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Make sure nd->path.mnt and nd->path.dentry are always valid pointers
Initialize them in set_nameidata() and make sure that terminate_walk() clears them
once the pointers become potentially invalid (i.e. we leave RCU mode or drop them
in non-RCU one). Currently we have "path_init() always initializes them and nobody
accesses them outside of path_init()/terminate_walk() segments", which is asking
for trouble.
With that change we would have nd->path.{mnt,dentry}
1) always valid - NULL or pointing to currently allocated objects.
2) non-NULL while we are successfully walking
3) NULL when we are not walking at all
4) contributing to refcounts whenever non-NULL outside of RCU mode.
Fixes: 6c6ec2b0a3
("fs: add support for LOOKUP_CACHED")
Reported-by: syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com
Tested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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@ -579,6 +579,8 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
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p->stack = p->internal;
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p->dfd = dfd;
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p->name = name;
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p->path.mnt = NULL;
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p->path.dentry = NULL;
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p->total_link_count = old ? old->total_link_count : 0;
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p->saved = old;
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current->nameidata = p;
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@ -652,6 +654,8 @@ static void terminate_walk(struct nameidata *nd)
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rcu_read_unlock();
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}
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nd->depth = 0;
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nd->path.mnt = NULL;
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nd->path.dentry = NULL;
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}
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/* path_put is needed afterwards regardless of success or failure */
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@ -2322,8 +2326,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
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}
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nd->root.mnt = NULL;
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nd->path.mnt = NULL;
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nd->path.dentry = NULL;
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/* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
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if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {
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