Message-ID: <CAG48ez2LW9zyiptNq8jApD3zeS05wvNPs-jj2zOeaCDQbZnD4g@mail.gmail.com>
Date: Mon, 9 Dec 2024 19:21:45 +0100
From: Jann Horn <jannh@...gle.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Delyan Kratunov <delyank@...com>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH bpf v2] bpf: Fix prog_array UAF in __uprobe_perf_func()

On Sat, Dec 7, 2024 at 12:15 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> On Fri, Dec 6, 2024 at 3:14 PM Jann Horn <jannh@...gle.com> wrote:
> > On Fri, Dec 6, 2024 at 11:43 PM Jann Horn <jannh@...gle.com> wrote:
> > > On Fri, Dec 6, 2024 at 11:30 PM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > > On Fri, Dec 6, 2024 at 2:25 PM Jann Horn <jannh@...gle.com> wrote:
> > > > >
> > > > > On Fri, Dec 6, 2024 at 11:15 PM Andrii Nakryiko
> > > > > <andrii.nakryiko@...il.com> wrote:
> > > > > > On Fri, Dec 6, 2024 at 12:45 PM Jann Horn <jannh@...gle.com> 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@...r.kernel.org
> > > > > > > Signed-off-by: Jann Horn <jannh@...gle.com>
> > > > > > > ---
> > > > > > > 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@google.com
> > > > > > > ---
> > > > > > >  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.
> > > >
> > > > there is really just one caller ("legacy" singular uprobe handler), so
> > > > I think this should be fine. Unless someone objects I'd keep it
> > > > consistent with other "prog_array_run" helpers
> > >
> > > Ack, I will make it consistent.
> > >
> > > > > > 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);
> > > >
> > > > Yeah, absolutely, forgot to move the RCU dereference part, good catch!
> > > > But I wouldn't do the _check() variant here, literally the previous
> > > > line does rcu_read_trace_lock(), so this check part seems like just
> > > > unnecessary verboseness, I'd go with a simple rcu_dereference().
> > >
> > > rcu_dereference() is not legal there - that asserts that we are in a
> > > normal RCU read-side critical section, which we are not.
> > > rcu_dereference_raw() would be, but I think it is nice to document the
> > > semantics to make it explicit under which lock we're operating.
>
> sure, I don't mind
>
> > >
> > > I'll send a v3 in a bit after testing it.
> >
> > Actually, now I'm still hitting a page fault with my WIP v3 fix
> > applied... I'll probably poke at this some more next week.
>
> OK, that's interesting, keep us posted!

If I replace the "uprobe/" in my reproducer with "uprobe.s/", the
reproducer stops crashing even on bpf/master without this fix -
because it happens that handle_swbp() is already holding a
rcu_read_lock_trace() lock way up the stack. So I think this fix
should still be applied, but it probably doesn't need to go into
stable unless there is another path to the buggy code that doesn't
come from handle_swbp(). I guess I probably should resend my patch
with an updated commit message pointing out this caveat?

The problem I'm actually hitting seems to be a use-after-free of a
"struct bpf_prog" because of mismatching RCU flavors. Uprobes always
use bpf_prog_run_array_uprobe() under tasks-trace-RCU protection. But
it is possible to attach a non-sleepable BPF program to a uprobe, and
non-sleepable BPF programs are freed via normal RCU (see
__bpf_prog_put_noref()). And that is what happens with the reproducer
from my initial post
(https://lore.kernel.org/all/20241206-bpf-fix-uprobe-uaf-v1-1-6869c8a17258@google.com/)
- I can see that __bpf_prog_put_noref runs with prog->sleepable==0.

So I think that while I am delaying execution in
bpf_prog_run_array_uprobe(), perf_event_detach_bpf_prog() NULLs out
the event->tp_event->prog_array pointer and does
bpf_prog_array_free_sleepable() followed by bpf_prog_put(), and then
the BPF program can be freed since the reader doesn't hold an RCU read
lock. This seems a bit annoying to fix - there could legitimately be
several versions of the bpf_prog_array that are still used by
tasks-trace-RCU readers, so I think we can't just NULL out the array
entry and use RCU for the bpf_prog_array access on the reader side. I
guess we could add another flag on BPF programs that answers "should
this program be freed via tasks-trace-RCU" (separately from whether
the program is sleepable)?

Powered by blists - more mailing lists