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configfs: fix a race in configfs_lookup() 

When configfs_lookup() is executing list_for_each_entry(),
it is possible that configfs_dir_lseek() is calling list_del().
Some unfortunate interleavings of them can cause a kernel NULL
pointer dereference error

Thread 1                  Thread 2
//configfs_dir_lseek()    //configfs_lookup()
list_del(&cursor->s_sibling);
                         list_for_each_entry(sd, ...)

Fix this by grabbing configfs_dirent_lock in configfs_lookup()
while iterating ->s_children.

Signed-off-by: Sishuai Gong <sishuai@purdue.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Sishuai Gong 2021-08-25 07:52:20 +02:00 committed by Christoph Hellwig
parent d07f132a22
commit c42dd069be
1 changed files with 4 additions and 3 deletions
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7
fs/configfs/dir.c
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@ -439,13 +439,13 @@ static struct dentry * configfs_lookup(struct inode *dir,
if (!configfs_dirent_is_ready(parent_sd))
return ERR_PTR(-ENOENT);
spin_lock(&configfs_dirent_lock);
list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
if ((sd->s_type & CONFIGFS_NOT_PINNED) &&
!strcmp(configfs_get_name(sd), dentry->d_name.name)) {
struct configfs_attribute *attr = sd->s_element;
umode_t mode = (attr->ca_mode & S_IALLUGO) | S_IFREG;
spin_lock(&configfs_dirent_lock);
dentry->d_fsdata = configfs_get(sd);
sd->s_dentry = dentry;
spin_unlock(&configfs_dirent_lock);
@ -462,10 +462,11 @@ static struct dentry * configfs_lookup(struct inode *dir,
inode->i_size = PAGE_SIZE;
inode->i_fop = &configfs_file_operations;
}
break;
goto done;
}
}
spin_unlock(&configfs_dirent_lock);
done:
d_add(dentry, inode);
return NULL;
}