MIPS: bpf: Return error code if the offset is a negative number

Previously, the negative offset was not checked leading to failures
due to trying to load data beyond the skb struct boundaries. Until we
have proper asm helpers in place, it's best if we return ENOSUPP if K
is negative when trying to JIT the filter or 0 during runtime if we
do an indirect load where the value of X is unknown during build time.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: netdev@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7123/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/net/bpf_jit.c

@@ -331,6 +331,12 @@ static inline void emit_srl(unsigned int dst, unsigned int src,
 	emit_instr(ctx, srl, dst, src, sa);
 }
 
+static inline void emit_slt(unsigned int dst, unsigned int src1,
+			    unsigned int src2, struct jit_ctx *ctx)
+{
+	emit_instr(ctx, slt, dst, src1, src2);
+}
+
 static inline void emit_sltu(unsigned int dst, unsigned int src1,
 			     unsigned int src2, struct jit_ctx *ctx)
 {
@@ -816,8 +822,21 @@ static int build_body(struct jit_ctx *ctx)
 			/* A <- P[k:1] */
 			load_order = 0;
 load:
+			/* the interpreter will deal with the negative K */
+			if ((int)k < 0)
+				return -ENOTSUPP;
+
 			emit_load_imm(r_off, k, ctx);
 load_common:
+			/*
+			 * We may got here from the indirect loads so
+			 * return if offset is negative.
+			 */
+			emit_slt(r_s0, r_off, r_zero, ctx);
+			emit_bcond(MIPS_COND_NE, r_s0, r_zero,
+				   b_imm(prog->len, ctx), ctx);
+			emit_reg_move(r_ret, r_zero, ctx);
+
 			ctx->flags |= SEEN_CALL | SEEN_OFF | SEEN_S0 |
 				SEEN_SKB | SEEN_A;
 
@@ -880,6 +899,10 @@ load_ind:
 			emit_load(r_X, r_skb, off, ctx);
 			break;
 		case BPF_LDX | BPF_B | BPF_MSH:
+			/* the interpreter will deal with the negative K */
+			if ((int)k < 0)
+				return -ENOTSUPP;
+
 			/* X <- 4 * (P[k:1] & 0xf) */
 			ctx->flags |= SEEN_X | SEEN_CALL | SEEN_S0 | SEEN_SKB;
 			/* Load offset to a1 */