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Subject[PATCH 5.2 089/137] ext4: fix potential use after free after remounting with noblock_validity
Date
From: zhangyi (F) <yi.zhang@huawei.com>

[ Upstream commit 7727ae52975d4f4ef7ff69ed8e6e25f6a4168158 ]

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: syzbot+1e470567330b7ad711d5@syzkaller.appspotmail.com
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/block_validity.c | 189 ++++++++++++++++++++++++++++-----------
fs/ext4/ext4.h | 10 ++-
2 files changed, 147 insertions(+), 52 deletions(-)

diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
index 8e83741b02e03..d4d4fdfac1a65 100644
--- a/fs/ext4/block_validity.c
+++ b/fs/ext4/block_validity.c
@@ -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,
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 1cb67859e0518..0014b1c5e6be1 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -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
*/
@@ -1420,7 +1428,7 @@ 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 */
--
2.20.1


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