Re: [PATCH] Validate pointer when copying mount namespace.

From: Al Viro
Date: Tue Apr 28 2015 - 22:36:24 EST

On Wed, Apr 29, 2015 at 09:56:43AM +0800, Leon Ma wrote:
> We encountered following panic. Validate the pointer to avoid this.

> @@ -2788,7 +2788,7 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
> q = next_mnt(q, new);
> if (!q)
> break;
> - while (p->mnt.mnt_root != q->mnt.mnt_root)
> + while (p && p->mnt.mnt_root != q->mnt.mnt_root)
> p = next_mnt(p, old);
> }
> namespace_unlock();

Details, please. How do you reproduce that behaviour?

I don't like that loop in its current form (it relies upon _not_ encountering
the same ->mnt_root in the parts of tree we hadn't copied), but your change
doesn't make it any better. Seeing a reproducer would be useful in sorting
it out; in this form the patch papers over the bug rather than fixing it.
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