bpf: Fix masking negation logic upon negative dst register

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 <daniel@iogearbox.net>
Tested-by: Piotr Krysiuk <piotras@gmail.com>
Reviewed-by: Piotr Krysiuk <piotras@gmail.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8fd552c..db347fb 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12391,14 +12391,10 @@ static int do_misc_fixups(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;