bpf: Prevent tailcall infinite loop caused by freplace

There is a potential infinite loop issue that can occur when using a
combination of tail calls and freplace.

In an upcoming selftest, the attach target for entry_freplace of
tailcall_freplace.c is subprog_tc of tc_bpf2bpf.c, while the tail call in
entry_freplace leads to entry_tc. This results in an infinite loop:

entry_tc -> subprog_tc -> entry_freplace --tailcall-> entry_tc.

The problem arises because the tail_call_cnt in entry_freplace resets to
zero each time entry_freplace is executed, causing the tail call mechanism
to never terminate, eventually leading to a kernel panic.

To fix this issue, the solution is twofold:

1. Prevent updating a program extended by an freplace program to a
   prog_array map.
2. Prevent extending a program that is already part of a prog_array map
   with an freplace program.

This ensures that:

* If a program or its subprogram has been extended by an freplace program,
  it can no longer be updated to a prog_array map.
* If a program has been added to a prog_array map, neither it nor its
  subprograms can be extended by an freplace program.

Moreover, an extension program should not be tailcalled. As such, return
-EINVAL if the program has a type of BPF_PROG_TYPE_EXT when adding it to a
prog_array map.

Additionally, fix a minor code style issue by replacing eight spaces with a
tab for proper formatting.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20241015150207.70264-2-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 13 insertions, 4 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 19d8ca8ac960..0c216e71cec7 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1292,8 +1292,12 @@ void *__bpf_dynptr_data_rw(const struct bpf_dynptr_kern *ptr, u32 len);
 bool __bpf_dynptr_is_rdonly(const struct bpf_dynptr_kern *ptr);
 
 #ifdef CONFIG_BPF_JIT
-int bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr);
-int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr);
+int bpf_trampoline_link_prog(struct bpf_tramp_link *link,
+			     struct bpf_trampoline *tr,
+			     struct bpf_prog *tgt_prog);
+int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
+			       struct bpf_trampoline *tr,
+			       struct bpf_prog *tgt_prog);
 struct bpf_trampoline *bpf_trampoline_get(u64 key,
 					  struct bpf_attach_target_info *tgt_info);
 void bpf_trampoline_put(struct bpf_trampoline *tr);
@@ -1374,12 +1378,14 @@ void bpf_jit_uncharge_modmem(u32 size);
 bool bpf_prog_has_trampoline(const struct bpf_prog *prog);
 #else
 static inline int bpf_trampoline_link_prog(struct bpf_tramp_link *link,
-					   struct bpf_trampoline *tr)
+					   struct bpf_trampoline *tr,
+					   struct bpf_prog *tgt_prog)
 {
 	return -ENOTSUPP;
 }
 static inline int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
-					     struct bpf_trampoline *tr)
+					     struct bpf_trampoline *tr,
+					     struct bpf_prog *tgt_prog)
 {
 	return -ENOTSUPP;
 }
@@ -1483,6 +1489,9 @@ struct bpf_prog_aux {
 	bool xdp_has_frags;
 	bool exception_cb;
 	bool exception_boundary;
+	bool is_extended; /* true if extended by freplace program */
+	u64 prog_array_member_cnt; /* counts how many times as member of prog_array */
+	struct mutex ext_mutex; /* mutex for is_extended and prog_array_member_cnt */
 	struct bpf_arena *arena;
 	/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
 	const struct btf_type *attach_func_proto;