with the existing bpf_prog_run_array(), but has the
downside of unnecessarily pushing responsibility up to the caller...
I'm fine with either.

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index eaee2a819f4c..4b8a9edd3727 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2193,26 +2193,25 @@ bpf_prog_run_array(const struct bpf_prog_array *array,
>   * rcu-protected dynamically sized maps.
>   */
>  static __always_inline u32
> -bpf_prog_run_array_uprobe(const struct bpf_prog_array __rcu *array_rcu,
> +bpf_prog_run_array_uprobe(const struct bpf_prog_array *array,
>                           const void *ctx, bpf_prog_run_fn run_prog)
>  {
>         const struct bpf_prog_array_item *item;
>         const struct bpf_prog *prog;
> -       const struct bpf_prog_array *array;
>         struct bpf_run_ctx *old_run_ctx;
>         struct bpf_trace_run_ctx run_ctx;
>         u32 ret = 1;
>
>         might_fault();
> +       RCU_LOCKDEP_WARN(!rcu_read_lock_trace_held(), "no rcu lock held");
> +
> +       if (unlikely(!array))
> +               goto out;
>
> -       rcu_read_lock_trace();
>         migrate_disable();
>
>         run_ctx.is_uprobe = true;
>
> -       array = rcu_dereference_check(array_rcu, rcu_read_lock_trace_held());
> -       if (unlikely(!array))
> -               goto out;
>         old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
>         item = &array->items[0];
>         while ((prog = READ_ONCE(item->prog))) {
> @@ -2229,7 +2228,6 @@ bpf_prog_run_array_uprobe(const struct
> bpf_prog_array __rcu *array_rcu,
>         bpf_reset_run_ctx(old_run_ctx);
>  out:
>         migrate_enable();
> -       rcu_read_unlock_trace();
>         return ret;
>  }
>
> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index fed382b7881b..87a2b8fefa90 100644
> --- a/kernel/trace/trace_uprobe.c
> +++ b/kernel/trace/trace_uprobe.c
> @@ -1404,7 +1404,9 @@ static void __uprobe_perf_func(struct trace_uprobe *tu,
>         if (bpf_prog_array_valid(call)) {
>                 u32 ret;
>
> +               rcu_read_lock_trace();
>                 ret = bpf_prog_run_array_uprobe(call->prog_array,
> regs, bpf_prog_run);

But then this should be something like this (possibly split across
multiple lines with a helper variable or such):

ret = bpf_prog_run_array_uprobe(rcu_dereference_check(call->prog_array,
rcu_read_lock_trace_held()), regs, bpf_prog_run);

> +               rcu_read_unlock_trace();
>                 if (!ret)
>                         return;
>         }
>
>