** Changed in: linux (Ubuntu)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1824735
Title:
shiftfs: use after free when checking mount options
Status in linux package in Ubuntu:
Fix Committed
Bug description:
SRU Justification
Impact: We currently keep a reference to the shiftfs mark mount's
shiftfs_super_info which was stashed in the superblock of the mark mount. The
problem is that we only take a reference to the mount of the underlay, i.e. the
filesystem that is *under* the shiftfs mark mount. This means when someone
performs a shiftfs mark mount, then a shiftfs overlay mount and then
immediately unmounts the shiftfs mark mount we muck with invalid memory since
shiftfs_put_super might have already been called freeing that memory.
Fix: Copy up the passthrough mount settings of the mark mount point to the
shiftfs overlay.
An alternative solution would be to start reference counting. But this is
overkill. We only care about the passthrough mount option of the mark mount.
And we only need it to verify that on remount the new passthrough options of
the shiftfs overlay are a subset of the mark mount's passthrough options. In
other scenarios we don't care. So copying up is good enough and also only needs
to happen once on mount, i.e. when a new superblock is created and the
.fill_super method is called.
Regression Potential: Limited to shiftfs, matches the behavior of
other stacked filesystems, and has been tested (see below).
Test Case: Built Ubuntu Disco Kernel with patch applied from source,
installed it, ran LXD and verified that passthrough mount option now
works correctly.
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