From 53e0db429b37a32b8fc706d0d90eb4583ad13848 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Fri, 30 Apr 2021 16:21:46 +0200 Subject: [PATCH] bpf: Fix masking negation logic upon negative dst register commit b9b34ddbe2076ade359cd5ce7537d5ed019e9807 upstream. The negation logic for the case where the off_reg is sitting in the dst register is not correct given then we cannot just invert the add to a sub or vice versa. As a fix, perform the final bitwise and-op unconditionally into AX from the off_reg, then move the pointer from the src to dst and finally use AX as the source for the original pointer arithmetic operation such that the inversion yields a correct result. The single non-AX mov in between is possible given constant blinding is retaining it as it's not an immediate based operation. Fixes: 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic") Signed-off-by: Daniel Borkmann Tested-by: Piotr Krysiuk Reviewed-by: Piotr Krysiuk Reviewed-by: John Fastabend Acked-by: Alexei Starovoitov Signed-off-by: Greg Kroah-Hartman --- kernel/bpf/verifier.c | 12 ++++-------- 1 file changed, 4 insertions(+), 8 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index bc439dcd438f..ff94a87573d2 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -9176,14 +9176,10 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env) *patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg); *patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0); *patch++ = BPF_ALU64_IMM(BPF_ARSH, BPF_REG_AX, 63); - if (issrc) { - *patch++ = BPF_ALU64_REG(BPF_AND, BPF_REG_AX, - off_reg); - insn->src_reg = BPF_REG_AX; - } else { - *patch++ = BPF_ALU64_REG(BPF_AND, off_reg, - BPF_REG_AX); - } + *patch++ = BPF_ALU64_REG(BPF_AND, BPF_REG_AX, off_reg); + if (!issrc) + *patch++ = BPF_MOV64_REG(insn->dst_reg, insn->src_reg); + insn->src_reg = BPF_REG_AX; if (isneg) insn->code = insn->code == code_add ? code_sub : code_add;