On 5/2/23 9:57 AM, Will Deacon wrote:
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@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: Krzesimir Nowak <krzesimir@xxxxxxxxxx>
Cc: Yonghong Song <yhs@xxxxxx>
Cc: Andrey Ignatov <rdna@xxxxxx>
Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Thanks for the fix! You didn't miss anything. It is a bug and we did not
find it probably because user always use 'u64 val = ctx->u64_field' in
their bpf code...
But I think the commit message can be improved. An example to show the
difference without and with this patch can explain the issue much better.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
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);
}
}
