[PATCH bpf-next v7 1/2] bpf: Prevent tailcall infinite loop caused by freplace

[Leon Hwang <leon.hwang@linux.dev>]

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.

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>
---
 include/linux/bpf.h     | 17 +++++++++++++----
 kernel/bpf/arraymap.c   | 23 ++++++++++++++++++++++-
 kernel/bpf/core.c       |  1 +
 kernel/bpf/syscall.c    |  7 ++++---
 kernel/bpf/trampoline.c | 37 +++++++++++++++++++++++++++++--------
 5 files changed, 69 insertions(+), 16 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 19d8ca8ac960f..0c216e71cec76 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;
diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 79660e3fca4c1..f9bd63a74eee7 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -947,6 +947,7 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map,
 				   struct file *map_file, int fd)
 {
 	struct bpf_prog *prog = bpf_prog_get(fd);
+	bool is_extended;
 
 	if (IS_ERR(prog))
 		return prog;
@@ -956,13 +957,33 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map,
 		return ERR_PTR(-EINVAL);
 	}
 
+	mutex_lock(&prog->aux->ext_mutex);
+	is_extended = prog->aux->is_extended;
+	if (!is_extended)
+		prog->aux->prog_array_member_cnt++;
+	mutex_unlock(&prog->aux->ext_mutex);
+	if (is_extended) {
+		/* Extended prog can not be tail callee. It's to prevent a
+		 * potential infinite loop like:
+		 * tail callee prog entry -> tail callee prog subprog ->
+		 * freplace prog entry --tailcall-> tail callee prog entry.
+		 */
+		bpf_prog_put(prog);
+		return ERR_PTR(-EBUSY);
+	}
+
 	return prog;
 }
 
 static void prog_fd_array_put_ptr(struct bpf_map *map, void *ptr, bool need_defer)
 {
+	struct bpf_prog *prog = ptr;
+
+	mutex_lock(&prog->aux->ext_mutex);
+	prog->aux->prog_array_member_cnt--;
+	mutex_unlock(&prog->aux->ext_mutex);
 	/* bpf_prog is freed after one RCU or tasks trace grace period */
-	bpf_prog_put(ptr);
+	bpf_prog_put(prog);
 }
 
 static u32 prog_fd_array_sys_lookup_elem(void *ptr)
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 5e77c58e06010..233ea78f8f1bd 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -131,6 +131,7 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
 	INIT_LIST_HEAD_RCU(&fp->aux->ksym_prefix.lnode);
 #endif
 	mutex_init(&fp->aux->used_maps_mutex);
+	mutex_init(&fp->aux->ext_mutex);
 	mutex_init(&fp->aux->dst_mutex);
 
 	return fp;
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a8f1808a1ca54..4d04d4d9c1f30 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;
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index f8302a5ca400d..99b86e3b28a41 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -523,7 +523,9 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
 	}
 }
 
-static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr)
+static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
+				      struct bpf_trampoline *tr,
+				      struct bpf_prog *tgt_prog)
 {
 	enum bpf_tramp_prog_type kind;
 	struct bpf_tramp_link *link_exiting;
@@ -544,6 +546,17 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
 		/* Cannot attach extension if fentry/fexit are in use. */
 		if (cnt)
 			return -EBUSY;
+		guard(mutex)(&tgt_prog->aux->ext_mutex);
+		if (tgt_prog->aux->prog_array_member_cnt)
+			/* Program extensions can not extend target prog when
+			 * the target prog has been updated to any prog_array
+			 * map as tail callee. It's to prevent a potential
+			 * infinite loop like:
+			 * tgt prog entry -> tgt prog subprog -> freplace prog
+			 * entry --tailcall-> tgt prog entry.
+			 */
+			return -EBUSY;
+		tgt_prog->aux->is_extended = true;
 		tr->extension_prog = link->link.prog;
 		return bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP, NULL,
 					  link->link.prog->bpf_func);
@@ -570,17 +583,21 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
 	return err;
 }
 
-int bpf_trampoline_link_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 err;
 
 	mutex_lock(&tr->mutex);
-	err = __bpf_trampoline_link_prog(link, tr);
+	err = __bpf_trampoline_link_prog(link, tr, tgt_prog);
 	mutex_unlock(&tr->mutex);
 	return err;
 }
 
-static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr)
+static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
+					struct bpf_trampoline *tr,
+					struct bpf_prog *tgt_prog)
 {
 	enum bpf_tramp_prog_type kind;
 	int err;
@@ -588,9 +605,11 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
 	kind = bpf_attach_type_to_tramp(link->link.prog);
 	if (kind == BPF_TRAMP_REPLACE) {
 		WARN_ON_ONCE(!tr->extension_prog);
+		guard(mutex)(&tgt_prog->aux->ext_mutex);
 		err = bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP,
 					 tr->extension_prog->bpf_func, NULL);
 		tr->extension_prog = NULL;
+		tgt_prog->aux->is_extended = false;
 		return err;
 	}
 	hlist_del_init(&link->tramp_hlist);
@@ -599,12 +618,14 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
 }
 
 /* bpf_trampoline_unlink_prog() should never fail. */
-int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr)
+int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
+			       struct bpf_trampoline *tr,
+			       struct bpf_prog *tgt_prog)
 {
 	int err;
 
 	mutex_lock(&tr->mutex);
-	err = __bpf_trampoline_unlink_prog(link, tr);
+	err = __bpf_trampoline_unlink_prog(link, tr, tgt_prog);
 	mutex_unlock(&tr->mutex);
 	return err;
 }
@@ -619,7 +640,7 @@ static void bpf_shim_tramp_link_release(struct bpf_link *link)
 	if (!shim_link->trampoline)
 		return;
 
-	WARN_ON_ONCE(bpf_trampoline_unlink_prog(&shim_link->link, shim_link->trampoline));
+	WARN_ON_ONCE(bpf_trampoline_unlink_prog(&shim_link->link, shim_link->trampoline, NULL));
 	bpf_trampoline_put(shim_link->trampoline);
 }
 
@@ -733,7 +754,7 @@ int bpf_trampoline_link_cgroup_shim(struct bpf_prog *prog,
 		goto err;
 	}
 
-	err = __bpf_trampoline_link_prog(&shim_link->link, tr);
+	err = __bpf_trampoline_link_prog(&shim_link->link, tr, NULL);
 	if (err)
 		goto err;
 
-- 
2.44.0