linux-kernel - [PATCH 5.6 191/194] bpf: Enforce returning 0 for fentry/fexit progs

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Message-Id: <20200518173547.306820163@linuxfoundation.org>
Date:   Mon, 18 May 2020 19:38:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Yonghong Song <yhs@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>
Subject: [PATCH 5.6 191/194] bpf: Enforce returning 0 for fentry/fexit progs

From: Yonghong Song <yhs@...com>

commit e92888c72fbdc6f9d07b3b0604c012e81d7c0da7 upstream.

Currently, tracing/fentry and tracing/fexit prog
return values are not enforced. In trampoline codes,
the fentry/fexit prog return values are ignored.
Let us enforce it to be 0 to avoid confusion and
allows potential future extension.

This patch also explicitly added return value
checking for tracing/raw_tp, tracing/fmod_ret,
and freplace programs such that these program
return values can be anything. The purpose are
two folds:
 1. to make it explicit about return value expectations
    for these programs in verifier.
 2. for tracing prog_type, if a future attach type
    is added, the default is -ENOTSUPP which will
    enforce to specify return value ranges explicitly.

Fixes: fec56f5890d9 ("bpf: Introduce BPF trampoline")
Signed-off-by: Yonghong Song <yhs@...com>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Andrii Nakryiko <andriin@...com>
Link: https://lore.kernel.org/bpf/20200514053206.1298415-1-yhs@fb.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/bpf/verifier.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6498,6 +6498,22 @@ static int check_return_code(struct bpf_
 			return 0;
 		range = tnum_const(0);
 		break;
+	case BPF_PROG_TYPE_TRACING:
+		switch (env->prog->expected_attach_type) {
+		case BPF_TRACE_FENTRY:
+		case BPF_TRACE_FEXIT:
+			range = tnum_const(0);
+			break;
+		case BPF_TRACE_RAW_TP:
+			return 0;
+		default:
+			return -ENOTSUPP;
+		}
+		break;
+	case BPF_PROG_TYPE_EXT:
+		/* freplace program can return anything as its return value
+		 * depends on the to-be-replaced kernel func or bpf program.
+		 */
 	default:
 		return 0;
 	}