bpf: add new arg_type that allows for 0 sized stack buffer

Currently, when we pass a buffer from the eBPF stack into a helper
function, the function proto indicates argument types as ARG_PTR_TO_STACK
and ARG_CONST_STACK_SIZE pair. If R<X> contains the former, then R<X+1>
must be of the latter type. Then, verifier checks whether the buffer
points into eBPF stack, is initialized, etc. The verifier also guarantees
that the constant value passed in R<X+1> is greater than 0, so helper
functions don't need to test for it and can always assume a non-NULL
initialized buffer as well as non-0 buffer size.

This patch adds a new argument types ARG_CONST_STACK_SIZE_OR_ZERO that
allows to also pass NULL as R<X> and 0 as R<X+1> into the helper function.
Such helper functions, of course, need to be able to handle these cases
internally then. Verifier guarantees that either R<X> == NULL && R<X+1> == 0
or R<X> != NULL && R<X+1> != 0 (like the case of ARG_CONST_STACK_SIZE), any
other combinations are not possible to load.

I went through various options of extending the verifier, and introducing
the type ARG_CONST_STACK_SIZE_OR_ZERO seems to have most minimal changes
needed to the verifier.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf.h

@@ -65,6 +65,7 @@ enum bpf_arg_type {
 	 */
 	ARG_PTR_TO_STACK,	/* any pointer to eBPF program stack */
 	ARG_CONST_STACK_SIZE,	/* number of bytes accessed from stack */
+	ARG_CONST_STACK_SIZE_OR_ZERO, /* number of bytes accessed from stack or 0 */
 
 	ARG_PTR_TO_CTX,		/* pointer to context */
 	ARG_ANYTHING,		/* any (initialized) argument is ok */

kernel/bpf/verifier.c

@@ -779,15 +779,24 @@ static int check_xadd(struct verifier_env *env, struct bpf_insn *insn)
  * bytes from that pointer, make sure that it's within stack boundary
  * and all elements of stack are initialized
  */
-static int check_stack_boundary(struct verifier_env *env,
-				int regno, int access_size)
+static int check_stack_boundary(struct verifier_env *env, int regno,
+				int access_size, bool zero_size_allowed)
 {
 	struct verifier_state *state = &env->cur_state;
 	struct reg_state *regs = state->regs;
 	int off, i;
 
-	if (regs[regno].type != PTR_TO_STACK)
+	if (regs[regno].type != PTR_TO_STACK) {
+		if (zero_size_allowed && access_size == 0 &&
+		    regs[regno].type == CONST_IMM &&
+		    regs[regno].imm  == 0)
+			return 0;
+
+		verbose("R%d type=%s expected=%s\n", regno,
+			reg_type_str[regs[regno].type],
+			reg_type_str[PTR_TO_STACK]);
 		return -EACCES;
+	}
 
 	off = regs[regno].imm;
 	if (off >= 0 || off < -MAX_BPF_STACK || off + access_size > 0 ||
@@ -830,15 +839,24 @@ static int check_func_arg(struct verifier_env *env, u32 regno,
 		return 0;
 	}
 
-	if (arg_type == ARG_PTR_TO_STACK || arg_type == ARG_PTR_TO_MAP_KEY ||
+	if (arg_type == ARG_PTR_TO_MAP_KEY ||
 	    arg_type == ARG_PTR_TO_MAP_VALUE) {
 		expected_type = PTR_TO_STACK;
-	} else if (arg_type == ARG_CONST_STACK_SIZE) {
+	} else if (arg_type == ARG_CONST_STACK_SIZE ||
+		   arg_type == ARG_CONST_STACK_SIZE_OR_ZERO) {
 		expected_type = CONST_IMM;
 	} else if (arg_type == ARG_CONST_MAP_PTR) {
 		expected_type = CONST_PTR_TO_MAP;
 	} else if (arg_type == ARG_PTR_TO_CTX) {
 		expected_type = PTR_TO_CTX;
+	} else if (arg_type == ARG_PTR_TO_STACK) {
+		expected_type = PTR_TO_STACK;
+		/* One exception here. In case function allows for NULL to be
+		 * passed in as argument, it's a CONST_IMM type. Final test
+		 * happens during stack boundary checking.
+		 */
+		if (reg->type == CONST_IMM && reg->imm == 0)
+			expected_type = CONST_IMM;
 	} else {
 		verbose("unsupported arg_type %d\n", arg_type);
 		return -EFAULT;
@@ -868,8 +886,8 @@ static int check_func_arg(struct verifier_env *env, u32 regno,
 			verbose("invalid map_ptr to access map->key\n");
 			return -EACCES;
 		}
-		err = check_stack_boundary(env, regno, (*mapp)->key_size);
-
+		err = check_stack_boundary(env, regno, (*mapp)->key_size,
+					   false);
 	} else if (arg_type == ARG_PTR_TO_MAP_VALUE) {
 		/* bpf_map_xxx(..., map_ptr, ..., value) call:
 		 * check [value, value + map->value_size) validity
@@ -879,9 +897,12 @@ static int check_func_arg(struct verifier_env *env, u32 regno,
 			verbose("invalid map_ptr to access map->value\n");
 			return -EACCES;
 		}
-		err = check_stack_boundary(env, regno, (*mapp)->value_size);
+		err = check_stack_boundary(env, regno, (*mapp)->value_size,
+					   false);
+	} else if (arg_type == ARG_CONST_STACK_SIZE ||
+		   arg_type == ARG_CONST_STACK_SIZE_OR_ZERO) {
+		bool zero_size_allowed = (arg_type == ARG_CONST_STACK_SIZE_OR_ZERO);
 
-	} else if (arg_type == ARG_CONST_STACK_SIZE) {
 		/* bpf_xxx(..., buf, len) call will access 'len' bytes
 		 * from stack pointer 'buf'. Check it
 		 * note: regno == len, regno - 1 == buf
@@ -891,7 +912,8 @@ static int check_func_arg(struct verifier_env *env, u32 regno,
 			verbose("ARG_CONST_STACK_SIZE cannot be first argument\n");
 			return -EACCES;
 		}
-		err = check_stack_boundary(env, regno - 1, reg->imm);
+		err = check_stack_boundary(env, regno - 1, reg->imm,
+					   zero_size_allowed);
 	}
 
 	return err;