Re: [PATCH] bpf: Prevent double bpf_prog_put call from bpf_tracing_prog_attach

On Mon, Jan 11, 2021 at 1:17 PM Jiri Olsa [off-list ref] wrote:

On Mon, Jan 11, 2021 at 12:34:48PM -0800, Andrii Nakryiko wrote:

quoted

On Mon, Jan 11, 2021 at 11:18 AM Jiri Olsa [off-list ref] wrote:

quoted

The bpf_tracing_prog_attach error path calls bpf_prog_put
on prog, which causes refcount underflow when it's called
from link_create function.

  link_create
    prog = bpf_prog_get              <-- get
    ...
    tracing_bpf_link_attach(prog..
      bpf_tracing_prog_attach(prog..
        out_put_prog:
          bpf_prog_put(prog);        <-- put

    if (ret < 0)
      bpf_prog_put(prog);            <-- put

Removing bpf_prog_put call from bpf_tracing_prog_attach
and making sure its callers call it instead.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---

I also double-checked all other attach functions called from
link_create, they all seem to be fine and don't put prog on failure,
so this should be the only needed fix. Also, missing:

it'd be easier to spot this if we use refcount_t instead of the atomic64_t,
I replaced it for this refcount and got nice console warning for this bug

then I saw:
  85192dbf4de0 bpf: Convert bpf_prog refcnt to atomic64_t

so I guess we need something like refcount64_t first

Having a non-failing refcount simplifies code quite a lot. I was
having a problem having to deal with potential refcount failure during
mmap()'ing, where it's impossible to communicate failure back to the
kernel. So if atomic64_t would never fail, but would generate a
warning on underflow, then yeah, it would be an improvement.

jirka

quoted

Fixes: 4a1e7c0c63e0 ("bpf: Support attaching freplace programs to
multiple attach points")

Acked-by: Andrii Nakryiko <andrii@kernel.org>

quoted

 kernel/bpf/syscall.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c3bb03c8371f..e5999d86c76e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -2712,7 +2712,6 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 out_put_prog:
        if (tgt_prog_fd && tgt_prog)
                bpf_prog_put(tgt_prog);
-       bpf_prog_put(prog);
        return err;
 }

@@ -2825,7 +2824,10 @@ static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
                        tp_name = prog->aux->attach_func_name;
                        break;
                }
-               return bpf_tracing_prog_attach(prog, 0, 0);
+               err = bpf_tracing_prog_attach(prog, 0, 0);
+               if (err >= 0)
+                       return err;
+               goto out_put_prog;
        case BPF_PROG_TYPE_RAW_TRACEPOINT:
        case BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE:
                if (strncpy_from_user(buf,
--

2.26.2