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ext4: fix race between ext4_sync_parent() and rename()

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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: d59729f4e7 ("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>
This commit is contained in:
Eric Biggers 2020-05-06 11:31:40 -07:00 committed by Greg Kroah-Hartman
parent aab1eab04a
commit 8f3f5ba25e
1 changed files with 13 additions and 15 deletions
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28
fs/ext4/fsync.c
View file

@ -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 *inode)
if (ret)
break;
}
iput(inode);
dput(dentry);
return ret;
}