linux-kernel - [RESEND PATCH bpf] bpf: verifier: fix addr_space

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Message-Id: <20240321153939.113996-1-puranjay12@gmail.com>
Date: Thu, 21 Mar 2024 15:39:39 +0000
From: Puranjay Mohan <puranjay12@...il.com>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: puranjay12@...il.com
Subject: [RESEND PATCH bpf] bpf: verifier: fix addr_space_cast from as(1) to as(0)

The verifier currently converts addr_space_cast from as(1) to as(0) that
is: BPF_ALU64 | BPF_MOV | BPF_X with off=1 and imm=1
to
BPF_ALU | BPF_MOV | BPF_X with imm=1 (32-bit mov)

Because of this imm=1, the JITs that have bpf_jit_needs_zext() == true,
interpret the converted instruction as BPF_ZEXT_REG(DST) which is a
special form of mov32, used for doing explicit zero extension on dst.
These JITs will just zero extend the dst reg and will not move the src to
dst before the zext.

Fix do_misc_fixups() to set imm=0 when converting addr_space_cast to a
normal mov32.

The JITs that have bpf_jit_needs_zext() == true rely on the verifier to
emit zext instructions. Mark dst_reg as subreg when doing cast from
as(1) to as(0) so the verifier emits a zext instruction after the mov.

Fixes: 6082b6c328b5 ("bpf: Recognize addr_space_cast instruction in the verifier.")
Signed-off-by: Puranjay Mohan <puranjay12@...il.com>
---
 kernel/bpf/verifier.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index de7813947981..ee796402ef60 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14046,8 +14046,11 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 				if (insn->imm) {
 					/* off == BPF_ADDR_SPACE_CAST */
 					mark_reg_unknown(env, regs, insn->dst_reg);
-					if (insn->imm == 1) /* cast from as(1) to as(0) */
+					if (insn->imm == 1) { /* cast from as(1) to as(0) */
 						dst_reg->type = PTR_TO_ARENA;
+						/* PTR_TO_ARENA is 32-bit */
+						dst_reg->subreg_def = env->insn_idx + 1;
+					}
 				} else if (insn->off == 0) {
 					/* case: R1 = R2
 					 * copy register state to dest reg
@@ -19606,8 +19609,9 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			    (((struct bpf_map *)env->prog->aux->arena)->map_flags & BPF_F_NO_USER_CONV)) {
 				/* convert to 32-bit mov that clears upper 32-bit */
 				insn->code = BPF_ALU | BPF_MOV | BPF_X;
-				/* clear off, so it's a normal 'wX = wY' from JIT pov */
+				/* clear off and imm, so it's a normal 'wX = wY' from JIT pov */
 				insn->off = 0;
+				insn->imm = 0;
 			} /* cast from as(0) to as(1) should be handled by JIT */
 			goto next_insn;
 		}
-- 
2.40.1