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authorAl Viro <viro@zeniv.linux.org.uk>2019-08-03 18:51:18 +0300
committerChristoph Hellwig <hch@lst.de>2019-09-11 13:45:49 +0300
commit351e5d869e5ac10cb40c78b5f2d7dfc816ad4587 (patch)
treebbff139362c2d49c1bfff6a843c7e32cfd32902b /fs/configfs/symlink.c
parentf74c2bb98776e2de508f4d607cd519873065118e (diff)
downloadlinux-351e5d869e5ac10cb40c78b5f2d7dfc816ad4587.tar.xz
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>
Diffstat (limited to 'fs/configfs/symlink.c')
-rw-r--r--fs/configfs/symlink.c33
1 files changed, 32 insertions, 1 deletions
diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 91eac6c55e07..f3881e4caedd 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -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);