bpf: Fix race between btf_put and btf_idr walk.

[ Upstream commit acf1c3d68e9a31f10d92bc67ad4673cdae5e8d92 ]

Florian and Eduard reported hard dead lock:
[   58.433327]  _raw_spin_lock_irqsave+0x40/0x50
[   58.433334]  btf_put+0x43/0x90
[   58.433338]  bpf_find_btf_id+0x157/0x240
[   58.433353]  btf_parse_fields+0x921/0x11c0

This happens since btf->refcount can be 1 at the time of btf_put() and
btf_put() will call btf_free_id() which will try to grab btf_idr_lock
and will dead lock.
Avoid the issue by doing btf_put() without locking.

Fixes: 3d78417b60fb ("bpf: Add bpf_btf_find_by_name_kind() helper.")
Fixes: 1e89106da253 ("bpf: Add bpf_core_add_cands() and wire it into bpf_core_apply_relo_insn().")
Reported-by: Florian Westphal <fw@strlen.de>
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20230421014901.70908-1-alexei.starovoitov@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index a83a3f2..a8838a3 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -572,8 +572,8 @@ static s32 bpf_find_btf_id(const char *name, u32 kind, struct btf **btf_p)
                        *btf_p = btf;
                        return ret;
                }
-               spin_lock_bh(&btf_idr_lock);
                btf_put(btf);
+               spin_lock_bh(&btf_idr_lock);
        }
        spin_unlock_bh(&btf_idr_lock);
        return ret;
@@ -7957,12 +7957,10 @@ check_modules:
                btf_get(mod_btf);
                spin_unlock_bh(&btf_idr_lock);
                cands = bpf_core_add_cands(cands, mod_btf, btf_nr_types(main_btf));
-               if (IS_ERR(cands)) {
-                       btf_put(mod_btf);
+               btf_put(mod_btf);
+               if (IS_ERR(cands))
                        return ERR_CAST(cands);
-               }
                spin_lock_bh(&btf_idr_lock);
-               btf_put(mod_btf);
        }
        spin_unlock_bh(&btf_idr_lock);
        /* cands is a pointer to kmalloced memory here if cands->cnt > 0