mpatch: be more careful about parsing binary patch data (SEC)

It appears to have been possible to trivially walk off the end of an
allocated region with a malformed patch. Oops.

Caught when writing an mpatch fuzzer for oss-fuzz.

This defect is OVE-20180430-0001. A CVE has not been obtained as of
this writing.

mercurial/mpatch.c

@@ -1,279 +1,281 b''
 /*
  mpatch.c - efficient binary patching for Mercurial
 
  This implements a patch algorithm that's O(m + nlog n) where m is the
  size of the output and n is the number of patches.
 
  Given a list of binary patches, it unpacks each into a hunk list,
  then combines the hunk lists with a treewise recursion to form a
  single hunk list. This hunk list is then applied to the original
  text.
 
  The text (or binary) fragments are copied directly from their source
  Python objects into a preallocated output string to avoid the
  allocation of intermediate Python objects. Working memory is about 2x
  the total number of hunks.
 
  Copyright 2005, 2006 Matt Mackall <[e-mail]>
 
  This software may be used and distributed according to the terms
  of the GNU General Public License, incorporated herein by reference.
 */
 
 #include <stdlib.h>
 #include <string.h>
 
 #include "bitmanipulation.h"
 #include "compat.h"
 #include "mpatch.h"
 
 static struct mpatch_flist *lalloc(ssize_t size)
 {
 	struct mpatch_flist *a = NULL;
 
 	if (size < 1)
 		size = 1;
 
 	a = (struct mpatch_flist *)malloc(sizeof(struct mpatch_flist));
 	if (a) {
 		a->base = (struct mpatch_frag *)malloc(
 		    sizeof(struct mpatch_frag) * size);
 		if (a->base) {
 			a->head = a->tail = a->base;
 			return a;
 		}
 		free(a);
 	}
 	return NULL;
 }
 
 void mpatch_lfree(struct mpatch_flist *a)
 {
 	if (a) {
 		free(a->base);
 		free(a);
 	}
 }
 
 static ssize_t lsize(struct mpatch_flist *a)
 {
 	return a->tail - a->head;
 }
 
 /* move hunks in source that are less cut to dest, compensating
    for changes in offset. the last hunk may be split if necessary.
 */
 static int gather(struct mpatch_flist *dest, struct mpatch_flist *src, int cut,
                   int offset)
 {
 	struct mpatch_frag *d = dest->tail, *s = src->head;
 	int postend, c, l;
 
 	while (s != src->tail) {
 		if (s->start + offset >= cut)
 			break; /* we've gone far enough */
 
 		postend = offset + s->start + s->len;
 		if (postend <= cut) {
 			/* save this hunk */
 			offset += s->start + s->len - s->end;
 			*d++ = *s++;
 		} else {
 			/* break up this hunk */
 			c = cut - offset;
 			if (s->end < c)
 				c = s->end;
 			l = cut - offset - s->start;
 			if (s->len < l)
 				l = s->len;
 
 			offset += s->start + l - c;
 
 			d->start = s->start;
 			d->end = c;
 			d->len = l;
 			d->data = s->data;
 			d++;
 			s->start = c;
 			s->len = s->len - l;
 			s->data = s->data + l;
 
 			break;
 		}
 	}
 
 	dest->tail = d;
 	src->head = s;
 	return offset;
 }
 
 /* like gather, but with no output list */
 static int discard(struct mpatch_flist *src, int cut, int offset)
 {
 	struct mpatch_frag *s = src->head;
 	int postend, c, l;
 
 	while (s != src->tail) {
 		if (s->start + offset >= cut)
 			break;
 
 		postend = offset + s->start + s->len;
 		if (postend <= cut) {
 			offset += s->start + s->len - s->end;
 			s++;
 		} else {
 			c = cut - offset;
 			if (s->end < c)
 				c = s->end;
 			l = cut - offset - s->start;
 			if (s->len < l)
 				l = s->len;
 
 			offset += s->start + l - c;
 			s->start = c;
 			s->len = s->len - l;
 			s->data = s->data + l;
 
 			break;
 		}
 	}
 
 	src->head = s;
 	return offset;
 }
 
 /* combine hunk lists a and b, while adjusting b for offset changes in a/
    this deletes a and b and returns the resultant list. */
 static struct mpatch_flist *combine(struct mpatch_flist *a,
                                     struct mpatch_flist *b)
 {
 	struct mpatch_flist *c = NULL;
 	struct mpatch_frag *bh, *ct;
 	int offset = 0, post;
 
 	if (a && b)
 		c = lalloc((lsize(a) + lsize(b)) * 2);
 
 	if (c) {
 
 		for (bh = b->head; bh != b->tail; bh++) {
 			/* save old hunks */
 			offset = gather(c, a, bh->start, offset);
 
 			/* discard replaced hunks */
 			post = discard(a, bh->end, offset);
 
 			/* insert new hunk */
 			ct = c->tail;
 			ct->start = bh->start - offset;
 			ct->end = bh->end - post;
 			ct->len = bh->len;
 			ct->data = bh->data;
 			c->tail++;
 			offset = post;
 		}
 
 		/* hold on to tail from a */
 		memcpy(c->tail, a->head, sizeof(struct mpatch_frag) * lsize(a));
 		c->tail += lsize(a);
 	}
 
 	mpatch_lfree(a);
 	mpatch_lfree(b);
 	return c;
 }
 
 /* decode a binary patch into a hunk list */
 int mpatch_decode(const char *bin, ssize_t len, struct mpatch_flist **res)
 {
 	struct mpatch_flist *l;
 	struct mpatch_frag *lt;
 	int pos = 0;
 
 	/* assume worst case size, we won't have many of these lists */
 	l = lalloc(len / 12 + 1);
 	if (!l)
 		return MPATCH_ERR_NO_MEM;
 
 	lt = l->tail;
 
-	while (pos >= 0 && pos < len) {
+	/* We check against len-11 to ensure we have at least 12 bytes
+	   left in the patch so we can read our three be32s out of it. */
+	while (pos >= 0 && pos < (len - 11)) {
 		lt->start = getbe32(bin + pos);
 		lt->end = getbe32(bin + pos + 4);
 		lt->len = getbe32(bin + pos + 8);
 		lt->data = bin + pos + 12;
 		pos += 12 + lt->len;
 		if (lt->start > lt->end || lt->len < 0)
 			break; /* sanity check */
 		lt++;
 	}
 
 	if (pos != len) {
 		mpatch_lfree(l);
 		return MPATCH_ERR_CANNOT_BE_DECODED;
 	}
 
 	l->tail = lt;
 	*res = l;
 	return 0;
 }
 
 /* calculate the size of resultant text */
 ssize_t mpatch_calcsize(ssize_t len, struct mpatch_flist *l)
 {
 	ssize_t outlen = 0, last = 0;
 	struct mpatch_frag *f = l->head;
 
 	while (f != l->tail) {
 		if (f->start < last || f->end > len) {
 			return MPATCH_ERR_INVALID_PATCH;
 		}
 		outlen += f->start - last;
 		last = f->end;
 		outlen += f->len;
 		f++;
 	}
 
 	outlen += len - last;
 	return outlen;
 }
 
 int mpatch_apply(char *buf, const char *orig, ssize_t len,
                  struct mpatch_flist *l)
 {
 	struct mpatch_frag *f = l->head;
 	int last = 0;
 	char *p = buf;
 
 	while (f != l->tail) {
 		if (f->start < last || f->end > len) {
 			return MPATCH_ERR_INVALID_PATCH;
 		}
 		memcpy(p, orig + last, f->start - last);
 		p += f->start - last;
 		memcpy(p, f->data, f->len);
 		last = f->end;
 		p += f->len;
 		f++;
 	}
 	memcpy(p, orig + last, len - last);
 	return 0;
 }
 
 /* recursively generate a patch of all bins between start and end */
 struct mpatch_flist *
 mpatch_fold(void *bins, struct mpatch_flist *(*get_next_item)(void *, ssize_t),
             ssize_t start, ssize_t end)
 {
 	ssize_t len;
 
 	if (start + 1 == end) {
 		/* trivial case, output a decoded list */
 		return get_next_item(bins, start);
 	}
 
 	/* divide and conquer, memory management is elsewhere */
 	len = (end - start) / 2;
 	return combine(mpatch_fold(bins, get_next_item, start, start + len),
 	               mpatch_fold(bins, get_next_item, start + len, end));
 }