linux-kernel - [PATCH 4.19 04/11] bpf: Do not mark insn as seen under speculative path verification

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Date:   Fri, 13 Aug 2021 17:07:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        bpf@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Benedict Schlueter <benedict.schlueter@....de>,
        Piotr Krysiuk <piotras@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Ovidiu Panait <ovidiu.panait@...driver.com>
Subject: [PATCH 4.19 04/11] bpf: Do not mark insn as seen under speculative path verification

From: Daniel Borkmann <daniel@...earbox.net>

commit fe9a5ca7e370e613a9a75a13008a3845ea759d6e upstream.

... in such circumstances, we do not want to mark the instruction as seen given
the goal is still to jmp-1 rewrite/sanitize dead code, if it is not reachable
from the non-speculative path verification. We do however want to verify it for
safety regardless.

With the patch as-is all the insns that have been marked as seen before the
patch will also be marked as seen after the patch (just with a potentially
different non-zero count). An upcoming patch will also verify paths that are
unreachable in the non-speculative domain, hence this extension is needed.

Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Reviewed-by: John Fastabend <john.fastabend@...il.com>
Reviewed-by: Benedict Schlueter <benedict.schlueter@....de>
Reviewed-by: Piotr Krysiuk <piotras@...il.com>
Acked-by: Alexei Starovoitov <ast@...nel.org>
[OP: - env->pass_cnt is not used in 4.19, so adjust sanitize_mark_insn_seen()
       to assign "true" instead
     - drop sanitize_insn_aux_data() comment changes, as the function is not
       present in 4.19]
Signed-off-by: Ovidiu Panait <ovidiu.panait@...driver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/bpf/verifier.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2901,6 +2901,19 @@ do_sim:
 	return !ret ? REASON_STACK : 0;
 }
 
+static void sanitize_mark_insn_seen(struct bpf_verifier_env *env)
+{
+	struct bpf_verifier_state *vstate = env->cur_state;
+
+	/* If we simulate paths under speculation, we don't update the
+	 * insn as 'seen' such that when we verify unreachable paths in
+	 * the non-speculative domain, sanitize_dead_code() can still
+	 * rewrite/sanitize them.
+	 */
+	if (!vstate->speculative)
+		env->insn_aux_data[env->insn_idx].seen = true;
+}
+
 static int sanitize_err(struct bpf_verifier_env *env,
 			const struct bpf_insn *insn, int reason,
 			const struct bpf_reg_state *off_reg,
@@ -5254,7 +5267,7 @@ static int do_check(struct bpf_verifier_
 		}
 
 		regs = cur_regs(env);
-		env->insn_aux_data[env->insn_idx].seen = true;
+		sanitize_mark_insn_seen(env);
 
 		if (class == BPF_ALU || class == BPF_ALU64) {
 			err = check_alu_op(env, insn);
@@ -5472,7 +5485,7 @@ process_bpf_exit:
 					return err;
 
 				env->insn_idx++;
-				env->insn_aux_data[env->insn_idx].seen = true;
+				sanitize_mark_insn_seen(env);
 			} else {
 				verbose(env, "invalid BPF_LD mode\n");
 				return -EINVAL;