Merge branch 'bpf: Fix to preserve reg parent/live fields when copying range info'

Alexei Starovoitov · Alexei Starovoitov · commit 2eecf81e149c · 2023-01-19T15:19:40.000-08:00
Eduard Zingerman says: ==================== Struct bpf_reg_state is copied directly in several places including: - check_stack_write_fixed_off() (via save_register_state()); - check_stack_read_fixed_off(); - find_equal_scalars(). However, a literal copy of this struct also copies the following fields: struct bpf_reg_state { ... struct bpf_reg_state *parent; ... enum bpf_reg_liveness live; ... }; This breaks register parentage chain and liveness marking logic. The commit message for the first patch has a detailed example. This patch-set replaces direct copies with a call to a function copy_register_state(dst,src), which preserves 'parent' and 'live' fields of the 'dst'. The fix comes with a significant verifier runtime penalty for some selftest binaries listed in tools/testing/selftests/bpf/veristat.cfg and cilium BPF binaries (see [1]): $ ./veristat -e file,prog,states -C -f 'states_diff>10' master-baseline.log current.log File Program States (A) States (B) States (DIFF) -------------------------- -------------------------------- ---------- ---------- --------------- bpf_host.o tail_handle_ipv4_from_host 231 299 +68 (+29.44%) bpf_host.o tail_handle_nat_fwd_ipv4 1088 1320 +232 (+21.32%) bpf_host.o tail_handle_nat_fwd_ipv6 716 729 +13 (+1.82%) bpf_host.o tail_nodeport_nat_ingress_ipv4 281 314 +33 (+11.74%) bpf_host.o tail_nodeport_nat_ingress_ipv6 245 256 +11 (+4.49%) bpf_lxc.o tail_handle_nat_fwd_ipv4 1088 1320 +232 (+21.32%) bpf_lxc.o tail_handle_nat_fwd_ipv6 716 729 +13 (+1.82%) bpf_lxc.o tail_ipv4_ct_egress 239 262 +23 (+9.62%) bpf_lxc.o tail_ipv4_ct_ingress 239 262 +23 (+9.62%) bpf_lxc.o tail_ipv4_ct_ingress_policy_only 239 262 +23 (+9.62%) bpf_lxc.o tail_ipv6_ct_egress 181 195 +14 (+7.73%) bpf_lxc.o tail_ipv6_ct_ingress 181 195 +14 (+7.73%) bpf_lxc.o tail_ipv6_ct_ingress_policy_only 181 195 +14 (+7.73%) bpf_lxc.o tail_nodeport_nat_ingress_ipv4 281 314 +33 (+11.74%) bpf_lxc.o tail_nodeport_nat_ingress_ipv6 245 256 +11 (+4.49%) bpf_overlay.o tail_handle_nat_fwd_ipv4 799 829 +30 (+3.75%) bpf_overlay.o tail_nodeport_nat_ingress_ipv4 281 314 +33 (+11.74%) bpf_overlay.o tail_nodeport_nat_ingress_ipv6 245 256 +11 (+4.49%) bpf_sock.o cil_sock4_connect 47 70 +23 (+48.94%) bpf_sock.o cil_sock4_sendmsg 45 68 +23 (+51.11%) bpf_sock.o cil_sock6_post_bind 31 42 +11 (+35.48%) bpf_xdp.o tail_lb_ipv4 4413 6457 +2044 (+46.32%) bpf_xdp.o tail_lb_ipv6 6876 7249 +373 (+5.42%) test_cls_redirect.bpf.o cls_redirect 4704 4799 +95 (+2.02%) test_tcp_hdr_options.bpf.o estab 180 206 +26 (+14.44%) xdp_synproxy_kern.bpf.o syncookie_tc 21059 21485 +426 (+2.02%) xdp_synproxy_kern.bpf.o syncookie_xdp 21857 23122 +1265 (+5.79%) -------------------------- -------------------------------- ---------- ---------- --------------- I looked through verification log for bpf_xdp.o tail_lb_ipv4 program in order to identify the reason for ~50% visited states increase. The slowdown is triggered by a difference in handling of three stack slots: fp-56, fp-72 and fp-80, with the main difference coming from fp-72. In fact the following change removes all the difference: @@ -3256,7 +3256,10 @@ static void save_register_state(struct bpf_func_state *state, { int i; - copy_register_state(&state->stack[spi].spilled_ptr, reg); + if ((spi == 6 /*56*/ || spi == 8 /*72*/ || spi == 9 /*80*/) && size != BPF_REG_SIZE) + state->stack[spi].spilled_ptr = *reg; + else + copy_register_state(&state->stack[spi].spilled_ptr, reg); For fp-56 I found the following pattern for divergences between verification logs with and w/o this patch: - At some point insn 1862 is reached and checkpoint is created; - At some other point insn 1862 is reached again: - with this patch: - the current state is considered *not* equivalent to the old checkpoint; - the reason for mismatch is the state of fp-56: - current state: fp-56=????mmmm - checkpoint: fp-56_rD=mmmmmmmm - without this patch the current state is considered equivalent to the checkpoint, the fp-56 is not present in the checkpoint. Here is a fragment of the verification log for when the checkpoint in question created at insn 1862: checkpoint 1862: ... fp-56=mmmmmmmm ... 1862: ... 1863: ... 1864: (61) r1 = *(u32 *)(r0 +0) 1865: ... 1866: (63) *(u32 *)(r10 -56) = r1 ; R1_w=scalar(...) R10=fp0 fp-56= 1867: (bf) r2 = r10 ; R2_w=fp0 R10=fp0 1868: (07) r2 += -56 ; R2_w=fp-56 ; return map_lookup_elem(&LB4_BACKEND_MAP_V2, &backend_id); 1869: (18) r1 = 0xffff888100286000 ; R1_w=map_ptr(off=0,ks=4,vs=8,imm=0) 1871: (85) call bpf_map_lookup_elem#1 - Without this patch: - at insn 1864 r1 liveness is set to REG_LIVE_WRITTEN; - at insn 1866 fp-56 liveness is set REG_LIVE_WRITTEN mark because of the direct r1 copy in save_register_state(); - at insn 1871 REG_LIVE_READ is not propagated to fp-56 at checkpoint 1862 because of the REG_LIVE_WRITTEN mark; - eventually fp-56 is pruned from checkpoint at 1862 in clean_func_state(). - With this patch: - at insn 1864 r1 liveness is set to REG_LIVE_WRITTEN; - at insn 1866 fp-56 liveness is *not* set to REG_LIVE_WRITTEN mark because write size is not equal to BPF_REG_SIZE; - at insn 1871 REG_LIVE_READ is propagated to fp-56 at checkpoint 1862. Hence more states have to be visited by verifier with this patch compared to current master. Similar patterns could be found for both fp-72 and fp-80, although these are harder to track trough the log because of a big number of insns between slot write and bpf_map_lookup_elem() call triggering read mark, boils down to the following C code: struct ipv4_frag_id frag_id = { .daddr = ip4->daddr, .saddr = ip4->saddr, .id = ip4->id, .proto = ip4->protocol, .pad = 0, }; ... map_lookup_elem(..., &frag_id); Where: - .id is mapped to fp-72, write of size u16; - .saddr is mapped to fp-80, write of size u32. This patch-set is a continuation of discussion from [2]. Changes v1 -> v2 (no changes in the code itself): - added analysis for the tail_lb_ipv4 verification slowdown; - rebase against fresh master branch. [1] git@github.com:anakryiko/cilium.git [2] https://lore.kernel.org/bpf/517af2c57ee4b9ce2d96a8cf33f7295f2d2dfe13.camel@gmail.com/ ==================== Acked-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
@@ -3243,13 +3243,24 @@ static bool __is_pointer_value(bool allow_ptr_leaks,
 	return reg->type != SCALAR_VALUE;
 }
 
+/* Copy src state preserving dst->parent and dst->live fields */
+static void copy_register_state(struct bpf_reg_state *dst, const struct bpf_reg_state *src)
+{
+	struct bpf_reg_state *parent = dst->parent;
+	enum bpf_reg_liveness live = dst->live;
+
+	*dst = *src;
+	dst->parent = parent;
+	dst->live = live;
+}
+
 static void save_register_state(struct bpf_func_state *state,
 				int spi, struct bpf_reg_state *reg,
 				int size)
 {
 	int i;
 
-	state->stack[spi].spilled_ptr = *reg;
+	copy_register_state(&state->stack[spi].spilled_ptr, reg);
 	if (size == BPF_REG_SIZE)
 		state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
 
@@ -3577,7 +3588,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 				 */
 				s32 subreg_def = state->regs[dst_regno].subreg_def;
 
-				state->regs[dst_regno] = *reg;
+				copy_register_state(&state->regs[dst_regno], reg);
 				state->regs[dst_regno].subreg_def = subreg_def;
 			} else {
 				for (i = 0; i < size; i++) {
@@ -3598,7 +3609,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 
 		if (dst_regno >= 0) {
 			/* restore register state from stack */
-			state->regs[dst_regno] = *reg;
+			copy_register_state(&state->regs[dst_regno], reg);
 			/* mark reg as written since spilled pointer state likely
 			 * has its liveness marks cleared by is_state_visited()
 			 * which resets stack/reg liveness for state transitions
@@ -9592,7 +9603,7 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
 	 */
 	if (!ptr_is_dst_reg) {
 		tmp = *dst_reg;
-		*dst_reg = *ptr_reg;
+		copy_register_state(dst_reg, ptr_reg);
 	}
 	ret = sanitize_speculative_path(env, NULL, env->insn_idx + 1,
 					env->insn_idx);
@@ -10845,7 +10856,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 					 * to propagate min/max range.
 					 */
 					src_reg->id = ++env->id_gen;
-				*dst_reg = *src_reg;
+				copy_register_state(dst_reg, src_reg);
 				dst_reg->live |= REG_LIVE_WRITTEN;
 				dst_reg->subreg_def = DEF_NOT_SUBREG;
 			} else {
@@ -10856,7 +10867,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 						insn->src_reg);
 					return -EACCES;
 				} else if (src_reg->type == SCALAR_VALUE) {
-					*dst_reg = *src_reg;
+					copy_register_state(dst_reg, src_reg);
 					/* Make sure ID is cleared otherwise
 					 * dst_reg min/max could be incorrectly
 					 * propagated into src_reg by find_equal_scalars()
@@ -11655,7 +11666,7 @@ static void find_equal_scalars(struct bpf_verifier_state *vstate,
 
 	bpf_for_each_reg_in_vstate(vstate, state, reg, ({
 		if (reg->type == SCALAR_VALUE && reg->id == known_reg->id)
-			*reg = *known_reg;
+			copy_register_state(reg, known_reg);
 	}));
 }
 
diff --git a/tools/testing/selftests/bpf/verifier/search_pruning.c b/tools/testing/selftests/bpf/verifier/search_pruning.c
@@ -225,3 +225,39 @@
 	.result_unpriv = ACCEPT,
 	.insn_processed = 15,
 },
+/* The test performs a conditional 64-bit write to a stack location
+ * fp[-8], this is followed by an unconditional 8-bit write to fp[-8],
+ * then data is read from fp[-8]. This sequence is unsafe.
+ *
+ * The test would be mistakenly marked as safe w/o dst register parent
+ * preservation in verifier.c:copy_register_state() function.
+ *
+ * Note the usage of BPF_F_TEST_STATE_FREQ to force creation of the
+ * checkpoint state after conditional 64-bit assignment.
+ */
+{
+	"write tracking and register parent chain bug",
+	.insns = {
+	/* r6 = ktime_get_ns() */
+	BPF_EMIT_CALL(BPF_FUNC_ktime_get_ns),
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+	/* r0 = ktime_get_ns() */
+	BPF_EMIT_CALL(BPF_FUNC_ktime_get_ns),
+	/* if r0 > r6 goto +1 */
+	BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_6, 1),
+	/* *(u64 *)(r10 - 8) = 0xdeadbeef */
+	BPF_ST_MEM(BPF_DW, BPF_REG_FP, -8, 0xdeadbeef),
+	/* r1 = 42 */
+	BPF_MOV64_IMM(BPF_REG_1, 42),
+	/* *(u8 *)(r10 - 8) = r1 */
+	BPF_STX_MEM(BPF_B, BPF_REG_FP, BPF_REG_1, -8),
+	/* r2 = *(u64 *)(r10 - 8) */
+	BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_FP, -8),
+	/* exit(0) */
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.flags = BPF_F_TEST_STATE_FREQ,
+	.errstr = "invalid read from stack off -8+1 size 8",
+	.result = REJECT,
+},
