riscv, bpf: Fix offset range checking for auipc+jalr on RV64

The existing code in emit_call on RV64 checks that the PC-relative offset
to the function fits in 32 bits before calling emit_jump_and_link to emit
an auipc+jalr pair. However, this check is incorrect because offsets in
the range [2^31 - 2^11, 2^31 - 1] cannot be encoded using auipc+jalr on
RV64 (see discussion [1]). The RISC-V spec has recently been updated
to reflect this fact [2, 3].

This patch fixes the problem by moving the check on the offset into
emit_jump_and_link and modifying it to the correct range of encodable
offsets, which is [-2^31 - 2^11, 2^31 - 2^11). This also enforces the
check on the offset to other uses of emit_jump_and_link (e.g., BPF_JA)
as well.

Currently, this bug is unlikely to be triggered, because the memory
region from which JITed images are allocated is close enough to kernel
text for the offsets to not become too large; and because the bounds on
BPF program size are small enough. This patch prevents this problem from
becoming an issue if either of these change.

[1]: https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/bwWFhBnnZFQ
[2]: b1e42e09ac
[3]: 4c1b2066eb

Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200406221604.18547-1-luke.r.nels@gmail.com

arch/riscv/net/bpf_jit_comp64.c

@@ -110,6 +110,16 @@ static bool is_32b_int(s64 val)
 	return -(1L << 31) <= val && val < (1L << 31);
 }
 
+static bool in_auipc_jalr_range(s64 val)
+{
+	/*
+	 * auipc+jalr can reach any signed PC-relative offset in the range
+	 * [-2^31 - 2^11, 2^31 - 2^11).
+	 */
+	return (-(1L << 31) - (1L << 11)) <= val &&
+		val < ((1L << 31) - (1L << 11));
+}
+
 static void emit_imm(u8 rd, s64 val, struct rv_jit_context *ctx)
 {
 	/* Note that the immediate from the add is sign-extended,
@@ -380,20 +390,24 @@ static void emit_sext_32_rd(u8 *rd, struct rv_jit_context *ctx)
 	*rd = RV_REG_T2;
 }
 
-static void emit_jump_and_link(u8 rd, s64 rvoff, bool force_jalr,
-			       struct rv_jit_context *ctx)
+static int emit_jump_and_link(u8 rd, s64 rvoff, bool force_jalr,
+			      struct rv_jit_context *ctx)
 {
 	s64 upper, lower;
 
 	if (rvoff && is_21b_int(rvoff) && !force_jalr) {
 		emit(rv_jal(rd, rvoff >> 1), ctx);
-		return;
+		return 0;
+	} else if (in_auipc_jalr_range(rvoff)) {
+		upper = (rvoff + (1 << 11)) >> 12;
+		lower = rvoff & 0xfff;
+		emit(rv_auipc(RV_REG_T1, upper), ctx);
+		emit(rv_jalr(rd, RV_REG_T1, lower), ctx);
+		return 0;
 	}
 
-	upper = (rvoff + (1 << 11)) >> 12;
-	lower = rvoff & 0xfff;
-	emit(rv_auipc(RV_REG_T1, upper), ctx);
-	emit(rv_jalr(rd, RV_REG_T1, lower), ctx);
+	pr_err("bpf-jit: target offset 0x%llx is out of range\n", rvoff);
+	return -ERANGE;
 }
 
 static bool is_signed_bpf_cond(u8 cond)
@@ -407,18 +421,16 @@ static int emit_call(bool fixed, u64 addr, struct rv_jit_context *ctx)
 	s64 off = 0;
 	u64 ip;
 	u8 rd;
+	int ret;
 
 	if (addr && ctx->insns) {
 		ip = (u64)(long)(ctx->insns + ctx->ninsns);
 		off = addr - ip;
-		if (!is_32b_int(off)) {
-			pr_err("bpf-jit: target call addr %pK is out of range\n",
-			       (void *)addr);
-			return -ERANGE;
-		}
 	}
 
-	emit_jump_and_link(RV_REG_RA, off, !fixed, ctx);
+	ret = emit_jump_and_link(RV_REG_RA, off, !fixed, ctx);
+	if (ret)
+		return ret;
 	rd = bpf_to_rv_reg(BPF_REG_0, ctx);
 	emit(rv_addi(rd, RV_REG_A0, 0), ctx);
 	return 0;
@@ -429,7 +441,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 {
 	bool is64 = BPF_CLASS(insn->code) == BPF_ALU64 ||
 		    BPF_CLASS(insn->code) == BPF_JMP;
-	int s, e, rvoff, i = insn - ctx->prog->insnsi;
+	int s, e, rvoff, ret, i = insn - ctx->prog->insnsi;
 	struct bpf_prog_aux *aux = ctx->prog->aux;
 	u8 rd = -1, rs = -1, code = insn->code;
 	s16 off = insn->off;
@@ -699,7 +711,9 @@ out_be:
 	/* JUMP off */
 	case BPF_JMP | BPF_JA:
 		rvoff = rv_offset(i, off, ctx);
-		emit_jump_and_link(RV_REG_ZERO, rvoff, false, ctx);
+		ret = emit_jump_and_link(RV_REG_ZERO, rvoff, false, ctx);
+		if (ret)
+			return ret;
 		break;
 
 	/* IF (dst COND src) JUMP off */
@@ -801,7 +815,6 @@ out_be:
 	case BPF_JMP | BPF_CALL:
 	{
 		bool fixed;
-		int ret;
 		u64 addr;
 
 		mark_call(ctx);
@@ -826,7 +839,9 @@ out_be:
 			break;
 
 		rvoff = epilogue_offset(ctx);
-		emit_jump_and_link(RV_REG_ZERO, rvoff, false, ctx);
+		ret = emit_jump_and_link(RV_REG_ZERO, rvoff, false, ctx);
+		if (ret)
+			return ret;
 		break;
 
 	/* dst = imm64 */