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mm/ksm: reset the unstable trees of all nodes

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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: ef53d16cde ("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>
This commit is contained in:
David Hildenbrand (Arm) 2026-10-31 10:40:08 +01:00
parent d03df42dda
commit 890e118e95
1 changed files with 3 additions and 3 deletions
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6
mm/ksm.c
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@ -475,6 +475,7 @@ static void advisor_stop_scan(void)
#ifdef CONFIG_NUMA
/* Zeroed when merging across nodes is not allowed */
static unsigned int ksm_merge_across_nodes = 1;
/* Number of allocated stable and unstable tree roots. */
static int ksm_nr_node_ids = 1;
#else
#define ksm_merge_across_nodes 1U
@ -3636,14 +3637,13 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
else {
root_stable_tree = buf;
root_unstable_tree = buf + nr_node_ids;
ksm_nr_node_ids = nr_node_ids;
/* Stable tree is empty but not the unstable */
root_unstable_tree[0] = one_unstable_tree[0];
}
}
if (!err) {
if (!err)
ksm_merge_across_nodes = knob;
ksm_nr_node_ids = knob ? 1 : nr_node_ids;
}
}
mutex_unlock(&ksm_thread_mutex);