Merge branch 'bpf-ptrs-beyond-pkt-end'

Alexei Starovoitov says:

====================
v1->v2:
- removed set-but-unused variable.
- added Jiri's Tested-by.

In some cases LLVM uses the knowledge that branch is taken to optimze the code
which causes the verifier to reject valid programs.
Teach the verifier to recognize that
r1 = skb->data;
r1 += 10;
r2 = skb->data_end;
if (r1 > r2) {
  here r1 points beyond packet_end and subsequent
  if (r1 > r2) // always evaluates to "true".
}
====================
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -45,7 +45,7 @@ struct bpf_reg_state {
 	enum bpf_reg_type type;
 	union {
 		/* valid when type == PTR_TO_PACKET */
-		u16 range;
+		int range;
 
 		/* valid when type == CONST_PTR_TO_MAP | PTR_TO_MAP_VALUE |
 		 *   PTR_TO_MAP_VALUE_OR_NULL

kernel/bpf/verifier.c

@@ -2739,7 +2739,9 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
 			regno);
 		return -EACCES;
 	}
-	err = __check_mem_access(env, regno, off, size, reg->range,
+
+	err = reg->range < 0 ? -EINVAL :
+	      __check_mem_access(env, regno, off, size, reg->range,
 				 zero_size_allowed);
 	if (err) {
 		verbose(env, "R%d offset is outside of the packet\n", regno);
@@ -4697,6 +4699,32 @@ static void clear_all_pkt_pointers(struct bpf_verifier_env *env)
 		__clear_all_pkt_pointers(env, vstate->frame[i]);
 }
 
+enum {
+	AT_PKT_END = -1,
+	BEYOND_PKT_END = -2,
+};
+
+static void mark_pkt_end(struct bpf_verifier_state *vstate, int regn, bool range_open)
+{
+	struct bpf_func_state *state = vstate->frame[vstate->curframe];
+	struct bpf_reg_state *reg = &state->regs[regn];
+
+	if (reg->type != PTR_TO_PACKET)
+		/* PTR_TO_PACKET_META is not supported yet */
+		return;
+
+	/* The 'reg' is pkt > pkt_end or pkt >= pkt_end.
+	 * How far beyond pkt_end it goes is unknown.
+	 * if (!range_open) it's the case of pkt >= pkt_end
+	 * if (range_open) it's the case of pkt > pkt_end
+	 * hence this pointer is at least 1 byte bigger than pkt_end
+	 */
+	if (range_open)
+		reg->range = BEYOND_PKT_END;
+	else
+		reg->range = AT_PKT_END;
+}
+
 static void release_reg_references(struct bpf_verifier_env *env,
 				   struct bpf_func_state *state,
 				   int ref_obj_id)
@@ -6708,7 +6736,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 
 static void __find_good_pkt_pointers(struct bpf_func_state *state,
 				     struct bpf_reg_state *dst_reg,
-				     enum bpf_reg_type type, u16 new_range)
+				     enum bpf_reg_type type, int new_range)
 {
 	struct bpf_reg_state *reg;
 	int i;
@@ -6733,8 +6761,7 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate,
 				   enum bpf_reg_type type,
 				   bool range_right_open)
 {
-	u16 new_range;
-	int i;
+	int new_range, i;
 
 	if (dst_reg->off < 0 ||
 	    (dst_reg->off == 0 && range_right_open))
@@ -6985,6 +7012,67 @@ static int is_branch_taken(struct bpf_reg_state *reg, u64 val, u8 opcode,
 	return is_branch64_taken(reg, val, opcode);
 }
 
+static int flip_opcode(u32 opcode)
+{
+	/* How can we transform "a <op> b" into "b <op> a"? */
+	static const u8 opcode_flip[16] = {
+		/* these stay the same */
+		[BPF_JEQ  >> 4] = BPF_JEQ,
+		[BPF_JNE  >> 4] = BPF_JNE,
+		[BPF_JSET >> 4] = BPF_JSET,
+		/* these swap "lesser" and "greater" (L and G in the opcodes) */
+		[BPF_JGE  >> 4] = BPF_JLE,
+		[BPF_JGT  >> 4] = BPF_JLT,
+		[BPF_JLE  >> 4] = BPF_JGE,
+		[BPF_JLT  >> 4] = BPF_JGT,
+		[BPF_JSGE >> 4] = BPF_JSLE,
+		[BPF_JSGT >> 4] = BPF_JSLT,
+		[BPF_JSLE >> 4] = BPF_JSGE,
+		[BPF_JSLT >> 4] = BPF_JSGT
+	};
+	return opcode_flip[opcode >> 4];
+}
+
+static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
+				   struct bpf_reg_state *src_reg,
+				   u8 opcode)
+{
+	struct bpf_reg_state *pkt;
+
+	if (src_reg->type == PTR_TO_PACKET_END) {
+		pkt = dst_reg;
+	} else if (dst_reg->type == PTR_TO_PACKET_END) {
+		pkt = src_reg;
+		opcode = flip_opcode(opcode);
+	} else {
+		return -1;
+	}
+
+	if (pkt->range >= 0)
+		return -1;
+
+	switch (opcode) {
+	case BPF_JLE:
+		/* pkt <= pkt_end */
+		fallthrough;
+	case BPF_JGT:
+		/* pkt > pkt_end */
+		if (pkt->range == BEYOND_PKT_END)
+			/* pkt has at last one extra byte beyond pkt_end */
+			return opcode == BPF_JGT;
+		break;
+	case BPF_JLT:
+		/* pkt < pkt_end */
+		fallthrough;
+	case BPF_JGE:
+		/* pkt >= pkt_end */
+		if (pkt->range == BEYOND_PKT_END || pkt->range == AT_PKT_END)
+			return opcode == BPF_JGE;
+		break;
+	}
+	return -1;
+}
+
 /* Adjusts the register min/max values in the case that the dst_reg is the
  * variable register that we are working on, and src_reg is a constant or we're
  * simply doing a BPF_K check.
@@ -7148,23 +7236,7 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
 				u64 val, u32 val32,
 				u8 opcode, bool is_jmp32)
 {
-	/* How can we transform "a <op> b" into "b <op> a"? */
-	static const u8 opcode_flip[16] = {
-		/* these stay the same */
-		[BPF_JEQ  >> 4] = BPF_JEQ,
-		[BPF_JNE  >> 4] = BPF_JNE,
-		[BPF_JSET >> 4] = BPF_JSET,
-		/* these swap "lesser" and "greater" (L and G in the opcodes) */
-		[BPF_JGE  >> 4] = BPF_JLE,
-		[BPF_JGT  >> 4] = BPF_JLT,
-		[BPF_JLE  >> 4] = BPF_JGE,
-		[BPF_JLT  >> 4] = BPF_JGT,
-		[BPF_JSGE >> 4] = BPF_JSLE,
-		[BPF_JSGT >> 4] = BPF_JSLT,
-		[BPF_JSLE >> 4] = BPF_JSGE,
-		[BPF_JSLT >> 4] = BPF_JSGT
-	};
-	opcode = opcode_flip[opcode >> 4];
+	opcode = flip_opcode(opcode);
 	/* This uses zero as "not present in table"; luckily the zero opcode,
 	 * BPF_JA, can't get here.
 	 */
@@ -7346,6 +7418,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_data' > pkt_end, pkt_meta' > pkt_data */
 			find_good_pkt_pointers(this_branch, dst_reg,
 					       dst_reg->type, false);
+			mark_pkt_end(other_branch, insn->dst_reg, true);
 		} else if ((dst_reg->type == PTR_TO_PACKET_END &&
 			    src_reg->type == PTR_TO_PACKET) ||
 			   (reg_is_init_pkt_pointer(dst_reg, PTR_TO_PACKET) &&
@@ -7353,6 +7426,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_end > pkt_data', pkt_data > pkt_meta' */
 			find_good_pkt_pointers(other_branch, src_reg,
 					       src_reg->type, true);
+			mark_pkt_end(this_branch, insn->src_reg, false);
 		} else {
 			return false;
 		}
@@ -7365,6 +7439,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_data' < pkt_end, pkt_meta' < pkt_data */
 			find_good_pkt_pointers(other_branch, dst_reg,
 					       dst_reg->type, true);
+			mark_pkt_end(this_branch, insn->dst_reg, false);
 		} else if ((dst_reg->type == PTR_TO_PACKET_END &&
 			    src_reg->type == PTR_TO_PACKET) ||
 			   (reg_is_init_pkt_pointer(dst_reg, PTR_TO_PACKET) &&
@@ -7372,6 +7447,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_end < pkt_data', pkt_data > pkt_meta' */
 			find_good_pkt_pointers(this_branch, src_reg,
 					       src_reg->type, false);
+			mark_pkt_end(other_branch, insn->src_reg, true);
 		} else {
 			return false;
 		}
@@ -7384,6 +7460,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_data' >= pkt_end, pkt_meta' >= pkt_data */
 			find_good_pkt_pointers(this_branch, dst_reg,
 					       dst_reg->type, true);
+			mark_pkt_end(other_branch, insn->dst_reg, false);
 		} else if ((dst_reg->type == PTR_TO_PACKET_END &&
 			    src_reg->type == PTR_TO_PACKET) ||
 			   (reg_is_init_pkt_pointer(dst_reg, PTR_TO_PACKET) &&
@@ -7391,6 +7468,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_end >= pkt_data', pkt_data >= pkt_meta' */
 			find_good_pkt_pointers(other_branch, src_reg,
 					       src_reg->type, false);
+			mark_pkt_end(this_branch, insn->src_reg, true);
 		} else {
 			return false;
 		}
@@ -7403,6 +7481,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_data' <= pkt_end, pkt_meta' <= pkt_data */
 			find_good_pkt_pointers(other_branch, dst_reg,
 					       dst_reg->type, false);
+			mark_pkt_end(this_branch, insn->dst_reg, true);
 		} else if ((dst_reg->type == PTR_TO_PACKET_END &&
 			    src_reg->type == PTR_TO_PACKET) ||
 			   (reg_is_init_pkt_pointer(dst_reg, PTR_TO_PACKET) &&
@@ -7410,6 +7489,7 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
 			/* pkt_end <= pkt_data', pkt_data <= pkt_meta' */
 			find_good_pkt_pointers(this_branch, src_reg,
 					       src_reg->type, true);
+			mark_pkt_end(other_branch, insn->src_reg, false);
 		} else {
 			return false;
 		}
@@ -7509,6 +7589,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 				       src_reg->var_off.value,
 				       opcode,
 				       is_jmp32);
+	} else if (reg_is_pkt_pointer_any(dst_reg) &&
+		   reg_is_pkt_pointer_any(src_reg) &&
+		   !is_jmp32) {
+		pred = is_pkt_ptr_branch_taken(dst_reg, src_reg, opcode);
 	}
 
 	if (pred >= 0) {
@@ -7517,7 +7601,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 		 */
 		if (!__is_pointer_value(false, dst_reg))
 			err = mark_chain_precision(env, insn->dst_reg);
-		if (BPF_SRC(insn->code) == BPF_X && !err)
+		if (BPF_SRC(insn->code) == BPF_X && !err &&
+		    !__is_pointer_value(false, src_reg))
 			err = mark_chain_precision(env, insn->src_reg);
 		if (err)
 			return err;

tools/testing/selftests/bpf/prog_tests/test_skb_pkt_end.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <test_progs.h>
+#include <network_helpers.h>
+#include "skb_pkt_end.skel.h"
+
+static int sanity_run(struct bpf_program *prog)
+{
+	__u32 duration, retval;
+	int err, prog_fd;
+
+	prog_fd = bpf_program__fd(prog);
+	err = bpf_prog_test_run(prog_fd, 1, &pkt_v4, sizeof(pkt_v4),
+				NULL, NULL, &retval, &duration);
+	if (CHECK(err || retval != 123, "test_run",
+		  "err %d errno %d retval %d duration %d\n",
+		  err, errno, retval, duration))
+		return -1;
+	return 0;
+}
+
+void test_test_skb_pkt_end(void)
+{
+	struct skb_pkt_end *skb_pkt_end_skel = NULL;
+	__u32 duration = 0;
+	int err;
+
+	skb_pkt_end_skel = skb_pkt_end__open_and_load();
+	if (CHECK(!skb_pkt_end_skel, "skb_pkt_end_skel_load", "skb_pkt_end skeleton failed\n"))
+		goto cleanup;
+
+	err = skb_pkt_end__attach(skb_pkt_end_skel);
+	if (CHECK(err, "skb_pkt_end_attach", "skb_pkt_end attach failed: %d\n", err))
+		goto cleanup;
+
+	if (sanity_run(skb_pkt_end_skel->progs.main_prog))
+		goto cleanup;
+
+cleanup:
+	skb_pkt_end__destroy(skb_pkt_end_skel);
+}

tools/testing/selftests/bpf/progs/skb_pkt_end.c

+// SPDX-License-Identifier: GPL-2.0
+#define BPF_NO_PRESERVE_ACCESS_INDEX
+#include <vmlinux.h>
+#include <bpf/bpf_core_read.h>
+#include <bpf/bpf_helpers.h>
+
+#define NULL 0
+#define INLINE __always_inline
+
+#define skb_shorter(skb, len) ((void *)(long)(skb)->data + (len) > (void *)(long)skb->data_end)
+
+#define ETH_IPV4_TCP_SIZE (14 + sizeof(struct iphdr) + sizeof(struct tcphdr))
+
+static INLINE struct iphdr *get_iphdr(struct __sk_buff *skb)
+{
+	struct iphdr *ip = NULL;
+	struct ethhdr *eth;
+
+	if (skb_shorter(skb, ETH_IPV4_TCP_SIZE))
+		goto out;
+
+	eth = (void *)(long)skb->data;
+	ip = (void *)(eth + 1);
+
+out:
+	return ip;
+}
+
+SEC("classifier/cls")
+int main_prog(struct __sk_buff *skb)
+{
+	struct iphdr *ip = NULL;
+	struct tcphdr *tcp;
+	__u8 proto = 0;
+
+	if (!(ip = get_iphdr(skb)))
+		goto out;
+
+	proto = ip->protocol;
+
+	if (proto != IPPROTO_TCP)
+		goto out;
+
+	tcp = (void*)(ip + 1);
+	if (tcp->dest != 0)
+		goto out;
+	if (!tcp)
+		goto out;
+
+	return tcp->urg_ptr;
+out:
+	return -1;
+}
+char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/verifier/ctx_skb.c

@@ -1089,3 +1089,45 @@
 	.errstr_unpriv = "R1 leaks addr",
 	.result = REJECT,
 },
+{
+       "pkt > pkt_end taken check",
+       .insns = {
+       BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,                //  0. r2 = *(u32 *)(r1 + data_end)
+                   offsetof(struct __sk_buff, data_end)),
+       BPF_LDX_MEM(BPF_W, BPF_REG_4, BPF_REG_1,                //  1. r4 = *(u32 *)(r1 + data)
+                   offsetof(struct __sk_buff, data)),
+       BPF_MOV64_REG(BPF_REG_3, BPF_REG_4),                    //  2. r3 = r4
+       BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, 42),                  //  3. r3 += 42
+       BPF_MOV64_IMM(BPF_REG_1, 0),                            //  4. r1 = 0
+       BPF_JMP_REG(BPF_JGT, BPF_REG_3, BPF_REG_2, 2),          //  5. if r3 > r2 goto 8
+       BPF_ALU64_IMM(BPF_ADD, BPF_REG_4, 14),                  //  6. r4 += 14
+       BPF_MOV64_REG(BPF_REG_1, BPF_REG_4),                    //  7. r1 = r4
+       BPF_JMP_REG(BPF_JGT, BPF_REG_3, BPF_REG_2, 1),          //  8. if r3 > r2 goto 10
+       BPF_LDX_MEM(BPF_H, BPF_REG_2, BPF_REG_1, 9),            //  9. r2 = *(u8 *)(r1 + 9)
+       BPF_MOV64_IMM(BPF_REG_0, 0),                            // 10. r0 = 0
+       BPF_EXIT_INSN(),                                        // 11. exit
+       },
+       .result = ACCEPT,
+       .prog_type = BPF_PROG_TYPE_SK_SKB,
+},
+{
+       "pkt_end < pkt taken check",
+       .insns = {
+       BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,                //  0. r2 = *(u32 *)(r1 + data_end)
+                   offsetof(struct __sk_buff, data_end)),
+       BPF_LDX_MEM(BPF_W, BPF_REG_4, BPF_REG_1,                //  1. r4 = *(u32 *)(r1 + data)
+                   offsetof(struct __sk_buff, data)),
+       BPF_MOV64_REG(BPF_REG_3, BPF_REG_4),                    //  2. r3 = r4
+       BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, 42),                  //  3. r3 += 42
+       BPF_MOV64_IMM(BPF_REG_1, 0),                            //  4. r1 = 0
+       BPF_JMP_REG(BPF_JGT, BPF_REG_3, BPF_REG_2, 2),          //  5. if r3 > r2 goto 8
+       BPF_ALU64_IMM(BPF_ADD, BPF_REG_4, 14),                  //  6. r4 += 14
+       BPF_MOV64_REG(BPF_REG_1, BPF_REG_4),                    //  7. r1 = r4
+       BPF_JMP_REG(BPF_JLT, BPF_REG_2, BPF_REG_3, 1),          //  8. if r2 < r3 goto 10
+       BPF_LDX_MEM(BPF_H, BPF_REG_2, BPF_REG_1, 9),            //  9. r2 = *(u8 *)(r1 + 9)
+       BPF_MOV64_IMM(BPF_REG_0, 0),                            // 10. r0 = 0
+       BPF_EXIT_INSN(),                                        // 11. exit
+       },
+       .result = ACCEPT,
+       .prog_type = BPF_PROG_TYPE_SK_SKB,
+},