[PATCH bpf-next 1/2] bpf: Fix sleepable check for tracing prog

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

When CONFIG_FUNCTION_ERROR_INJECTION is disabled, a sleepable tracing prog
is allowed to attach to '__x64_'-alike prefix symbols.

It is because the verifier does not verify whether the symbol is a kernel
function or a bpf prog. That said, a sleepable tracing prog is allowed to
attach to a bpf prog target whose name has '__x64_'-alike prefix.

For example, a sleepable fentry prog attaches to a '__x64_sys_nop' XDP
prog, and copies buffer from a user pointer with bpf_copy_from_user()
helper. After attaching the XDP prog to lo interface, the kernel BUG
could be triggered by 'ping -c 1 -W 1 127.0.0.1':

[    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324

Fix it by disallowing sleepable tracing prog always when its target is
bpf prog.

Fixes: 16d9c5660692 ("bpf: Always allow sleepable programs on syscalls")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/verifier.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 52be0a118cce..40d567b90d24 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -18935,13 +18935,16 @@ static bool is_tracing_multi_id(const struct bpf_prog *prog, u32 btf_id)
 }
 
 static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct bpf_prog *prog,
-				  const struct btf *btf)
+				  const struct btf *btf, const struct bpf_prog *tgt_prog)
 {
 	const struct btf_type *t;
 	const char *tname;
 
 	switch (prog->type) {
 	case BPF_PROG_TYPE_TRACING:
+		if (tgt_prog)
+			return prog->sleepable ? -EINVAL : 0;
+
 		t = btf_type_by_id(btf, btf_id);
 		if (!t)
 			return -EINVAL;
@@ -19324,7 +19327,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 		}
 
 		if (prog->sleepable) {
-			ret = btf_id_allow_sleepable(btf_id, addr, prog, btf);
+			ret = btf_id_allow_sleepable(btf_id, addr, prog, btf, tgt_prog);
 			if (ret) {
 				module_put(mod);
 				bpf_log(log, "%s is not sleepable\n", tname);
@@ -19575,7 +19578,7 @@ int bpf_check_attach_btf_id_multi(struct btf *btf, struct bpf_prog *prog, u32 bt
 
 	/* Check sleepable program attachment. */
 	if (prog->sleepable) {
-		err = btf_id_allow_sleepable(btf_id, addr, prog, btf);
+		err = btf_id_allow_sleepable(btf_id, addr, prog, btf, NULL);
 		if (err)
 			return err;
 	}
-- 
2.55.0