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Subject[PATCH v2] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields
Date
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. For example:

0: 61 10 00 00 00 00 00 00 r0 = *(u32 *)(r1 + 0)

results in the following code generation for a 64-bit field:

ldr x7, [x7] // 64-bit load
mov x10, #0xffffffffffffffff
and x7, x7, x10

Fix the mask generation so that narrow loads always perform a 32-bit AND
operation:

ldr x7, [x7] // 64-bit load
mov w10, #0xffffffff
and w7, w7, w10

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Krzesimir Nowak <krzesimir@kinvolk.io>
Cc: Andrey Ignatov <rdna@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
Signed-off-by: Will Deacon <will@kernel.org>
---

v2: Improve commit message and add Acked-by.

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);
}
}
--
2.40.1.698.g37aff9b760-goog
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