bpf: Ensure RCU lock is held around bpf_prog_ksym_find

Add a warning to ensure RCU lock is held around tree lookup, and then
fix one of the invocations in bpf_stack_walker. The program has an
active stack frame and won't disappear. Use the opportunity to remove
unneeded invocation of is_bpf_text_address.

Fixes: f18b03f ("bpf: Implement BPF exceptions")
Reviewed-by: Emil Tsalapatis <[email protected]>
Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: kkdwivedi

kernel/bpf/core.c

@@ -782,7 +782,10 @@ bool is_bpf_text_address(unsigned long addr)
 
 struct bpf_prog *bpf_prog_ksym_find(unsigned long addr)
 {
-	struct bpf_ksym *ksym = bpf_ksym_find(addr);
+	struct bpf_ksym *ksym;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+	ksym = bpf_ksym_find(addr);
 
 	return ksym && ksym->prog ?
 	       container_of(ksym, struct bpf_prog_aux, ksym)->prog :

kernel/bpf/helpers.c

@@ -2981,9 +2981,16 @@ static bool bpf_stack_walker(void *cookie, u64 ip, u64 sp, u64 bp)
 	struct bpf_throw_ctx *ctx = cookie;
 	struct bpf_prog *prog;
 
-	if (!is_bpf_text_address(ip))
-		return !ctx->cnt;
+	/*
+	 * The RCU read lock is held to safely traverse the latch tree, but we
+	 * don't need its protection when accessing the prog, since it has an
+	 * active stack frame on the current stack trace, and won't disappear.
+	 */
+	rcu_read_lock();
 	prog = bpf_prog_ksym_find(ip);
+	rcu_read_unlock();
+	if (!prog)
+		return !ctx->cnt;
 	ctx->cnt++;
 	if (bpf_is_subprog(prog))
 		return true;