Do not clone unmounted child mounts in cloneMountTree. - #14420
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When a mount is unmounted, it may temporarily remain linked in its parent's `children` map pending deferred cleanup (e.g. during dead mount point removal via forgetDeadMountpoint or across user mount namespace boundaries). Previously, `cloneMountTree` traversed all mounts in `parent.children` without checking whether they had been marked unmounted (`c.umounted`). As a result, cloning an active parent mount could recursively clone and resurrect destroyed descendant mounts and dentries. In overlayfs, resurrecting a destroyed dentry bypassed its reference-counting, which could lead to use-after-free and premature release of the underlying MemoryFile while guest VMAs were still referencing it. This change fixes the issue by: - Skipping unmounted child mounts during `cloneMountTree`. - Ensuring `BindAt` rejects recursive bind-mounts when the source mount is not valid in a mount namespace (`!vfs.validInMountNS`). - Updated `overlay.dentry.IncRef` to panic if incrementing a destroyed dentry (`refs < 0`). - Added a test to ensure unmounted children are not cloned. PiperOrigin-RevId: 970865845
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Do not clone unmounted child mounts in cloneMountTree.
When a mount is unmounted, it may temporarily remain linked in its parent's
childrenmap pending deferred cleanup (e.g. during dead mount point removalvia forgetDeadMountpoint or across user mount namespace boundaries).
Previously,
cloneMountTreetraversed all mounts inparent.childrenwithoutchecking whether they had been marked unmounted (
c.umounted). As a result,cloning an active parent mount could recursively clone and resurrect destroyed
descendant mounts and dentries. In overlayfs, resurrecting a destroyed dentry
bypassed its reference-counting, which could lead to use-after-free and
premature release of the underlying MemoryFile while guest VMAs were still
referencing it.
This change fixes the issue by:
cloneMountTree.BindAtrejects recursive bind-mounts when the source mount is notvalid in a mount namespace (
!vfs.validInMountNS).overlay.dentry.IncRefto panic if incrementing a destroyed dentry(
refs < 0).