bpf: verifier: Refactor helper access type tracking

JIRA: https://issues.redhat.com/browse/RHEL-93930

commit 37cce22
Author: Daniel Xu <dxu@dxuuu.xyz>
Date:   Tue Jan 14 13:28:44 2025 -0700

    bpf: verifier: Refactor helper access type tracking

    Previously, the verifier was treating all PTR_TO_STACK registers passed
    to a helper call as potentially written to by the helper. However, all
    calls to check_stack_range_initialized() already have precise access type
    information available.

    Rather than treat ACCESS_HELPER as a proxy for BPF_WRITE, pass
    enum bpf_access_type to check_stack_range_initialized() to more
    precisely track helper arguments.

    One benefit from this precision is that registers tracked as valid
    spills and passed as a read-only helper argument remain tracked after
    the call.  Rather than being marked STACK_MISC afterwards.

    An additional benefit is the verifier logs are also more precise. For
    this particular error, users will enjoy a slightly clearer message. See
    included selftest updates for examples.

    Acked-by: Eduard Zingerman <eddyz87@gmail.com>
    Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
    Link: https://lore.kernel.org/r/ff885c0e5859e0cd12077c3148ff0754cad4f7ed.1736886479.git.dxu@dxuuu.xyz
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/bpf/verifier.c

@@ -5128,7 +5128,7 @@ enum bpf_access_src {
 static int check_stack_range_initialized(struct bpf_verifier_env *env,
 					 int regno, int off, int access_size,
 					 bool zero_size_allowed,
-					 enum bpf_access_src type,
+					 enum bpf_access_type type,
 					 struct bpf_call_arg_meta *meta);
 
 static struct bpf_reg_state *reg_state(struct bpf_verifier_env *env, int regno)
@@ -5161,7 +5161,7 @@ static int check_stack_read_var_off(struct bpf_verifier_env *env,
 	/* Note that we pass a NULL meta, so raw access will not be permitted.
 	 */
 	err = check_stack_range_initialized(env, ptr_regno, off, size,
-					    false, ACCESS_DIRECT, NULL);
+					    false, BPF_READ, NULL);
 	if (err)
 		return err;
 
@@ -7011,7 +7011,7 @@ static int check_stack_slot_within_bounds(struct bpf_verifier_env *env,
 static int check_stack_access_within_bounds(
 		struct bpf_verifier_env *env,
 		int regno, int off, int access_size,
-		enum bpf_access_src src, enum bpf_access_type type)
+		enum bpf_access_type type)
 {
 	struct bpf_reg_state *regs = cur_regs(env);
 	struct bpf_reg_state *reg = regs + regno;
@@ -7020,10 +7020,7 @@ static int check_stack_access_within_bounds(
 	int err;
 	char *err_extra;
 
-	if (src == ACCESS_HELPER)
-		/* We don't know if helpers are reading or writing (or both). */
-		err_extra = " indirect access to";
-	else if (type == BPF_READ)
+	if (type == BPF_READ)
 		err_extra = " read from";
 	else
 		err_extra = " write to";
@@ -7241,7 +7238,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 
 	} else if (reg->type == PTR_TO_STACK) {
 		/* Basic bounds checks. */
-		err = check_stack_access_within_bounds(env, regno, off, size, ACCESS_DIRECT, t);
+		err = check_stack_access_within_bounds(env, regno, off, size, t);
 		if (err)
 			return err;
 
@@ -7461,13 +7458,11 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 static int check_stack_range_initialized(
 		struct bpf_verifier_env *env, int regno, int off,
 		int access_size, bool zero_size_allowed,
-		enum bpf_access_src type, struct bpf_call_arg_meta *meta)
+		enum bpf_access_type type, struct bpf_call_arg_meta *meta)
 {
 	struct bpf_reg_state *reg = reg_state(env, regno);
 	struct bpf_func_state *state = func(env, reg);
 	int err, min_off, max_off, i, j, slot, spi;
-	char *err_extra = type == ACCESS_HELPER ? " indirect" : "";
-	enum bpf_access_type bounds_check_type;
 	/* Some accesses can write anything into the stack, others are
 	 * read-only.
 	 */
@@ -7478,18 +7473,10 @@ static int check_stack_range_initialized(
 		return -EACCES;
 	}
 
-	if (type == ACCESS_HELPER) {
-		/* The bounds checks for writes are more permissive than for
-		 * reads. However, if raw_mode is not set, we'll do extra
-		 * checks below.
-		 */
-		bounds_check_type = BPF_WRITE;
+	if (type == BPF_WRITE)
 		clobber = true;
-	} else {
-		bounds_check_type = BPF_READ;
-	}
-	err = check_stack_access_within_bounds(env, regno, off, access_size,
-					       type, bounds_check_type);
+
+	err = check_stack_access_within_bounds(env, regno, off, access_size, type);
 	if (err)
 		return err;
 
@@ -7506,8 +7493,8 @@ static int check_stack_range_initialized(
 			char tn_buf[48];
 
 			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
-			verbose(env, "R%d%s variable offset stack access prohibited for !root, var_off=%s\n",
-				regno, err_extra, tn_buf);
+			verbose(env, "R%d variable offset stack access prohibited for !root, var_off=%s\n",
+				regno, tn_buf);
 			return -EACCES;
 		}
 		/* Only initialized buffer on stack is allowed to be accessed
@@ -7588,14 +7575,14 @@ static int check_stack_range_initialized(
 		}
 
 		if (tnum_is_const(reg->var_off)) {
-			verbose(env, "invalid%s read from stack R%d off %d+%d size %d\n",
-				err_extra, regno, min_off, i - min_off, access_size);
+			verbose(env, "invalid read from stack R%d off %d+%d size %d\n",
+				regno, min_off, i - min_off, access_size);
 		} else {
 			char tn_buf[48];
 
 			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
-			verbose(env, "invalid%s read from stack R%d var_off %s+%d size %d\n",
-				err_extra, regno, tn_buf, i - min_off, access_size);
+			verbose(env, "invalid read from stack R%d var_off %s+%d size %d\n",
+				regno, tn_buf, i - min_off, access_size);
 		}
 		return -EACCES;
 mark:
@@ -7670,7 +7657,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 		return check_stack_range_initialized(
 				env,
 				regno, reg->off, access_size,
-				zero_size_allowed, ACCESS_HELPER, meta);
+				zero_size_allowed, access_type, meta);
 	case PTR_TO_BTF_ID:
 		return check_ptr_to_btf_access(env, regs, regno, reg->off,
 					       access_size, BPF_READ, -1);

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

@@ -192,7 +192,7 @@ int ringbuf_invalid_api(void *ctx)
 
 /* Can't add a dynptr to a map */
 SEC("?raw_tp")
-__failure __msg("invalid indirect read from stack")
+__failure __msg("invalid read from stack")
 int add_dynptr_to_map1(void *ctx)
 {
 	struct bpf_dynptr ptr;
@@ -210,7 +210,7 @@ int add_dynptr_to_map1(void *ctx)
 
 /* Can't add a struct with an embedded dynptr to a map */
 SEC("?raw_tp")
-__failure __msg("invalid indirect read from stack")
+__failure __msg("invalid read from stack")
 int add_dynptr_to_map2(void *ctx)
 {
 	struct test_info x;
@@ -398,7 +398,7 @@ int data_slice_missing_null_check2(void *ctx)
  * dynptr argument
  */
 SEC("?raw_tp")
-__failure __msg("invalid indirect read from stack")
+__failure __msg("invalid read from stack")
 int invalid_helper1(void *ctx)
 {
 	struct bpf_dynptr ptr;

tools/testing/selftests/bpf/progs/test_global_func10.c

@@ -26,7 +26,7 @@ __noinline int foo(const struct Big *big)
 }
 
 SEC("cgroup_skb/ingress")
-__failure __msg("invalid indirect access to stack")
+__failure __msg("invalid read from stack")
 int global_func10(struct __sk_buff *skb)
 {
 	const struct Small small = {.x = skb->len };

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

@@ -70,7 +70,8 @@ __naked int helper_uninit_to_misc(void *ctx)
 		r1 = r10;				\
 		r1 += -128;				\
 		r2 = 32;				\
-		call %[bpf_trace_printk];		\
+		r3 = 0;					\
+		call %[bpf_probe_read_user];		\
 		/* Call to dummy() forces print_verifier_state(..., true),	\
 		 * thus showing the stack state, matched by __msg().		\
 		 */					\
@@ -79,7 +80,7 @@ __naked int helper_uninit_to_misc(void *ctx)
 		exit;					\
 "
 		      :
-		      : __imm(bpf_trace_printk),
+		      : __imm(bpf_probe_read_user),
 			__imm(dummy)
 		      : __clobber_all);
 }

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

@@ -28,7 +28,7 @@ __naked void stack_out_of_bounds(void)
 SEC("socket")
 __description("uninitialized stack1")
 __success __log_level(4) __msg("stack depth 8")
-__failure_unpriv __msg_unpriv("invalid indirect read from stack")
+__failure_unpriv __msg_unpriv("invalid read from stack")
 __naked void uninitialized_stack1(void)
 {
 	asm volatile ("					\

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

@@ -25,7 +25,7 @@ __naked void constant_should_keep_constant_type(void)
 
 SEC("tracepoint")
 __description("constant register |= constant should not bypass stack boundary checks")
-__failure __msg("invalid indirect access to stack R1 off=-48 size=58")
+__failure __msg("invalid write to stack R1 off=-48 size=58")
 __naked void not_bypass_stack_boundary_checks_1(void)
 {
 	asm volatile ("					\
@@ -62,7 +62,7 @@ __naked void register_should_keep_constant_type(void)
 
 SEC("tracepoint")
 __description("constant register |= constant register should not bypass stack boundary checks")
-__failure __msg("invalid indirect access to stack R1 off=-48 size=58")
+__failure __msg("invalid write to stack R1 off=-48 size=58")
 __naked void not_bypass_stack_boundary_checks_2(void)
 {
 	asm volatile ("					\

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

@@ -67,7 +67,7 @@ SEC("socket")
 __description("helper access to variable memory: stack, bitwise AND, zero included")
 /* in privileged mode reads from uninitialized stack locations are permitted */
 __success __failure_unpriv
-__msg_unpriv("invalid indirect read from stack R2 off -64+0 size 64")
+__msg_unpriv("invalid read from stack R2 off -64+0 size 64")
 __retval(0)
 __naked void stack_bitwise_and_zero_included(void)
 {
@@ -100,7 +100,7 @@ __naked void stack_bitwise_and_zero_included(void)
 
 SEC("tracepoint")
 __description("helper access to variable memory: stack, bitwise AND + JMP, wrong max")
-__failure __msg("invalid indirect access to stack R1 off=-64 size=65")
+__failure __msg("invalid write to stack R1 off=-64 size=65")
 __naked void bitwise_and_jmp_wrong_max(void)
 {
 	asm volatile ("					\
@@ -187,7 +187,7 @@ l0_%=:	r0 = 0;						\
 
 SEC("tracepoint")
 __description("helper access to variable memory: stack, JMP, bounds + offset")
-__failure __msg("invalid indirect access to stack R1 off=-64 size=65")
+__failure __msg("invalid write to stack R1 off=-64 size=65")
 __naked void memory_stack_jmp_bounds_offset(void)
 {
 	asm volatile ("					\
@@ -211,7 +211,7 @@ l0_%=:	r0 = 0;						\
 
 SEC("tracepoint")
 __description("helper access to variable memory: stack, JMP, wrong max")
-__failure __msg("invalid indirect access to stack R1 off=-64 size=65")
+__failure __msg("invalid write to stack R1 off=-64 size=65")
 __naked void memory_stack_jmp_wrong_max(void)
 {
 	asm volatile ("					\
@@ -260,7 +260,7 @@ SEC("socket")
 __description("helper access to variable memory: stack, JMP, no min check")
 /* in privileged mode reads from uninitialized stack locations are permitted */
 __success __failure_unpriv
-__msg_unpriv("invalid indirect read from stack R2 off -64+0 size 64")
+__msg_unpriv("invalid read from stack R2 off -64+0 size 64")
 __retval(0)
 __naked void stack_jmp_no_min_check(void)
 {
@@ -750,7 +750,7 @@ SEC("socket")
 __description("helper access to variable memory: 8 bytes leak")
 /* in privileged mode reads from uninitialized stack locations are permitted */
 __success __failure_unpriv
-__msg_unpriv("invalid indirect read from stack R2 off -64+32 size 64")
+__msg_unpriv("invalid read from stack R2 off -64+32 size 64")
 __retval(0)
 __naked void variable_memory_8_bytes_leak(void)
 {

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

@@ -96,7 +96,7 @@ __naked void arg_ptr_to_long_misaligned(void)
 
 SEC("cgroup/sysctl")
 __description("arg pointer to long size < sizeof(long)")
-__failure __msg("invalid indirect access to stack R4 off=-4 size=8")
+__failure __msg("invalid write to stack R4 off=-4 size=8")
 __naked void to_long_size_sizeof_long(void)
 {
 	asm volatile ("					\

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

@@ -8,7 +8,7 @@ SEC("tc/ingress")
 __description("uninit/mtu: write rejected")
 __success
 __caps_unpriv(CAP_BPF|CAP_NET_ADMIN)
-__failure_unpriv __msg_unpriv("invalid indirect read from stack")
+__failure_unpriv __msg_unpriv("invalid read from stack")
 int tc_uninit_mtu(struct __sk_buff *ctx)
 {
 	__u32 mtu;

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

@@ -236,7 +236,7 @@ __naked void load_bytes_spilled_regs_data(void)
 
 SEC("tc")
 __description("raw_stack: skb_load_bytes, invalid access 1")
-__failure __msg("invalid indirect access to stack R3 off=-513 size=8")
+__failure __msg("invalid write to stack R3 off=-513 size=8")
 __naked void load_bytes_invalid_access_1(void)
 {
 	asm volatile ("					\
@@ -255,7 +255,7 @@ __naked void load_bytes_invalid_access_1(void)
 
 SEC("tc")
 __description("raw_stack: skb_load_bytes, invalid access 2")
-__failure __msg("invalid indirect access to stack R3 off=-1 size=8")
+__failure __msg("invalid write to stack R3 off=-1 size=8")
 __naked void load_bytes_invalid_access_2(void)
 {
 	asm volatile ("					\