[PATCH 14/16] X.509: Add an ASN.1 decoder

[David Howells]

Add an ASN.1 BER/DER/CER decoder.  This uses the bytecode from the ASN.1
compiler in the previous patch to inform it as to what to expect to find in the
encoded byte stream.  The output from the compiler also tells it what functions
to call on what tags, thus allowing the caller to retrieve information.

The decoder is called as follows:

	int asn1_decoder(const struct asn1_decoder *decoder,
			 void *context,
			 const unsigned char *data,
			 size_t datalen);

The decoder argument points to the bytecode from the ASN.1 compiler.  context
is the caller's context and is passed to the action functions.  data and
datalen define the byte stream to be decoded.


Note that the decoder is currently limited to datalen being less than 64K.
This reduces the amount of stack space used by the decoder because ASN.1 is a
nested construct.  Similarly, the decoder is limited to a maximum of 10 levels
of constructed data outside of a leaf node also in an effort to keep stack
usage down.

These restrictions can be raised if necessary.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 include/linux/asn1_decoder.h |   24 ++
 lib/Makefile                 |    2 
 lib/asn1_decoder.c           |  473 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 499 insertions(+)
 create mode 100644 include/linux/asn1_decoder.h
 create mode 100644 lib/asn1_decoder.c


diff --git a/include/linux/asn1_decoder.h b/include/linux/asn1_decoder.h
new file mode 100644
index 0000000..fa2ff5b
--- /dev/null
+++ b/include/linux/asn1_decoder.h
@@ -0,0 +1,24 @@
+/* ASN.1 decoder
+ *
+ * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@xxxxxxxxxx)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#ifndef _LINUX_ASN1_DECODER_H
+#define _LINUX_ASN1_DECODER_H
+
+#include <linux/asn1.h>
+
+struct asn1_decoder;
+
+extern int asn1_ber_decoder(const struct asn1_decoder *decoder,
+			    void *context,
+			    const unsigned char *data,
+			    size_t datalen);
+
+#endif /* _LINUX_ASN1_DECODER_H */
diff --git a/lib/Makefile b/lib/Makefile
index b042896..ca856b6 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -140,6 +140,8 @@ $(foreach file, $(libfdt_files), \
 	$(eval CFLAGS_$(file) = -I$(src)/../scripts/dtc/libfdt))
 lib-$(CONFIG_LIBFDT) += $(libfdt_files)
 
+obj-$(CONFIG_ASN1) += asn1_decoder.o
+
 hostprogs-y	:= gen_crc32table
 clean-files	:= crc32table.h
 
diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
new file mode 100644
index 0000000..8179f3c
--- /dev/null
+++ b/lib/asn1_decoder.c
@@ -0,0 +1,473 @@
+/* Decoder for ASN.1 BER/DER/CER encoded bytestream
+ *
+ * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@xxxxxxxxxx)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#include <linux/export.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/asn1_decoder.h>
+#include <linux/asn1_ber_bytecode.h>
+
+static const unsigned char asn1_op_lengths[ASN1_OP__NR] = {
+	/*					OPC TAG JMP ACT */
+	[ASN1_OP_MATCH]				= 1 + 1,
+	[ASN1_OP_MATCH_OR_SKIP]			= 1 + 1,
+	[ASN1_OP_MATCH_ACT]			= 1 + 1     + 1,
+	[ASN1_OP_MATCH_ACT_OR_SKIP]		= 1 + 1     + 1,
+	[ASN1_OP_MATCH_JUMP]			= 1 + 1 + 1,
+	[ASN1_OP_MATCH_JUMP_OR_SKIP]		= 1 + 1 + 1,
+	[ASN1_OP_MATCH_ANY]			= 1,
+	[ASN1_OP_MATCH_ANY_ACT]			= 1         + 1,
+	[ASN1_OP_COND_MATCH_OR_SKIP]		= 1 + 1,
+	[ASN1_OP_COND_MATCH_ACT_OR_SKIP]	= 1 + 1     + 1,
+	[ASN1_OP_COND_MATCH_JUMP_OR_SKIP]	= 1 + 1 + 1,
+	[ASN1_OP_COND_MATCH_ANY]		= 1,
+	[ASN1_OP_COND_MATCH_ANY_ACT]		= 1         + 1,
+	[ASN1_OP_COND_FAIL]			= 1,
+	[ASN1_OP_COMPLETE]			= 1,
+	[ASN1_OP_ACT]				= 1         + 1,
+	[ASN1_OP_RETURN]			= 1,
+	[ASN1_OP_END_SEQ]			= 1,
+	[ASN1_OP_END_SEQ_OF]			= 1     + 1,
+	[ASN1_OP_END_SET]			= 1,
+	[ASN1_OP_END_SET_OF]			= 1     + 1,
+	[ASN1_OP_END_SEQ_ACT]			= 1         + 1,
+	[ASN1_OP_END_SEQ_OF_ACT]		= 1     + 1 + 1,
+	[ASN1_OP_END_SET_ACT]			= 1         + 1,
+	[ASN1_OP_END_SET_OF_ACT]		= 1     + 1 + 1,
+};
+
+/*
+ * Find the length of an indefinite length object
+ */
+static int asn1_find_indefinite_length(const unsigned char *data, size_t datalen,
+				       const char **_errmsg, size_t *_err_dp)
+{
+	unsigned char tag, tmp;
+	size_t dp = 0, len, n;
+	int indef_level = 1;
+
+next_tag:
+	if (unlikely(datalen - dp < 2)) {
+		if (datalen == dp)
+			goto missing_eoc;
+		goto data_overrun_error;
+	}
+
+	/* Extract a tag from the data */
+	tag = data[dp++];
+	if (tag == 0) {
+		/* It appears to be an EOC. */
+		if (data[dp++] != 0)
+			goto invalid_eoc;
+		if (--indef_level <= 0)
+			return dp;
+		goto next_tag;
+	}
+
+	if (unlikely((tag & 0x1f) == 0x1f)) {
+		do {
+			if (unlikely(datalen - dp < 2))
+				goto data_overrun_error;
+			tmp = data[dp++];
+		} while (tmp & 0x80);
+	}
+
+	/* Extract the length */
+	len = data[dp++];
+	if (len < 0x7f) {
+		dp += len;
+		goto next_tag;
+	}
+
+	if (unlikely(len == 0x80)) {
+		/* Indefinite length */
+		if (unlikely((tag & ASN1_CONS_BIT) == ASN1_PRIM << 5))
+			goto indefinite_len_primitive;
+		indef_level++;
+		goto next_tag;
+	}
+
+	n = len - 0x80;
+	if (unlikely(n > sizeof(size_t) - 1))
+		goto length_too_long;
+	if (unlikely(n > datalen - dp))
+		goto data_overrun_error;
+	for (len = 0; n > 0; n--) {
+		len <<= 8;
+		len |= data[dp++];
+	}
+	dp += len;
+	goto next_tag;
+
+length_too_long:
+	*_errmsg = "Unsupported length";
+	goto error;
+indefinite_len_primitive:
+	*_errmsg = "Indefinite len primitive not permitted";
+	goto error;
+invalid_eoc:
+	*_errmsg = "Invalid length EOC";
+	goto error;
+data_overrun_error:
+	*_errmsg = "Data overrun error";
+	goto error;
+missing_eoc:
+	*_errmsg = "Missing EOC in indefinite len cons";
+error:
+	*_err_dp = dp;
+	return -1;
+}
+
+/**
+ * asn1_ber_decoder - Decoder BER/DER/CER ASN.1 according to pattern
+ * @decoder: The decoder definition (produced by asn1_compiler)
+ * @context: The caller's context (to be passed to the action functions)
+ * @data: The encoded data
+ * @datasize: The size of the encoded data
+ *
+ * Decode BER/DER/CER encoded ASN.1 data according to a bytecode pattern
+ * produced by asn1_compiler.  Action functions are called on marked tags to
+ * allow the caller to retrieve significant data.
+ *
+ * LIMITATIONS:
+ *
+ * To keep down the amount of stack used by this function, the following limits
+ * have been imposed:
+ *
+ *  (1) This won't handle datalen > 65535 without increasing the size of the
+ *	cons stack elements and length_too_long checking.
+ *
+ *  (2) The stack of constructed types is 10 deep.  If the depth of non-leaf
+ *	constructed types exceeds this, the decode will fail.
+ *
+ *  (3) The SET type (not the SET OF type) isn't really supported as tracking
+ *	what members of the set have been seen is a pain.
+ */
+int asn1_ber_decoder(const struct asn1_decoder *decoder,
+		     void *context,
+		     const unsigned char *data,
+		     size_t datalen)
+{
+	const unsigned char *machine = decoder->machine;
+	const asn1_action_t *actions = decoder->actions;
+	size_t machlen = decoder->machlen;
+	enum asn1_opcode op;
+	unsigned char tag = 0, csp = 0, jsp = 0, optag = 0;
+	const char *errmsg;
+	size_t pc = 0, dp = 0, tdp = 0, len = 0, hdr = 0;
+	int ret;
+
+	unsigned char flags = 0;
+#define FLAG_INDEFINITE_LENGTH	0x01
+#define FLAG_MATCHED		0x02
+#define FLAG_CONS		0x20 /* Corresponds to CONS bit in the opcode tag
+				      * - ie. whether or not we are going to parse
+				      *   a compound type.
+				      */
+
+#define NR_CONS_STACK 10
+	unsigned short cons_dp_stack[NR_CONS_STACK];
+	unsigned short cons_datalen_stack[NR_CONS_STACK];
+#define NR_JUMP_STACK 10
+	unsigned char jump_stack[NR_JUMP_STACK];
+
+	if (datalen > 65535)
+		return -EMSGSIZE;
+
+next_op:
+	pr_debug("next_op: pc=\e[32m%zu\e[m/%zu dp=\e[33m%zu\e[m/%zu C=%d J=%d\n",
+		 pc, machlen, dp, datalen, csp, jsp);
+	if (unlikely(pc >= machlen))
+		goto machine_overrun_error;
+	op = machine[pc];
+	if (unlikely(pc + asn1_op_lengths[op] > machlen))
+		goto machine_overrun_error;
+
+	/* If this command is meant to match a tag, then do that before
+	 * evaluating the command.
+	 */
+	if (op <= ASN1_OP__MATCHES_TAG) {
+		unsigned char tmp;
+
+		/* Skip conditional matches if possible */
+		if ((op & ASN1_OP_MATCH__COND &&
+		     flags & FLAG_MATCHED) ||
+		    dp == datalen) {
+			pc += asn1_op_lengths[op];
+			goto next_op;
+		}
+
+		flags = 0;
+		hdr = dp;
+
+		/* Extract a tag from the data */
+		if (unlikely(dp >= datalen - 1))
+			goto data_overrun_error;
+		tag = data[dp++];
+		if (unlikely((tag & 0x1f) == 0x1f))
+			goto long_tag_not_supported;
+
+		if (op & ASN1_OP_MATCH__ANY) {
+			pr_debug("- any %02x\n", tag);
+		} else {
+			/* Extract the tag from the machine
+			 * - Either CONS or PRIM are permitted in the data if
+			 *   CONS is not set in the op stream, otherwise CONS
+			 *   is mandatory.
+			 */
+			optag = machine[pc + 1];
+			flags |= optag & FLAG_CONS;
+
+			/* Determine whether the tag matched */
+			tmp = optag ^ tag;
+			tmp &= ~(optag & ASN1_CONS_BIT);
+			pr_debug("- match? %02x %02x %02x\n", tag, optag, tmp);
+			if (tmp != 0) {
+				/* All odd-numbered tags are MATCH_OR_SKIP. */
+				if (op & ASN1_OP_MATCH__SKIP) {
+					pc += asn1_op_lengths[op];
+					dp--;
+					goto next_op;
+				}
+				goto tag_mismatch;
+			}
+		}
+		flags |= FLAG_MATCHED;
+
+		len = data[dp++];
+		if (len > 0x7f) {
+			if (unlikely(len == 0x80)) {
+				/* Indefinite length */
+				if (unlikely(!(tag & ASN1_CONS_BIT)))
+					goto indefinite_len_primitive;
+				flags |= FLAG_INDEFINITE_LENGTH;
+				if (unlikely(2 > datalen - dp))
+					goto data_overrun_error;
+			} else {
+				int n = len - 0x80;
+				if (unlikely(n > 2))
+					goto length_too_long;
+				if (unlikely(dp >= datalen - n))
+					goto data_overrun_error;
+				for (len = 0; n > 0; n--) {
+					len <<= 8;
+					len |= data[dp++];
+				}
+				if (unlikely(len > datalen - dp))
+					goto data_overrun_error;
+			}
+		}
+
+		if (flags & FLAG_CONS) {
+			/* For expected compound forms, we stack the positions
+			 * of the start and end of the data.
+			 */
+			if (unlikely(csp >= NR_CONS_STACK))
+				goto cons_stack_overflow;
+			cons_dp_stack[csp] = dp;
+			if (!(flags & FLAG_INDEFINITE_LENGTH)) {
+				cons_datalen_stack[csp] = datalen;
+				datalen = dp + len;
+			} else {
+				cons_datalen_stack[csp] = 0;
+			}
+			csp++;
+		}
+
+		pr_debug("- TAG: %02x %zu%s\n",
+			 tag, len, flags & FLAG_CONS ? " CONS" : "");
+		tdp = dp;
+	}
+
+	/* Decide how to handle the operation */
+	switch (op) {
+	case ASN1_OP_MATCH_ANY_ACT:
+	case ASN1_OP_COND_MATCH_ANY_ACT:
+		ret = actions[machine[pc + 1]](context, dp - hdr, tag, data + dp, len);
+		if (ret < 0)
+			return ret;
+		goto skip_data;
+
+	case ASN1_OP_MATCH_ACT:
+	case ASN1_OP_MATCH_ACT_OR_SKIP:
+	case ASN1_OP_COND_MATCH_ACT_OR_SKIP:
+		ret = actions[machine[pc + 2]](context, dp - hdr, tag, data + dp, len);
+		if (ret < 0)
+			return ret;
+		goto skip_data;
+
+	case ASN1_OP_MATCH:
+	case ASN1_OP_MATCH_OR_SKIP:
+	case ASN1_OP_MATCH_ANY:
+	case ASN1_OP_COND_MATCH_OR_SKIP:
+	case ASN1_OP_COND_MATCH_ANY:
+	skip_data:
+		if (!(flags & FLAG_CONS)) {
+			if (flags & FLAG_INDEFINITE_LENGTH) {
+				len = asn1_find_indefinite_length(
+					data + dp, datalen - dp, &errmsg, &dp);
+				if (len < 0)
+					goto error;
+			}
+			pr_debug("- LEAF: %zu\n", len);
+			dp += len;
+		}
+		pc += asn1_op_lengths[op];
+		goto next_op;
+
+	case ASN1_OP_MATCH_JUMP:
+	case ASN1_OP_MATCH_JUMP_OR_SKIP:
+	case ASN1_OP_COND_MATCH_JUMP_OR_SKIP:
+		pr_debug("- MATCH_JUMP\n");
+		if (unlikely(jsp == NR_JUMP_STACK))
+			goto jump_stack_overflow;
+		jump_stack[jsp++] = pc + asn1_op_lengths[op];
+		pc = machine[pc + 2];
+		goto next_op;
+
+	case ASN1_OP_COND_FAIL:
+		if (unlikely(!(flags & FLAG_MATCHED)))
+			goto tag_mismatch;
+		pc += asn1_op_lengths[op];
+		goto next_op;
+
+	case ASN1_OP_COMPLETE:
+		if (unlikely(jsp != 0 || csp != 0)) {
+			pr_err("ASN.1 decoder error: Stacks not empty at completion (%u, %u)\n",
+			       jsp, csp);
+			return -EBADMSG;
+		}
+		return 0;
+
+	case ASN1_OP_END_SET:
+	case ASN1_OP_END_SET_ACT:
+		if (unlikely(!(flags & FLAG_MATCHED)))
+			goto tag_mismatch;
+	case ASN1_OP_END_SEQ:
+	case ASN1_OP_END_SET_OF:
+	case ASN1_OP_END_SEQ_OF:
+	case ASN1_OP_END_SEQ_ACT:
+	case ASN1_OP_END_SET_OF_ACT:
+	case ASN1_OP_END_SEQ_OF_ACT:
+		if (unlikely(csp <= 0))
+			goto cons_stack_underflow;
+		csp--;
+		tdp = cons_dp_stack[csp];
+		len = datalen;
+		datalen = cons_datalen_stack[csp];
+		pr_debug("- end cons t=%zu dp=%zu l=%zu/%zu\n",
+			 tdp, dp, len, datalen);
+		if (datalen == 0) {
+			/* Indefinite length - check for the EOC. */
+			datalen = len;
+			if (unlikely(datalen - dp < 2))
+				goto data_overrun_error;
+			if (data[dp++] != 0) {
+				if (op & ASN1_OP_END__OF) {
+					dp--;
+					csp++;
+					pc = machine[pc + 1];
+					pr_debug("- continue\n");
+					goto next_op;
+				}
+				goto missing_eoc;
+			}
+			if (data[dp++] != 0)
+				goto invalid_eoc;
+			len = dp - tdp - 2;
+		} else {
+			if (dp < len && (op & ASN1_OP_END__OF)) {
+				datalen = len;
+				csp++;
+				pc = machine[pc + 1];
+				pr_debug("- continue\n");
+				goto next_op;
+			}
+			if (dp != len)
+				goto cons_length_error;
+			len -= tdp;
+			pr_debug("- cons len l=%zu d=%zu\n", len, dp - tdp);
+		}
+
+		if (op & ASN1_OP_END__ACT) {
+			unsigned char act;
+			if (op & ASN1_OP_END__OF)
+				act = machine[pc + 2];
+			else
+				act = machine[pc + 1];
+			ret = actions[act](context, dp - hdr, 0, data + tdp, len);
+		}
+		pc += asn1_op_lengths[op];
+		goto next_op;
+
+	case ASN1_OP_ACT:
+		ret = actions[machine[pc + 1]](context, dp - hdr, tag, data + tdp, len);
+		pc += asn1_op_lengths[op];
+		goto next_op;
+
+	case ASN1_OP_RETURN:
+		if (unlikely(jsp <= 0))
+			goto jump_stack_underflow;
+		pc = jump_stack[--jsp];
+		goto next_op;
+
+	default:
+		break;
+	}
+
+	/* Shouldn't reach here */
+	pr_err("ASN.1 decoder error: Found reserved opcode (%u)\n", op);
+	return -EBADMSG;
+
+data_overrun_error:
+	errmsg = "Data overrun error";
+	goto error;
+machine_overrun_error:
+	errmsg = "Machine overrun error";
+	goto error;
+jump_stack_underflow:
+	errmsg = "Jump stack underflow";
+	goto error;
+jump_stack_overflow:
+	errmsg = "Jump stack overflow";
+	goto error;
+cons_stack_underflow:
+	errmsg = "Cons stack underflow";
+	goto error;
+cons_stack_overflow:
+	errmsg = "Cons stack overflow";
+	goto error;
+cons_length_error:
+	errmsg = "Cons length error";
+	goto error;
+missing_eoc:
+	errmsg = "Missing EOC in indefinite len cons";
+	goto error;
+invalid_eoc:
+	errmsg = "Invalid length EOC";
+	goto error;
+length_too_long:
+	errmsg = "Unsupported length";
+	goto error;
+indefinite_len_primitive:
+	errmsg = "Indefinite len primitive not permitted";
+	goto error;
+tag_mismatch:
+	errmsg = "Unexpected tag";
+	goto error;
+long_tag_not_supported:
+	errmsg = "Long tag not supported";
+error:
+	pr_debug("\nASN1: %s [m=%zu d=%zu ot=%02x t=%02x l=%zu]\n",
+		 errmsg, pc, dp, optag, tag, len);
+	return -EBADMSG;
+}
+EXPORT_SYMBOL_GPL(asn1_ber_decoder);

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