[PATCH 38/46] mm/ksm: reset the unstable trees of all nodes

[sashiko-bot at kernel.org]

> Each full scan resets ksm_nr_node_ids unstable tree roots. Enabling
> merge_across_nodes resets this count to 1, even if per-node roots were
> previously allocated.
> 
> So when toggling "merge_across_nodes" 0 -> 1 -> 0, a scan that starts
> while the setting is 1 and continues after switching back to 0 can
> encounter stale unstable tree roots, resulting in UAF.
> 
> Only root is allowed to change "merge_across_nodes", and the documented
> sequence is: "merge_across_nodes setting can be changed only when there
> are no ksm shared pages in the system: set run 2 to unmerge pages first,
> then to 1 after changing merge_across_nodes, to remerge according to the
> new setting."
> 
> Let's fix it by tracking in ksm_nr_node_ids the actual number of
> allocated tree roots.
> 
> remove_all_stable_nodes() and ksm_check_stable_tree() will now scan
> more tree roots (skipping the empty ones), which is harmless.
> 
> Fixes: ef53d16cded7 ("ksm: allocate roots when needed")
> Cc: stable at vger.kernel.org
> Reported-by: Josef Bacik <josef at toxicpanda.com>
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org?part=38