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Commit 7727ae52 authored by zhangyi (F)'s avatar zhangyi (F) Committed by Theodore Ts'o
Browse files

ext4: fix potential use after free after remounting with noblock_validity 



Remount process will release system zone which was allocated before if
"noblock_validity" is specified. If we mount an ext4 file system to two
mountpoints with default mount options, and then remount one of them
with "noblock_validity", it may trigger a use after free problem when
someone accessing the other one.

 # mount /dev/sda foo
 # mount /dev/sda bar

User access mountpoint "foo"   |   Remount mountpoint "bar"
                               |
ext4_map_blocks()              |   ext4_remount()
check_block_validity()         |   ext4_setup_system_zone()
ext4_data_block_valid()        |   ext4_release_system_zone()
                               |   free system_blks rb nodes
access system_blks rb nodes    |
trigger use after free         |

This problem can also be reproduced by one mountpint, At the same time,
add_system_zone() can get called during remount as well so there can be
racing ext4_data_block_valid() reading the rbtree at the same time.

This patch add RCU to protect system zone from releasing or building
when doing a remount which inverse current "noblock_validity" mount
option. It assign the rbtree after the whole tree was complete and
do actual freeing after rcu grace period, avoid any intermediate state.

Reported-by: default avatar <syzbot+1e470567330b7ad711d5@syzkaller.appspotmail.com>
Signed-off-by: default avatarzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
parent 4c273352

fs/ext4/block_validity.c

+138 −51
Original line number Diff line number Diff line
@@ -38,6 +38,7 @@ int __init ext4_init_system_zone(void) 


void ext4_exit_system_zone(void)

{

	rcu_barrier();

	kmem_cache_destroy(ext4_system_zone_cachep);

}
@@ -49,17 +50,26 @@ static inline int can_merge(struct ext4_system_zone *entry1, 
	return 0;

}



static void release_system_zone(struct ext4_system_blocks *system_blks)

{

	struct ext4_system_zone	*entry, *n;



	rbtree_postorder_for_each_entry_safe(entry, n,

				&system_blks->root, node)

		kmem_cache_free(ext4_system_zone_cachep, entry);

}



/*

 * Mark a range of blocks as belonging to the "system zone" --- that

 * is, filesystem metadata blocks which should never be used by

 * inodes.

 */

static int add_system_zone(struct ext4_sb_info *sbi,

static int add_system_zone(struct ext4_system_blocks *system_blks,

			   ext4_fsblk_t start_blk,

			   unsigned int count)

{

	struct ext4_system_zone *new_entry = NULL, *entry;

	struct rb_node **n = &sbi->system_blks.rb_node, *node;

	struct rb_node **n = &system_blks->root.rb_node, *node;

	struct rb_node *parent = NULL, *new_node = NULL;



	while (*n) {
@@ -91,7 +101,7 @@ static int add_system_zone(struct ext4_sb_info *sbi, 
		new_node = &new_entry->node;



		rb_link_node(new_node, parent, n);

		rb_insert_color(new_node, &sbi->system_blks);

		rb_insert_color(new_node, &system_blks->root);

	}



	/* Can we merge to the left? */
@@ -101,7 +111,7 @@ static int add_system_zone(struct ext4_sb_info *sbi, 
		if (can_merge(entry, new_entry)) {

			new_entry->start_blk = entry->start_blk;

			new_entry->count += entry->count;

			rb_erase(node, &sbi->system_blks);

			rb_erase(node, &system_blks->root);

			kmem_cache_free(ext4_system_zone_cachep, entry);

		}

	}
@@ -112,7 +122,7 @@ static int add_system_zone(struct ext4_sb_info *sbi, 
		entry = rb_entry(node, struct ext4_system_zone, node);

		if (can_merge(new_entry, entry)) {

			new_entry->count += entry->count;

			rb_erase(node, &sbi->system_blks);

			rb_erase(node, &system_blks->root);

			kmem_cache_free(ext4_system_zone_cachep, entry);

		}

	}
@@ -126,7 +136,7 @@ static void debug_print_tree(struct ext4_sb_info *sbi) 
	int first = 1;



	printk(KERN_INFO "System zones: ");

	node = rb_first(&sbi->system_blks);

	node = rb_first(&sbi->system_blks->root);

	while (node) {

		entry = rb_entry(node, struct ext4_system_zone, node);

		printk(KERN_CONT "%s%llu-%llu", first ? "" : ", ",
@@ -137,7 +147,47 @@ static void debug_print_tree(struct ext4_sb_info *sbi) 
	printk(KERN_CONT "\n");

}



static int ext4_protect_reserved_inode(struct super_block *sb, u32 ino)

/*

 * Returns 1 if the passed-in block region (start_blk,

 * start_blk+count) is valid; 0 if some part of the block region

 * overlaps with filesystem metadata blocks.

 */

static int ext4_data_block_valid_rcu(struct ext4_sb_info *sbi,

				     struct ext4_system_blocks *system_blks,

				     ext4_fsblk_t start_blk,

				     unsigned int count)

{

	struct ext4_system_zone *entry;

	struct rb_node *n;



	if ((start_blk <= le32_to_cpu(sbi->s_es->s_first_data_block)) ||

	    (start_blk + count < start_blk) ||

	    (start_blk + count > ext4_blocks_count(sbi->s_es))) {

		sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);

		return 0;

	}



	if (system_blks == NULL)

		return 1;



	n = system_blks->root.rb_node;

	while (n) {

		entry = rb_entry(n, struct ext4_system_zone, node);

		if (start_blk + count - 1 < entry->start_blk)

			n = n->rb_left;

		else if (start_blk >= (entry->start_blk + entry->count))

			n = n->rb_right;

		else {

			sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);

			return 0;

		}

	}

	return 1;

}



static int ext4_protect_reserved_inode(struct super_block *sb,

				       struct ext4_system_blocks *system_blks,

				       u32 ino)

{

	struct inode *inode;

	struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -163,14 +213,15 @@ static int ext4_protect_reserved_inode(struct super_block *sb, u32 ino) 
		if (n == 0) {

			i++;

		} else {

			if (!ext4_data_block_valid(sbi, map.m_pblk, n)) {

			if (!ext4_data_block_valid_rcu(sbi, system_blks,

						map.m_pblk, n)) {

				ext4_error(sb, "blocks %llu-%llu from inode %u "

					   "overlap system zone", map.m_pblk,

					   map.m_pblk + map.m_len - 1, ino);

				err = -EFSCORRUPTED;

				break;

			}

			err = add_system_zone(sbi, map.m_pblk, n);

			err = add_system_zone(system_blks, map.m_pblk, n);

			if (err < 0)

				break;

			i += n;
@@ -180,94 +231,130 @@ static int ext4_protect_reserved_inode(struct super_block *sb, u32 ino) 
	return err;

}



static void ext4_destroy_system_zone(struct rcu_head *rcu)

{

	struct ext4_system_blocks *system_blks;



	system_blks = container_of(rcu, struct ext4_system_blocks, rcu);

	release_system_zone(system_blks);

	kfree(system_blks);

}



/*

 * Build system zone rbtree which is used for block validity checking.

 *

 * The update of system_blks pointer in this function is protected by

 * sb->s_umount semaphore. However we have to be careful as we can be

 * racing with ext4_data_block_valid() calls reading system_blks rbtree

 * protected only by RCU. That's why we first build the rbtree and then

 * swap it in place.

 */

int ext4_setup_system_zone(struct super_block *sb)

{

	ext4_group_t ngroups = ext4_get_groups_count(sb);

	struct ext4_sb_info *sbi = EXT4_SB(sb);

	struct ext4_system_blocks *system_blks;

	struct ext4_group_desc *gdp;

	ext4_group_t i;

	int flex_size = ext4_flex_bg_size(sbi);

	int ret;



	if (!test_opt(sb, BLOCK_VALIDITY)) {

		if (sbi->system_blks.rb_node)

		if (sbi->system_blks)

			ext4_release_system_zone(sb);

		return 0;

	}

	if (sbi->system_blks.rb_node)

	if (sbi->system_blks)

		return 0;



	system_blks = kzalloc(sizeof(*system_blks), GFP_KERNEL);

	if (!system_blks)

		return -ENOMEM;



	for (i=0; i < ngroups; i++) {

		cond_resched();

		if (ext4_bg_has_super(sb, i) &&

		    ((i < 5) || ((i % flex_size) == 0)))

			add_system_zone(sbi, ext4_group_first_block_no(sb, i),

			add_system_zone(system_blks,

					ext4_group_first_block_no(sb, i),

					ext4_bg_num_gdb(sb, i) + 1);

		gdp = ext4_get_group_desc(sb, i, NULL);

		ret = add_system_zone(sbi, ext4_block_bitmap(sb, gdp), 1);

		ret = add_system_zone(system_blks,

				ext4_block_bitmap(sb, gdp), 1);

		if (ret)

			return ret;

		ret = add_system_zone(sbi, ext4_inode_bitmap(sb, gdp), 1);

			goto err;

		ret = add_system_zone(system_blks,

				ext4_inode_bitmap(sb, gdp), 1);

		if (ret)

			return ret;

		ret = add_system_zone(sbi, ext4_inode_table(sb, gdp),

			goto err;

		ret = add_system_zone(system_blks,

				ext4_inode_table(sb, gdp),

				sbi->s_itb_per_group);

		if (ret)

			return ret;

			goto err;

	}

	if (ext4_has_feature_journal(sb) && sbi->s_es->s_journal_inum) {

		ret = ext4_protect_reserved_inode(sb,

		ret = ext4_protect_reserved_inode(sb, system_blks,

				le32_to_cpu(sbi->s_es->s_journal_inum));

		if (ret)

			return ret;

			goto err;

	}



	/*

	 * System blks rbtree complete, announce it once to prevent racing

	 * with ext4_data_block_valid() accessing the rbtree at the same

	 * time.

	 */

	rcu_assign_pointer(sbi->system_blks, system_blks);



	if (test_opt(sb, DEBUG))

		debug_print_tree(sbi);

	return 0;

err:

	release_system_zone(system_blks);

	kfree(system_blks);

	return ret;

}



/* Called when the filesystem is unmounted */

/*

 * Called when the filesystem is unmounted or when remounting it with

 * noblock_validity specified.

 *

 * The update of system_blks pointer in this function is protected by

 * sb->s_umount semaphore. However we have to be careful as we can be

 * racing with ext4_data_block_valid() calls reading system_blks rbtree

 * protected only by RCU. So we first clear the system_blks pointer and

 * then free the rbtree only after RCU grace period expires.

 */

void ext4_release_system_zone(struct super_block *sb)

{

	struct ext4_system_zone	*entry, *n;

	struct ext4_system_blocks *system_blks;



	rbtree_postorder_for_each_entry_safe(entry, n,

			&EXT4_SB(sb)->system_blks, node)

		kmem_cache_free(ext4_system_zone_cachep, entry);

	system_blks = rcu_dereference_protected(EXT4_SB(sb)->system_blks,

					lockdep_is_held(&sb->s_umount));

	rcu_assign_pointer(EXT4_SB(sb)->system_blks, NULL);



	EXT4_SB(sb)->system_blks = RB_ROOT;

	if (system_blks)

		call_rcu(&system_blks->rcu, ext4_destroy_system_zone);

}



/*

 * Returns 1 if the passed-in block region (start_blk,

 * start_blk+count) is valid; 0 if some part of the block region

 * overlaps with filesystem metadata blocks.

 */

int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,

			  unsigned int count)

{

	struct ext4_system_zone *entry;

	struct rb_node *n = sbi->system_blks.rb_node;

	struct ext4_system_blocks *system_blks;

	int ret;



	if ((start_blk <= le32_to_cpu(sbi->s_es->s_first_data_block)) ||

	    (start_blk + count < start_blk) ||

	    (start_blk + count > ext4_blocks_count(sbi->s_es))) {

		sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);

		return 0;

	}

	while (n) {

		entry = rb_entry(n, struct ext4_system_zone, node);

		if (start_blk + count - 1 < entry->start_blk)

			n = n->rb_left;

		else if (start_blk >= (entry->start_blk + entry->count))

			n = n->rb_right;

		else {

			sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);

			return 0;

		}

	}

	return 1;

	/*

	 * Lock the system zone to prevent it being released concurrently

	 * when doing a remount which inverse current "[no]block_validity"

	 * mount option.

	 */

	rcu_read_lock();

	system_blks = rcu_dereference(sbi->system_blks);

	ret = ext4_data_block_valid_rcu(sbi, system_blks, start_blk,

					count);

	rcu_read_unlock();

	return ret;

}



int ext4_check_blockref(const char *function, unsigned int line,

fs/ext4/ext4.h

+9 −1
Original line number Diff line number Diff line
@@ -184,6 +184,14 @@ struct ext4_map_blocks { 
	unsigned int m_flags;

};



/*

 * Block validity checking, system zone rbtree.

 */

struct ext4_system_blocks {

	struct rb_root root;

	struct rcu_head rcu;

};



/*

 * Flags for ext4_io_end->flags

 */
@@ -1431,7 +1439,7 @@ struct ext4_sb_info { 
	int s_jquota_fmt;			/* Format of quota to use */

#endif

	unsigned int s_want_extra_isize; /* New inodes should reserve # bytes */

	struct rb_root system_blks;

	struct ext4_system_blocks __rcu *system_blks;



#ifdef EXTENTS_STATS

	/* ext4 extents stats */