mm/ksm: reset the unstable trees of all nodes

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@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-39-d6437171ecc3@kernel.org
Reviewed-by: Xu Xin <xu.xin@linux.dev>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

0 files changed, 0 insertions, 0 deletions