netdev - [PATCH bpf 6/9] bpf: force strict alignment checks for stack pointers

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Date:   Mon, 18 Dec 2017 20:11:58 -0800
From:   Alexei Starovoitov <ast@...nel.org>
To:     "David S . Miller" <davem@...emloft.net>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Edward Cree <ecree@...arflare.com>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: [PATCH bpf 6/9] bpf: force strict alignment checks for stack pointers

From: Jann Horn <jannh@...gle.com>

Force strict alignment checks for stack pointers because the tracking of
stack spills relies on it; unaligned stack accesses can lead to corruption
of spilled registers, which is exploitable.

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 77e4b5223867..102c519836f6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1059,6 +1059,11 @@ static int check_ptr_alignment(struct bpf_verifier_env *env,
 		break;
 	case PTR_TO_STACK:
 		pointer_desc = "stack ";
+		/* The stack spill tracking logic in check_stack_write()
+		 * and check_stack_read() relies on stack accesses being
+		 * aligned.
+		 */
+		strict = true;
 		break;
 	default:
 		break;
-- 
2.9.5