riscv, bpf: Add BPF exception tables

When a tracing BPF program attempts to read memory without using the
bpf_probe_read() helper, the verifier marks the load instruction with
the BPF_PROBE_MEM flag. Since the riscv JIT does not currently recognize
this flag it falls back to the interpreter.

Add support for BPF_PROBE_MEM, by appending an exception table to the
BPF program. If the load instruction causes a data abort, the fixup
infrastructure finds the exception table and fixes up the fault, by
clearing the destination register and jumping over the faulting
instruction.

A more generic solution would add a "handler" field to the table entry,
like on x86 and s390. The same issue in ARM64 is fixed in 80083428
("bpf, arm64: Add BPF exception tables").
Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Pu Lehui <pulehui@huawei.com>
Tested-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20211027111822.3801679-1-tongtiangen@huawei.com

arch/riscv/mm/extable.c

@@ -11,14 +11,23 @@
 #include <linux/module.h>
 #include <linux/uaccess.h>
 
+#ifdef CONFIG_BPF_JIT
+int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
+#endif
+
 int fixup_exception(struct pt_regs *regs)
 {
 	const struct exception_table_entry *fixup;
 
 	fixup = search_exception_tables(regs->epc);
-	if (fixup) {
+	if (!fixup)
+		return 0;
+
+#ifdef CONFIG_BPF_JIT
+	if (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END)
+		return rv_bpf_fixup_exception(fixup, regs);
+#endif
+
 	regs->epc = fixup->fixup;
 	return 1;
-	}
-	return 0;
 }

arch/riscv/net/bpf_jit.h

@@ -71,6 +71,7 @@ struct rv_jit_context {
 	int ninsns;
 	int epilogue_offset;
 	int *offset;		/* BPF to RV */
+	int nexentries;
 	unsigned long flags;
 	int stack_size;
 };

arch/riscv/net/bpf_jit_comp64.c

@@ -5,6 +5,7 @@
  *
  */
 
+#include <linux/bitfield.h>
 #include <linux/bpf.h>
 #include <linux/filter.h>
 #include "bpf_jit.h"
@@ -27,6 +28,21 @@ static const int regmap[] = {
 	[BPF_REG_AX] =	RV_REG_T0,
 };
 
+static const int pt_regmap[] = {
+	[RV_REG_A0] = offsetof(struct pt_regs, a0),
+	[RV_REG_A1] = offsetof(struct pt_regs, a1),
+	[RV_REG_A2] = offsetof(struct pt_regs, a2),
+	[RV_REG_A3] = offsetof(struct pt_regs, a3),
+	[RV_REG_A4] = offsetof(struct pt_regs, a4),
+	[RV_REG_A5] = offsetof(struct pt_regs, a5),
+	[RV_REG_S1] = offsetof(struct pt_regs, s1),
+	[RV_REG_S2] = offsetof(struct pt_regs, s2),
+	[RV_REG_S3] = offsetof(struct pt_regs, s3),
+	[RV_REG_S4] = offsetof(struct pt_regs, s4),
+	[RV_REG_S5] = offsetof(struct pt_regs, s5),
+	[RV_REG_T0] = offsetof(struct pt_regs, t0),
+};
+
 enum {
 	RV_CTX_F_SEEN_TAIL_CALL =	0,
 	RV_CTX_F_SEEN_CALL =		RV_REG_RA,
@@ -440,6 +456,69 @@ static int emit_call(bool fixed, u64 addr, struct rv_jit_context *ctx)
 	return 0;
 }
 
+#define BPF_FIXUP_OFFSET_MASK   GENMASK(26, 0)
+#define BPF_FIXUP_REG_MASK      GENMASK(31, 27)
+
+int rv_bpf_fixup_exception(const struct exception_table_entry *ex,
+				struct pt_regs *regs)
+{
+	off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup);
+	int regs_offset = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup);
+
+	*(unsigned long *)((void *)regs + pt_regmap[regs_offset]) = 0;
+	regs->epc = (unsigned long)&ex->fixup - offset;
+
+	return 1;
+}
+
+/* For accesses to BTF pointers, add an entry to the exception table */
+static int add_exception_handler(const struct bpf_insn *insn,
+				 struct rv_jit_context *ctx,
+				 int dst_reg, int insn_len)
+{
+	struct exception_table_entry *ex;
+	unsigned long pc;
+	off_t offset;
+
+	if (!ctx->insns || !ctx->prog->aux->extable || BPF_MODE(insn->code) != BPF_PROBE_MEM)
+		return 0;
+
+	if (WARN_ON_ONCE(ctx->nexentries >= ctx->prog->aux->num_exentries))
+		return -EINVAL;
+
+	if (WARN_ON_ONCE(insn_len > ctx->ninsns))
+		return -EINVAL;
+
+	if (WARN_ON_ONCE(!rvc_enabled() && insn_len == 1))
+		return -EINVAL;
+
+	ex = &ctx->prog->aux->extable[ctx->nexentries];
+	pc = (unsigned long)&ctx->insns[ctx->ninsns - insn_len];
+
+	offset = pc - (long)&ex->insn;
+	if (WARN_ON_ONCE(offset >= 0 || offset < INT_MIN))
+		return -ERANGE;
+	ex->insn = pc;
+
+	/*
+	 * Since the extable follows the program, the fixup offset is always
+	 * negative and limited to BPF_JIT_REGION_SIZE. Store a positive value
+	 * to keep things simple, and put the destination register in the upper
+	 * bits. We don't need to worry about buildtime or runtime sort
+	 * modifying the upper bits because the table is already sorted, and
+	 * isn't part of the main exception table.
+	 */
+	offset = (long)&ex->fixup - (pc + insn_len * sizeof(u16));
+	if (!FIELD_FIT(BPF_FIXUP_OFFSET_MASK, offset))
+		return -ERANGE;
+
+	ex->fixup = FIELD_PREP(BPF_FIXUP_OFFSET_MASK, offset) |
+		FIELD_PREP(BPF_FIXUP_REG_MASK, dst_reg);
+
+	ctx->nexentries++;
+	return 0;
+}
+
 int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		      bool extra_pass)
 {
@@ -893,52 +972,86 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 
 	/* LDX: dst = *(size *)(src + off) */
 	case BPF_LDX | BPF_MEM | BPF_B:
+	case BPF_LDX | BPF_MEM | BPF_H:
+	case BPF_LDX | BPF_MEM | BPF_W:
+	case BPF_LDX | BPF_MEM | BPF_DW:
+	case BPF_LDX | BPF_PROBE_MEM | BPF_B:
+	case BPF_LDX | BPF_PROBE_MEM | BPF_H:
+	case BPF_LDX | BPF_PROBE_MEM | BPF_W:
+	case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
+	{
+		int insn_len, insns_start;
+
+		switch (BPF_SIZE(code)) {
+		case BPF_B:
 			if (is_12b_int(off)) {
+				insns_start = ctx->ninsns;
 				emit(rv_lbu(rd, off, rs), ctx);
+				insn_len = ctx->ninsns - insns_start;
 				break;
 			}
 
 			emit_imm(RV_REG_T1, off, ctx);
 			emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
+			insns_start = ctx->ninsns;
 			emit(rv_lbu(rd, 0, RV_REG_T1), ctx);
+			insn_len = ctx->ninsns - insns_start;
 			if (insn_is_zext(&insn[1]))
 				return 1;
 			break;
-	case BPF_LDX | BPF_MEM | BPF_H:
+		case BPF_H:
 			if (is_12b_int(off)) {
+				insns_start = ctx->ninsns;
 				emit(rv_lhu(rd, off, rs), ctx);
+				insn_len = ctx->ninsns - insns_start;
 				break;
 			}
 
 			emit_imm(RV_REG_T1, off, ctx);
 			emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
+			insns_start = ctx->ninsns;
 			emit(rv_lhu(rd, 0, RV_REG_T1), ctx);
+			insn_len = ctx->ninsns - insns_start;
 			if (insn_is_zext(&insn[1]))
 				return 1;
 			break;
-	case BPF_LDX | BPF_MEM | BPF_W:
+		case BPF_W:
 			if (is_12b_int(off)) {
+				insns_start = ctx->ninsns;
 				emit(rv_lwu(rd, off, rs), ctx);
+				insn_len = ctx->ninsns - insns_start;
 				break;
 			}
 
 			emit_imm(RV_REG_T1, off, ctx);
 			emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
+			insns_start = ctx->ninsns;
 			emit(rv_lwu(rd, 0, RV_REG_T1), ctx);
+			insn_len = ctx->ninsns - insns_start;
 			if (insn_is_zext(&insn[1]))
 				return 1;
 			break;
-	case BPF_LDX | BPF_MEM | BPF_DW:
+		case BPF_DW:
 			if (is_12b_int(off)) {
+				insns_start = ctx->ninsns;
 				emit_ld(rd, off, rs, ctx);
+				insn_len = ctx->ninsns - insns_start;
 				break;
 			}
 
 			emit_imm(RV_REG_T1, off, ctx);
 			emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
+			insns_start = ctx->ninsns;
 			emit_ld(rd, 0, RV_REG_T1, ctx);
+			insn_len = ctx->ninsns - insns_start;
 			break;
+		}
 
+		ret = add_exception_handler(insn, ctx, rd, insn_len);
+		if (ret)
+			return ret;
+		break;
+	}
 	/* speculation barrier */
 	case BPF_ST | BPF_NOSPEC:
 		break;

arch/riscv/net/bpf_jit_core.c

@@ -41,12 +41,12 @@ bool bpf_jit_needs_zext(void)
 
 struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 {
+	unsigned int prog_size = 0, extable_size = 0;
 	bool tmp_blinded = false, extra_pass = false;
 	struct bpf_prog *tmp, *orig_prog = prog;
 	int pass = 0, prev_ninsns = 0, i;
 	struct rv_jit_data *jit_data;
 	struct rv_jit_context *ctx;
-	unsigned int image_size = 0;
 
 	if (!prog->jit_requested)
 		return orig_prog;
@@ -73,7 +73,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 
 	if (ctx->offset) {
 		extra_pass = true;
-		image_size = sizeof(*ctx->insns) * ctx->ninsns;
+		prog_size = sizeof(*ctx->insns) * ctx->ninsns;
 		goto skip_init_ctx;
 	}
 
@@ -102,10 +102,13 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		if (ctx->ninsns == prev_ninsns) {
 			if (jit_data->header)
 				break;
+			/* obtain the actual image size */
+			extable_size = prog->aux->num_exentries *
+				sizeof(struct exception_table_entry);
+			prog_size = sizeof(*ctx->insns) * ctx->ninsns;
 
-			image_size = sizeof(*ctx->insns) * ctx->ninsns;
 			jit_data->header =
-				bpf_jit_binary_alloc(image_size,
+				bpf_jit_binary_alloc(prog_size + extable_size,
 						     &jit_data->image,
 						     sizeof(u32),
 						     bpf_fill_ill_insns);
@@ -130,9 +133,13 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		goto out_offset;
 	}
 
+	if (extable_size)
+		prog->aux->extable = (void *)ctx->insns + prog_size;
+
 skip_init_ctx:
 	pass++;
 	ctx->ninsns = 0;
+	ctx->nexentries = 0;
 
 	bpf_jit_build_prologue(ctx);
 	if (build_body(ctx, extra_pass, NULL)) {
@@ -143,11 +150,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	bpf_jit_build_epilogue(ctx);
 
 	if (bpf_jit_enable > 1)
-		bpf_jit_dump(prog->len, image_size, pass, ctx->insns);
+		bpf_jit_dump(prog->len, prog_size, pass, ctx->insns);
 
 	prog->bpf_func = (void *)ctx->insns;
 	prog->jited = 1;
-	prog->jited_len = image_size;
+	prog->jited_len = prog_size;
 
 	bpf_flush_icache(jit_data->header, ctx->insns + ctx->ninsns);