[PATCH bpf-next v3 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 btf
is not kernel's btf.

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

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b274004fccfd..7bb541e343b2 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19019,6 +19019,9 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b
 
 	switch (prog->type) {
 	case BPF_PROG_TYPE_TRACING:
+		if (!btf_is_kernel(btf))
+			return -EINVAL;
+
 		t = btf_type_by_id(btf, btf_id);
 		if (!t)
 			return -EINVAL;
-- 
2.55.0