[PATCH 5.12 05/17] bpf: Fix masking negation logic upon negative dst register

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Daniel Borkmann <daniel@iogearbox.net>

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 <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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/bpf/verifier.c |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11760,14 +11760,10 @@ static int fixup_bpf_calls(struct bpf_ve
 			*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;