Return to roff_escape.c CVS log

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Fix a buffer overrun in the roff(7) escape sequence parser that could
be triggered by macro arguments ending in double backslashes, for
example if people wrote .Sq "\\" instead of the correct .Sq "\e".

The bug was hard to find because it caused a segfault only very rarely,
according to my measurements with a probability of less than one permille.
I'm sorry that the first one to hit the bug was an arm64 release build
run by deraadt@.  Thanks to bluhm@ for providing access to an arm64
machine for debugging purposes.  In the end, the bug turned out to be
architecture-independent.

The reason for the bug was that i assumed an invariant that does not exist.
The function roff_parse_comment() is very careful to make sure that the
input buffer does not end in an escape character before passing it on,
so i assumed this is still true when reaching roff_expand() immediately
afterwards.  But roff_expand() can also be reached from roff_getarg(),
in which case there *can* be a lone escape character at the end of the
buffer in case copy mode processing found and converted a double
backslash.

Fix this by handling a trailing escape character correctly in the
function roff_escape().

The lesson here probably is to refrain from assuming an invariant
unless verifying that the invariant actually holds is reasonably
simple.  In some cases, in particular for invariants that are important
but not simple, it might also make sense to assert(3) rather than just
assume the invariant.  An assertion failure is so much better than a
buffer overrun...

/* $OpenBSD$ */
/*
 * Copyright (c) 2011, 2012, 2013, 2014, 2015, 2017, 2018, 2020, 2022
 *               Ingo Schwarze <schwarze@openbsd.org>
 * Copyright (c) 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 * Parser for roff(7) escape sequences.
 * To be used by all mandoc(1) parsers and formatters.
 */
#include <assert.h>
#include <ctype.h>
#include <limits.h>
#include <stdio.h>
#include <string.h>

#include "mandoc.h"
#include "roff.h"
#include "roff_int.h"

/*
 * Traditional escape sequence interpreter for general use
 * including in high-level formatters.  This function does not issue
 * diagnostics and is not usable for expansion in the roff(7) parser.
 * It is documented in the mandoc_escape(3) manual page.
 */
enum mandoc_esc
mandoc_escape(const char **rendarg, const char **rarg, int *rargl)
{
        int		 iarg, iendarg, iend;
        enum mandoc_esc  rval;

        rval = roff_escape(--*rendarg, 0, 0, NULL, &iarg, &iendarg, &iend);
        assert(rval != ESCAPE_EXPAND);
        if (rarg != NULL)
	       *rarg = *rendarg + iarg;
        if (rargl != NULL)
	       *rargl = iendarg - iarg;
        *rendarg += iend;
        return rval;
}

/*
 * Full-featured escape sequence parser.
 * If it encounters a nested escape sequence that requires expansion
 * by the parser and re-parsing, the positions of that inner escape
 * sequence are returned in *resc ... *rend.
 * Otherwise, *resc is set to aesc and the positions of the escape
 * sequence starting at aesc are returned.
 * Diagnostic messages are generated if and only if resc != NULL,
 * that is, if and only if called by roff_expand().
 */
enum mandoc_esc
roff_escape(const char *buf, const int ln, const int aesc,
    int *resc, int *rarg, int *rendarg, int *rend)
{
	int		 iesc;		/* index of leading escape char */
	int		 iarg;		/* index beginning the argument */
	int		 iendarg;	/* index right after the argument */
	int		 iend;		/* index right after the sequence */
	int		 sesc, sarg, sendarg, send; /* for sub-escape */
	int		 maxl;		/* expected length of the argument */
	int		 argl;		/* actual length of the argument */
	int		 c, i;		/* for \[char...] parsing */
	int 		 valid_A;	/* for \A parsing */
	enum mandoc_esc	 rval;		/* return value */
	enum mandocerr	 err;		/* diagnostic code */
	char		 esc_name;
	char		 term;		/* byte terminating the argument */

	/*
	 * Treat "\E" just like "\";
	 * it only makes a difference in copy mode.
	 */

	iesc = iarg = aesc;
	do {
		iarg++;
	} while (buf[iarg] == 'E');

	/*
	 * Sort the following cases first by syntax category,
	 * then by escape sequence type, and finally by ASCII code.
	 */

	esc_name = buf[iarg];
	iendarg = iend = ++iarg;
	maxl = INT_MAX;
	term = '\0';
	switch (esc_name) {

	/* Escape sequences taking no arguments at all. */

	case '!':
	case '?':
	case 'r':
		rval = ESCAPE_UNSUPP;
		goto out;

	case '%':
	case '&':
	case ')':
	case ',':
	case '/':
	case '^':
	case 'a':
	case 'd':
	case 't':
	case 'u':
	case '{':
	case '|':
	case '}':
		rval = ESCAPE_IGNORE;
		goto out;

	case '\0':
		iendarg = --iend;
		/* FALLTHROUGH */
	case '\\':
	default:
		iarg--;
		rval = ESCAPE_UNDEF;
		goto out;

	case ' ':
	case '\'':
	case '-':
	case '.':
	case '0':
	case ':':
	case '_':
	case '`':
	case 'e':
	case '~':
		iarg--;
		argl = 1;
		rval = ESCAPE_SPECIAL;
		goto out;
	case 'p':
		rval = ESCAPE_BREAK;
		goto out;
	case 'c':
		rval = ESCAPE_NOSPACE;
		goto out;
	case 'z':
		rval = ESCAPE_SKIPCHAR;
		goto out;

	/* Standard argument format. */

	case '$':
	case '*':
	case 'V':
	case 'g':
	case 'n':
		rval = ESCAPE_EXPAND;
		break;
	case 'F':
	case 'M':
	case 'O':
	case 'Y':
	case 'k':
	case 'm':
		rval = ESCAPE_IGNORE;
		break;
	case '(':
	case '[':
		rval = ESCAPE_SPECIAL;
		iendarg = iend = --iarg;
		break;
	case 'f':
		rval = ESCAPE_FONT;
		break;

	/* Quoted arguments */

	case 'A':
	case 'B':
	case 'w':
		rval = ESCAPE_EXPAND;
		term = '\b';
		break;
	case 'D':
	case 'H':
	case 'L':
	case 'R':
	case 'S':
	case 'X':
	case 'Z':
	case 'b':
	case 'v':
	case 'x':
		rval = ESCAPE_IGNORE;
		term = '\b';
		break;
	case 'C':
		if (buf[iarg] != '\'') {
			rval = ESCAPE_ERROR;
			goto out;
		}
		rval = ESCAPE_SPECIAL;
		term = '\b';
		break;
	case 'N':
		rval = ESCAPE_NUMBERED;
		term = '\b';
		break;
	case 'h':
		rval = ESCAPE_HORIZ;
		term = '\b';
		break;
	case 'l':
		rval = ESCAPE_HLINE;
		term = '\b';
		break;
	case 'o':
		rval = ESCAPE_OVERSTRIKE;
		term = '\b';
		break;

	/* Sizes support both forms, with additional peculiarities. */

	case 's':
		rval = ESCAPE_IGNORE;
		if (buf[iarg] == '+' || buf[iarg] == '-'||
		    buf[iarg] == ASCII_HYPH)
			iarg++;
		switch (buf[iarg]) {
		case '(':
			maxl = 2;
			iarg++;
			break;
		case '[':
			term = ']';
			iarg++;
			break;
		case '\'':
			term = '\'';
			iarg++;
			break;
		case '1':
		case '2':
		case '3':
			if (buf[iarg - 1] == 's' &&
			    isdigit((unsigned char)buf[iarg + 1])) {
				maxl = 2;
				break;
			}
			/* FALLTHROUGH */
		default:
			maxl = 1;
			break;
		}
		iendarg = iend = iarg;
	}

	/* Decide how to end the argument. */

	if ((term == '\b' || (term == '\0' && maxl == INT_MAX)) &&
	    buf[iarg] == buf[iesc] && roff_escape(buf, ln, iendarg,
	    &sesc, &sarg, &sendarg, &send) == ESCAPE_EXPAND)
		goto out_sub;

	if (term == '\b') {
		if ((esc_name == 'N' && isdigit((unsigned char)buf[iarg])) ||
		    (esc_name == 'h' && strchr(" %&()*+-./0123456789:<=>",
		     buf[iarg]) != NULL)) {
			iendarg = iend = iarg + 1;
			rval = ESCAPE_ERROR;
			goto out;
		}
		term = buf[iarg++];
	} else if (term == '\0' && maxl == INT_MAX) {
		if (esc_name == 'n' && (buf[iarg] == '+' || buf[iarg] == '-'))
			iarg++;
		switch (buf[iarg]) {
		case '(':
			maxl = 2;
			iarg++;
			break;
		case '[':
			if (buf[++iarg] == ' ') {
				iendarg = iend = iarg + 1;
				rval = ESCAPE_ERROR;
				goto out;
			}
			term = ']';
			break;
		default:
			maxl = 1;
			break;
		}
	}

	/* Advance to the end of the argument. */

	valid_A = 1;
	iendarg = iarg;
	while (maxl > 0) {
		if (buf[iendarg] == '\0') {
			/* Ignore an incomplete argument except for \w. */
			if (esc_name != 'w')
				iendarg = iarg;
			break;
		}
		if (buf[iendarg] == term) {
			iend = iendarg + 1;
			break;
		}
		if (esc_name == 'N' &&
		    isdigit((unsigned char)buf[iendarg]) == 0) {
			iend = iendarg + 1;
			break;
		}
		if (buf[iendarg] == buf[iesc]) {
			switch (roff_escape(buf, ln, iendarg,
			    &sesc, &sarg, &sendarg, &send)) {
			case ESCAPE_EXPAND:
				goto out_sub;
			case ESCAPE_UNDEF:
				break;
			default:
				valid_A = 0;
				break;
			}
			iendarg = iend = send;
		} else {
			if (buf[iendarg] == ' ' || buf[iendarg] == '\t')
				valid_A = 0;
			if (maxl != INT_MAX)
				maxl--;
			iend = ++iendarg;
		}
	}
	if (resc != NULL && ((maxl != INT_MAX && maxl != 0) ||
	    (term != '\0' && buf[iendarg] != term)))
		mandoc_msg(MANDOCERR_ESC_BAD, ln, iesc, "%s", buf + iesc);

	/* Post-process depending on the content of the argument. */

	argl = iendarg - iarg;
	switch (esc_name) {
	case '*':
		if (resc == NULL && argl == 2 &&
		    buf[iarg] == '.' && buf[iarg + 1] == 'T')
			rval = ESCAPE_DEVICE;
		break;
	case 'A':
		if (valid_A == 0)
			iendarg = iarg;
		break;
	case 'O':
		switch (buf[iarg]) {
		case '0':
			rval = ESCAPE_UNSUPP;
			break;
		case '1':
		case '2':
		case '3':
		case '4':
			rval = argl == 1 ? ESCAPE_IGNORE : ESCAPE_ERROR;
			break;
		case '5':
			rval = buf[iarg - 1] == '[' ? ESCAPE_UNSUPP :
			    ESCAPE_ERROR;
			break;
		default:
			rval = ESCAPE_ERROR;
			break;
		}
		break;
	default:
		break;
	}

	switch (rval) {
	case ESCAPE_FONT:
		rval = mandoc_font(buf + iarg, argl);
		break;

	case ESCAPE_SPECIAL:

		/*
		 * The file chars.c only provides one common list of
		 * character names, but \[-] == \- is the only one of
		 * the characters with one-byte names that allows
		 * enclosing the name in brackets.
		 */

		if (term != '\0' && argl == 1 && buf[iarg] != '-') {
			rval = ESCAPE_ERROR;
			break;
		}

		/* Treat \[char...] as an alias for \N'...'. */

		if (buf[iarg] == 'c') {
			if (argl < 6 || argl > 7 ||
			    strncmp(buf + iarg, "char", 4) != 0 ||
			    (int)strspn(buf + iarg + 4, "0123456789")
			     + 4 < argl)
				break;
			c = 0;
			for (i = iarg; i < iendarg; i++)
				c = 10 * c + (buf[i] - '0');
			if (c < 0x21 || (c > 0x7e && c < 0xa0) || c > 0xff)
				break;
			iarg += 4;
			rval = ESCAPE_NUMBERED;
			break;
		}

		/*
		 * Unicode escapes are defined in groff as \[u0000]
		 * to \[u10FFFF], where the contained value must be
		 * a valid Unicode codepoint.  Here, however, only
		 * check the length and range.
		 */

		if (buf[iarg] != 'u' || argl < 5 || argl > 7)
			break;
		if (argl == 7 &&
		    (buf[iarg + 1] != '1' || buf[iarg + 2] != '0'))
			break;
		if (argl == 6 && buf[iarg + 1] == '0')
			break;
		if (argl == 5 && buf[iarg + 1] == 'D' &&
		    strchr("89ABCDEF", buf[iarg + 2]) != NULL)
			break;
		if ((int)strspn(buf + iarg + 1, "0123456789ABCDEFabcdef")
		    + 1 == argl)
			rval = ESCAPE_UNICODE;
		break;
	default:
		break;
	}
	goto out;

out_sub:
	iesc = sesc;
	iarg = sarg;
	iendarg = sendarg;
	iend = send;
	rval = ESCAPE_EXPAND;

out:
	if (rarg != NULL)
		*rarg = iarg;
	if (rendarg != NULL)
		*rendarg = iendarg;
	if (rend != NULL)
		*rend = iend;
	if (resc == NULL)
		return rval;

	/*
	 * Diagnostic messages are only issued when called
	 * from the parser, not when called from the formatters.
	 */

	*resc = iesc;
	switch (rval) {
	case ESCAPE_ERROR:
		err = MANDOCERR_ESC_BAD;
		break;
	case ESCAPE_UNSUPP:
		err = MANDOCERR_ESC_UNSUPP;
		break;
	case ESCAPE_UNDEF:
		if (esc_name == '\\')
			return rval;
		err = MANDOCERR_ESC_UNDEF;
		break;
	case ESCAPE_SPECIAL:
		if (mchars_spec2cp(buf + iarg, argl) >= 0)
			return rval;
		err = MANDOCERR_ESC_BAD;
		break;
	default:
		return rval;
	}
	mandoc_msg(err, ln, iesc, "%.*s", iend - iesc, buf + iesc);
	return rval;
}