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Message-Id: <20230502165754.16728-1-will@kernel.org>
Date: Tue, 2 May 2023 17:57:54 +0100
From: Will Deacon <will@...nel.org>
To: bpf@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
Will Deacon <will@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
John Fastabend <john.fastabend@...il.com>,
Krzesimir Nowak <krzesimir@...volk.io>,
Yonghong Song <yhs@...com>, Andrey Ignatov <rdna@...com>
Subject: [PATCH] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields
A narrow load from a 64-bit context field results in a 64-bit load
followed potentially by a 64-bit right-shift and then a bitwise AND
operation to extract the relevant data.
In the case of a 32-bit access, an immediate mask of 0xffffffff is used
to construct a 64-bit BPP_AND operation which then sign-extends the mask
value and effectively acts as a glorified no-op.
Fix the mask generation so that narrow loads always perform a 32-bit AND
operation.
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: John Fastabend <john.fastabend@...il.com>
Cc: Krzesimir Nowak <krzesimir@...volk.io>
Cc: Yonghong Song <yhs@...com>
Cc: Andrey Ignatov <rdna@...com>
Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
Signed-off-by: Will Deacon <will@...nel.org>
---
I spotted this while playing around with the JIT on arm64. I can't
figure out why 31fd85816dbe special-cases 8-byte ctx fields in the
first place, so I fear I may be missing something...
kernel/bpf/verifier.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index fbcf5a4e2fcd..5871aa78d01a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17033,7 +17033,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH,
insn->dst_reg,
shift);
- insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
+ insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
(1ULL << size * 8) - 1);
}
}
--
2.40.1.495.gc816e09b53d-goog
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