[PATCH 4.19 198/267] ext4: fix race between ext4_sync_parent() and rename()

Greg Kroah-Hartman
2020-06-19T15:02:23Z

From: Eric Biggers <ebiggers@google.com>

commit 08adf452e628b0e2ce9a01048cfbec52353703d7 upstream.

'igrab(d_inode(dentry->d_parent))' without holding dentry->d_lock is
broken because without d_lock, d_parent can be concurrently changed due
to a rename(). Then if the old directory is immediately deleted, old
d_parent->inode can be NULL. That causes a NULL dereference in igrab().

To fix this, use dget_parent() to safely grab a reference to the parent
dentry, which pins the inode. This also eliminates the need to use
d_find_any_alias() other than for the initial inode, as we no longer
throw away the dentry at each step.

This is an extremely hard race to hit, but it is possible. Adding a
udelay() in between the reads of ->d_parent and its ->d_inode makes it
reproducible on a no-journal filesystem using the following program:

#include <fcntl.h>
#include <unistd.h>

int main()
{
if (fork()) {
for (;;) {
mkdir("dir1", 0700);
int fd = open("dir1/file", O_RDWR|O_CREAT|O_SYNC);
write(fd, "X", 1);
close(fd);
}
} else {
mkdir("dir2", 0700);
for (;;) {
rename("dir1/file", "dir2/file");
rmdir("dir1");
}
}
}

Fixes: d59729f4e794 ("ext4: fix races in ext4_sync_parent()")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200506183140.541194-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/fsync.c | 28 +++++++++++++---------------
1 file changed, 13 insertions(+), 15 deletions(-)

--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -44,30 +44,28 @@
*/
static int ext4_sync_parent(struct inode *inode)
{
- struct dentry *dentry = NULL;
- struct inode *next;
+ struct dentry *dentry, *next;
int ret = 0;

if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
return 0;
- inode = igrab(inode);
+ dentry = d_find_any_alias(inode);
+ if (!dentry)
+ return 0;
while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
- dentry = d_find_any_alias(inode);
- if (!dentry)
- break;
- next = igrab(d_inode(dentry->d_parent));
+
+ next = dget_parent(dentry);
dput(dentry);
- if (!next)
- break;
- iput(inode);
- inode = next;
+ dentry = next;
+ inode = dentry->d_inode;
+
/*
* The directory inode may have gone through rmdir by now. But
* the inode itself and its blocks are still allocated (we hold
- * a reference to the inode so it didn't go through
- * ext4_evict_inode()) and so we are safe to flush metadata
- * blocks and the inode.
+ * a reference to the inode via its dentry), so it didn't go
+ * through ext4_evict_inode()) and so we are safe to flush
+ * metadata blocks and the inode.
*/
ret = sync_mapping_buffers(inode->i_mapping);
if (ret)
@@ -76,7 +74,7 @@ static int ext4_sync_parent(struct inode
if (ret)
break;
}
- iput(inode);
+ dput(dentry);
return ret;
}