configfs: fix a deadlock in configfs_symlink()
Configfs abuses symlink(2). Unlike the normal filesystems, it wants the target resolved at symlink(2) time, like link(2) would've done. The problem is that ->symlink() is called with the parent directory locked exclusive, so resolving the target inside the ->symlink() is easily deadlocked. Short of really ugly games in sys_symlink() itself, all we can do is to unlock the parent before resolving the target and relock it after. However, that invalidates the checks done by the caller of ->symlink(), so we have to * check that dentry is still where it used to be (it couldn't have been moved, but it could've been unhashed) * recheck that it's still negative (somebody else might've successfully created a symlink with the same name while we were looking the target up) * recheck the permissions on the parent directory. Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
parent
f74c2bb987
commit
351e5d869e
|
@ -143,11 +143,42 @@ int configfs_symlink(struct inode *dir, struct dentry *dentry, const char *symna
|
|||
!type->ct_item_ops->allow_link)
|
||||
goto out_put;
|
||||
|
||||
/*
|
||||
* This is really sick. What they wanted was a hybrid of
|
||||
* link(2) and symlink(2) - they wanted the target resolved
|
||||
* at syscall time (as link(2) would've done), be a directory
|
||||
* (which link(2) would've refused to do) *AND* be a deep
|
||||
* fucking magic, making the target busy from rmdir POV.
|
||||
* symlink(2) is nothing of that sort, and the locking it
|
||||
* gets matches the normal symlink(2) semantics. Without
|
||||
* attempts to resolve the target (which might very well
|
||||
* not even exist yet) done prior to locking the parent
|
||||
* directory. This perversion, OTOH, needs to resolve
|
||||
* the target, which would lead to obvious deadlocks if
|
||||
* attempted with any directories locked.
|
||||
*
|
||||
* Unfortunately, that garbage is userland ABI and we should've
|
||||
* said "no" back in 2005. Too late now, so we get to
|
||||
* play very ugly games with locking.
|
||||
*
|
||||
* Try *ANYTHING* of that sort in new code, and you will
|
||||
* really regret it. Just ask yourself - what could a BOFH
|
||||
* do to me and do I want to find it out first-hand?
|
||||
*
|
||||
* AV, a thoroughly annoyed bastard.
|
||||
*/
|
||||
inode_unlock(dir);
|
||||
ret = get_target(symname, &path, &target_item, dentry->d_sb);
|
||||
inode_lock(dir);
|
||||
if (ret)
|
||||
goto out_put;
|
||||
|
||||
ret = type->ct_item_ops->allow_link(parent_item, target_item);
|
||||
if (dentry->d_inode || d_unhashed(dentry))
|
||||
ret = -EEXIST;
|
||||
else
|
||||
ret = inode_permission(dir, MAY_WRITE | MAY_EXEC);
|
||||
if (!ret)
|
||||
ret = type->ct_item_ops->allow_link(parent_item, target_item);
|
||||
if (!ret) {
|
||||
mutex_lock(&configfs_symlink_mutex);
|
||||
ret = create_link(parent_item, target_item, dentry);
|
||||
|
|
Loading…
Reference in New Issue