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, 4 insertions, 3 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a8f1808a1ca5..4d04d4d9c1f3 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3214,7 +3214,8 @@ static void bpf_tracing_link_release(struct bpf_link *link)
 		container_of(link, struct bpf_tracing_link, link.link);
 
 	WARN_ON_ONCE(bpf_trampoline_unlink_prog(&tr_link->link,
-						tr_link->trampoline));
+						tr_link->trampoline,
+						tr_link->tgt_prog));
 
 	bpf_trampoline_put(tr_link->trampoline);
 
@@ -3354,7 +3355,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 	 *   in prog->aux
 	 *
 	 * - if prog->aux->dst_trampoline is NULL, the program has already been
-         *   attached to a target and its initial target was cleared (below)
+	 *   attached to a target and its initial target was cleared (below)
 	 *
 	 * - if tgt_prog != NULL, the caller specified tgt_prog_fd +
 	 *   target_btf_id using the link_create API.
@@ -3429,7 +3430,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 	if (err)
 		goto out_unlock;
 
-	err = bpf_trampoline_link_prog(&link->link, tr);
+	err = bpf_trampoline_link_prog(&link->link, tr, tgt_prog);
 	if (err) {
 		bpf_link_cleanup(&link_primer);
 		link = NULL;