linux-kernel - [PATCH 1/5] bpf: Clear callee saved regs after updating REG0

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Date:   Mon,  8 Aug 2022 08:53:37 -0700
From:   David Vernet <void@...ifault.com>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        john.fastabend@...il.com, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, tj@...nel.org, joannelkoong@...il.com,
        linux-kernel@...r.kernel.org, Kernel-team@...com
Subject: [PATCH 1/5] bpf: Clear callee saved regs after updating REG0

In the verifier, we currently reset all of the registers containing caller
saved args before updating the callee's return register (REG0). In a
follow-on patch, we will need to be able to be able to inspect the caller
saved registers when updating REG0 to determine if a dynptr that's passed
to a helper function was allocated by a helper, or allocated by a program.

This patch therefore updates check_helper_call() to clear the caller saved
regs after updating REG0.

Signed-off-by: David Vernet <void@...ifault.com>
---
 kernel/bpf/verifier.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 096fdac70165..938ba1536249 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7348,11 +7348,9 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 	if (err)
 		return err;
 
-	/* reset caller saved regs */
-	for (i = 0; i < CALLER_SAVED_REGS; i++) {
-		mark_reg_not_init(env, regs, caller_saved[i]);
-		check_reg_arg(env, caller_saved[i], DST_OP_NO_MARK);
-	}
+	/* reset return reg */
+	mark_reg_not_init(env, regs, BPF_REG_0);
+	check_reg_arg(env, BPF_REG_0, DST_OP_NO_MARK);
 
 	/* helper call returns 64-bit value. */
 	regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
@@ -7488,6 +7486,13 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		regs[BPF_REG_0].ref_obj_id = dynptr_id;
 	}
 
+	/* reset remaining caller saved regs */
+	BUILD_BUG_ON(caller_saved[0] != BPF_REG_0);
+	for (i = 1; i < CALLER_SAVED_REGS; i++) {
+		mark_reg_not_init(env, regs, caller_saved[i]);
+		check_reg_arg(env, caller_saved[i], DST_OP_NO_MARK);
+	}
+
 	do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
 
 	err = check_map_func_compatibility(env, meta.map_ptr, func_id);
-- 
2.30.2