CVE-2021-41072: Improper Link Resolution Before File Access ('Link Following') (CWE-59)
value: squashfs_opendir in unsquash-2.c in Squashfs-Tools 4.5 allows Directory Traversal, a different vulnerability than CVE-2021-40153. A squashfs filesystem that has been crafted to include a symbolic link and then contents under the same filename in a filesystem can cause unsquashfs to first create the symbolic link pointing outside the expected directory, and then the subsequent write operation will cause the unsquashfs process to write through the symbolic link elsewhere in the filesystem.
CWE description: The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
CVSS v3 base score: 8.1
Published: 2021-09-14T01:15Z
Repository: https://github.com/plougher/squashfs-tools
Language: C
Files changed: squashfs-tools/Makefile, squashfs-tools/unsquash-1.c, squashfs-tools/unsquash-12.c, squashfs-tools/unsquash-1234.c, squashfs-tools/unsquash-2.c, squashfs-tools/unsquash-3.c, squashfs-tools/unsquash-4.c, squashfs-tools/unsquashfs.h
Commit date: 2021-09-13 07:26:09 +0100
Commit message: Unsquashfs: additional write outside destination directory exploit fix

An issue on github (https://github.com/plougher/squashfs-tools/issues/72)
showed how some specially crafted Squashfs filesystems containing
invalid file names (with '/' and '..') can cause Unsquashfs to write
files outside of the destination directory.

Since then it has been shown that specially crafted Squashfs filesystems
that contain a symbolic link pointing outside of the destination directory,
coupled with an identically named file within the same directory, can
cause Unsquashfs to write files outside of the destination directory.

Specifically the symbolic link produces a pathname pointing outside
of the destination directory, which is then followed when writing the
duplicate identically named file within the directory.

This commit fixes this exploit by explictly checking for duplicate
filenames within a directory.  As directories in v2.1, v3.x, and v4.0
filesystems are sorted, this is achieved by checking for consecutively
identical filenames.  Additionally directories are checked to
ensure they are sorted, to avoid attempts to evade the duplicate
check.

Version 1.x and 2.0 filesystems (where the directories were unsorted)
are sorted and then the above duplicate filename check is applied.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

Diff: diff --git a/squashfs-tools/Makefile b/squashfs-tools/Makefile
index 7262a2e0..1b544ed6 100755
--- a/squashfs-tools/Makefile
+++ b/squashfs-tools/Makefile
@@ -158,12 +158,12 @@ INSTALL_DIR = /usr/local/bin
 MKSQUASHFS_OBJS = mksquashfs.o read_fs.o action.o swap.o pseudo.o compressor.o \
 	sort.o progressbar.o info.o restore.o process_fragments.o \
 	caches-queues-lists.o reader.o tar.o
 
 UNSQUASHFS_OBJS = unsquashfs.o unsquash-1.o unsquash-2.o unsquash-3.o \
-	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o swap.o \
-	compressor.o unsquashfs_info.o
+	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o unsquash-12.o \
+	swap.o compressor.o unsquashfs_info.o
 
 CFLAGS ?= -O2
 CFLAGS += $(EXTRA_CFLAGS) $(INCLUDEDIR) -D_FILE_OFFSET_BITS=64 \
 	-D_LARGEFILE_SOURCE -D_GNU_SOURCE -DCOMP_DEFAULT=\"$(COMP_DEFAULT)\" \
 	-Wall
@@ -391,10 +391,12 @@ unsquash-123.o: unsquashfs.h unsquash-123.c squashfs_fs.h squashfs_compat.h unsq
 
 unsquash-34.o: unsquashfs.h unsquash-34.c unsquashfs_error.h
 
 unsquash-1234.o: unsquash-1234.c unsquashfs_error.h
 
+unsquash-1234.o: unsquash-12.c
+
 unsquashfs_xattr.o: unsquashfs_xattr.c unsquashfs.h squashfs_fs.h xattr.h unsquashfs_error.h
 
 unsquashfs_info.o: unsquashfs.h squashfs_fs.h unsquashfs_error.h
 
 .PHONY: clean
diff --git a/squashfs-tools/unsquash-1.c b/squashfs-tools/unsquash-1.c
index b6044341..88866fcd 100644
--- a/squashfs-tools/unsquash-1.c
+++ b/squashfs-tools/unsquash-1.c
@@ -368,10 +368,16 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 			dir->dir_count ++;
 			bytes += dire->size + 1;
 		}
 	}
 
+	/* check directory for duplicate names.  Need to sort directory first */
+	sort_directory(dir);
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names\n");
+		goto corrupted;
+	}
 	return dir;
 
 corrupted:
 	squashfs_closedir(dir);
 	return NULL;
diff --git a/squashfs-tools/unsquash-12.c b/squashfs-tools/unsquash-12.c
new file mode 100644
index 00000000..61bf1284
--- /dev/null
+++ b/squashfs-tools/unsquash-12.c
@@ -0,0 +1,110 @@
+/*
+ * Unsquash a squashfs filesystem.  This is a highly compressed read only
+ * filesystem.
+ *
+ * Copyright (c) 2021
+ * Phillip Lougher <phillip@squashfs.org.uk>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2,
+ * or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * unsquash-12.c
+ *
+ * Helper functions used by unsquash-1 and unsquash-2.
+ */
+
+#include "unsquashfs.h"
+
+/*
+ * Bottom up linked list merge sort.
+ *
+ */
+void sort_directory(struct dir *dir)
+{
+	struct dir_ent *cur, *l1, *l2, *next;
+	int len1, len2, stride = 1;
+
+	if(dir->dir_count < 2)
+		return;
+
+	/*
+	 * We can consider our linked-list to be made up of stride length
+	 * sublists.  Eacn iteration around this loop merges adjacent
+	 * stride length sublists into larger 2*stride sublists.  We stop
+	 * when stride becomes equal to the entire list.
+	 *
+	 * Initially stride = 1 (by definition a sublist of 1 is sorted), and
+	 * these 1 element sublists are merged into 2 element sublists,  which
+	 * are then merged into 4 element sublists and so on.
+	 */
+	do {
+		l2 = dir->dirs; /* head of current linked list */
+		cur = NULL; /* empty output list */
+
+		/*
+		 * Iterate through the linked list, merging adjacent sublists.
+		 * On each interation l2 points to the next sublist pair to be
+		 * merged (if there's only one sublist left this is simply added
+		 * to the output list)
+		 */
+		while(l2) {
+			l1 = l2;
+			for(len1 = 0; l2 && len1 < stride; len1 ++, l2 = l2->next);
+			len2 = stride;
+
+			/*
+			 * l1 points to first sublist.
+			 * l2 points to second sublist.
+			 * Merge them onto the output list
+			 */
+			while(len1 && l2 && len2) {
+				if(strcmp(l1->name, l2->name) <= 0) {
+					next = l1;
+					l1 = l1->next;
+					len1 --;
+				} else {
+					next = l2;
+					l2 = l2->next;
+					len2 --;
+				}
+
+				if(cur) {
+					cur->next = next;
+					cur = next;
+				} else
+					dir->dirs = cur = next;
+			}
+			/*
+			 * One sublist is now empty, copy the other one onto the
+			 * output list
+			 */
+			for(; len1; len1 --, l1 = l1->next) {
+				if(cur) {
+					cur->next = l1;
+					cur = l1;
+				} else
+					dir->dirs = cur = l1;
+			}
+			for(; l2 && len2; len2 --, l2 = l2->next) {
+				if(cur) {
+					cur->next = l2;
+					cur = l2;
+				} else
+					dir->dirs = cur = l2;
+			}
+		}
+		cur->next = NULL;
+		stride = stride << 1;
+	} while(stride < dir->dir_count);
+}
diff --git a/squashfs-tools/unsquash-1234.c b/squashfs-tools/unsquash-1234.c
index e389f8d6..98a81ed0 100644
--- a/squashfs-tools/unsquash-1234.c
+++ b/squashfs-tools/unsquash-1234.c
@@ -70,5 +70,26 @@ void squashfs_closedir(struct dir *dir)
 		free(tmp);
 	}
 
 	free(dir);
 }
+
+
+/*
+ * Check directory for duplicate names.  As the directory should be sorted,
+ * duplicates will be consecutive.  Obviously we also need to check if the
+ * directory has been deliberately unsorted, to evade this check.
+ */
+int check_directory(struct dir *dir)
+{
+	int i;
+	struct dir_ent *ent;
+
+	if(dir->dir_count < 2)
+		return TRUE;
+
+	for(ent = dir->dirs, i = 0; i < dir->dir_count - 1; ent = ent->next, i++)
+		if(strcmp(ent->name, ent->next->name) >= 0)
+			return FALSE;
+
+	return TRUE;
+}
diff --git a/squashfs-tools/unsquash-2.c b/squashfs-tools/unsquash-2.c
index 956f96f4..0e36f7d5 100644
--- a/squashfs-tools/unsquash-2.c
+++ b/squashfs-tools/unsquash-2.c
@@ -27,10 +27,11 @@
 #include "compressor.h"
 
 static squashfs_fragment_entry_2 *fragment_table;
 static unsigned int *uid_table, *guid_table;
 static squashfs_operations ops;
+static int needs_sorting = FALSE;
 
 
 static void read_block_list(unsigned int *block_list, long long start,
 					unsigned int offset, int blocks)
 {
@@ -461,10 +462,21 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 			dir->dir_count ++;
 			bytes += dire->size + 1;
 		}
 	}
 
+	if(needs_sorting)
+		sort_directory(dir);
+
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		if(needs_sorting)
+			ERROR("File system corrupted: directory has duplicate names\n");
+		else
+			ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
 	return dir;
 
 corrupted:
 	squashfs_closedir(dir);
 	return NULL;
@@ -594,10 +606,14 @@ int read_super_2(squashfs_operations **s_ops, void *s)
 
 	/*
 	 * 2.x filesystems use gzip compression.
 	 */
 	comp = lookup_compressor("gzip");
+
+	if(sBlk_3->s_minor == 0)
+		needs_sorting = TRUE;
+
 	return TRUE;
 }
 
 
 static void squashfs_stat(char *source)
diff --git a/squashfs-tools/unsquash-3.c b/squashfs-tools/unsquash-3.c
index 835a574c..01235626 100644
--- a/squashfs-tools/unsquash-3.c
+++ b/squashfs-tools/unsquash-3.c
@@ -495,10 +495,16 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 			dir->dir_count ++;
 			bytes += dire->size + 1;
 		}
 	}
 
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
+
 	return dir;
 
 corrupted:
 	squashfs_closedir(dir);
 	return NULL;
diff --git a/squashfs-tools/unsquash-4.c b/squashfs-tools/unsquash-4.c
index 694783da..c615bb83 100644
--- a/squashfs-tools/unsquash-4.c
+++ b/squashfs-tools/unsquash-4.c
@@ -432,10 +432,16 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 			dir->dir_count ++;
 			bytes += dire->size + 1;
 		}
 	}
 
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
+
 	return dir;
 
 corrupted:
 	squashfs_closedir(dir);
 	return NULL;
diff --git a/squashfs-tools/unsquashfs.h b/squashfs-tools/unsquashfs.h
index f8cf78cb..bf2a80d0 100644
--- a/squashfs-tools/unsquashfs.h
+++ b/squashfs-tools/unsquashfs.h
@@ -291,6 +291,10 @@ extern int read_ids(int, long long, long long, unsigned int **);
 extern long long *alloc_index_table(int);
 
 /* unsquash-1234.c */
 extern int check_name(char *, int);
 extern void squashfs_closedir(struct dir *);
+extern int check_directory(struct dir *);
+
+/* unsquash-12.c */
+extern void sort_directory(struct dir *);
 #endif

Vulnerable code: 	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o swap.o \
	compressor.o unsquashfs_info.o
Fixed code: 	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o unsquash-12.o \
	swap.o compressor.o unsquashfs_info.o
unsquash-1234.o: unsquash-12.c

	/* check directory for duplicate names.  Need to sort directory first */
	sort_directory(dir);
	if(check_directory(dir) == FALSE) {
		ERROR("File system corrupted: directory has duplicate names\n");
		goto corrupted;
	}
/*
 * Unsquash a squashfs filesystem.  This is a highly compressed read only
 * filesystem.
 *
 * Copyright (c) 2021
 * Phillip Lougher <phillip@squashfs.org.uk>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * unsquash-12.c
 *
 * Helper functions used by unsquash-1 and unsquash-2.
 */

#include "unsquashfs.h"

/*
 * Bottom up linked list merge sort.
 *
 */
void sort_directory(struct dir *dir)
{
	struct dir_ent *cur, *l1, *l2, *next;
	int len1, len2, stride = 1;

	if(dir->dir_count < 2)
		return;

	/*
	 * We can consider our linked-list to be made up of stride length
	 * sublists.  Eacn iteration around this loop merges adjacent
	 * stride length sublists into larger 2*stride sublists.  We stop
	 * when stride becomes equal to the entire list.
	 *
	 * Initially stride = 1 (by definition a sublist of 1 is sorted), and
	 * these 1 element sublists are merged into 2 element sublists,  which
	 * are then merged into 4 element sublists and so on.
	 */
	do {
		l2 = dir->dirs; /* head of current linked list */
		cur = NULL; /* empty output list */

		/*
		 * Iterate through the linked list, merging adjacent sublists.
		 * On each interation l2 points to the next sublist pair to be
		 * merged (if there's only one sublist left this is simply added
		 * to the output list)
		 */
		while(l2) {
			l1 = l2;
			for(len1 = 0; l2 && len1 < stride; len1 ++, l2 = l2->next);
			len2 = stride;

			/*
			 * l1 points to first sublist.
			 * l2 points to second sublist.
			 * Merge them onto the output list
			 */
			while(len1 && l2 && len2) {
				if(strcmp(l1->name, l2->name) <= 0) {
					next = l1;
					l1 = l1->next;
					len1 --;
				} else {
					next = l2;
					l2 = l2->next;
					len2 --;
				}

				if(cur) {
					cur->next = next;
					cur = next;
				} else
					dir->dirs = cur = next;
			}
			/*
			 * One sublist is now empty, copy the other one onto the
			 * output list
			 */
			for(; len1; len1 --, l1 = l1->next) {
				if(cur) {
					cur->next = l1;
					cur = l1;
				} else
					dir->dirs = cur = l1;
			}
			for(; l2 && len2; len2 --, l2 = l2->next) {
				if(cur) {
					cur->next = l2;
					cur = l2;
				} else
					dir->dirs = cur = l2;
			}
		}
		cur->next = NULL;
		stride = stride << 1;
	} while(stride < dir->dir_count);
}


/*
 * Check directory for duplicate names.  As the directory should be sorted,
 * duplicates will be consecutive.  Obviously we also need to check if the
 * directory has been deliberately unsorted, to evade this check.
 */
int check_directory(struct dir *dir)
{
	int i;
	struct dir_ent *ent;

	if(dir->dir_count < 2)
		return TRUE;

	for(ent = dir->dirs, i = 0; i < dir->dir_count - 1; ent = ent->next, i++)
		if(strcmp(ent->name, ent->next->name) >= 0)
			return FALSE;

	return TRUE;
}
static int needs_sorting = FALSE;
	if(needs_sorting)
		sort_directory(dir);

	/* check directory for duplicate names and sorting */
	if(check_directory(dir) == FALSE) {
		if(needs_sorting)
			ERROR("File system corrupted: directory has duplicate names\n");
		else
			ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
		goto corrupted;
	}

	if(sBlk_3->s_minor == 0)
		needs_sorting = TRUE;

	/* check directory for duplicate names and sorting */
	if(check_directory(dir) == FALSE) {
		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
		goto corrupted;
	}

	/* check directory for duplicate names and sorting */
	if(check_directory(dir) == FALSE) {
		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
		goto corrupted;
	}

extern int check_directory(struct dir *);

/* unsquash-12.c */
extern void sort_directory(struct dir *);