bpf: Fix verifier support for validation of async callbacks
Commit bfc6bb74e4f1 ("bpf: Implement verifier support for validation of async callbacks.")
added support for BPF_FUNC_timer_set_callback to
the __check_func_call() function. The test in __check_func_call() is
flaweed because it can mis-interpret a regular BPF-to-BPF pseudo-call
as a BPF_FUNC_timer_set_callback callback call.
Consider the conditional in the code:
if (insn->code == (BPF_JMP | BPF_CALL) &&
insn->imm == BPF_FUNC_timer_set_callback) {
The BPF_FUNC_timer_set_callback has value 170. This means that if you
have a BPF program that contains a pseudo-call with an instruction delta
of 170, this conditional will be found to be true by the verifier, and
it will interpret the pseudo-call as a callback. This leads to a mess
with the verification of the program because it makes the wrong
assumptions about the nature of this call.
Solution: include an explicit check to ensure that insn->src_reg == 0.
This ensures that calls cannot be mis-interpreted as an async callback
call.
Fixes: bfc6bb74e4f1 ("bpf: Implement verifier support for validation of async callbacks.")
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220105210150.GH1559@oracle.com
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c8d9e76..bfb4538 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6031,6 +6031,7 @@ static int __check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn
}
if (insn->code == (BPF_JMP | BPF_CALL) &&
+ insn->src_reg == 0 &&
insn->imm == BPF_FUNC_timer_set_callback) {
struct bpf_verifier_state *async_cb;