linux-kernel - A possible divide by zero bug in alloc_nodes

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Date:   Fri, 14 May 2021 19:31:24 +0800
From:   Yiyuan guo <yguoaz@...il.com>
To:     tglx@...utronix.de
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: A possible divide by zero bug in alloc_nodes_vectors

In kernel/irq/affinity.c, the function alloc_nodes_vectors has the
following code:

static void alloc_nodes_vectors(unsigned int numvecs,
                cpumask_var_t *node_to_cpumask,
                const struct cpumask *cpu_mask,
                const nodemask_t nodemsk,
                struct cpumask *nmsk,
                struct node_vectors *node_vectors) {
    unsigned n, remaining_ncpus = 0;
    ...
    for_each_node_mask(n, nodemsk) {
        ...
        ncpus = cpumask_weight(nmsk);

        if (!ncpus)
            continue;
        remaining_ncpus += ncpus;
        ...
    }

    numvecs = min_t(unsigned, remaining_ncpus, numvecs);
    ...
    for (n = 0; n < nr_node_ids; n++) {
        ...
        WARN_ON_ONCE(numvecs == 0);
        ...
        nvectors = max_t(unsigned, 1,
                       numvecs * ncpus / remaining_ncpus);
    }
}

The variable remaining_ncpus may remain 0 if cpumask_weight(nmsk)
keeps returning 0 in the for loop. However, remaining_ncpus is used as
a divisor, leading to a potential divide by zero problem.

Notice that the code explicitly warns about numvecs being zero. And
since it is likely that numvecs equals to remaining_ncpus (because of
assignment: numvecs = min_t(unsigned, remaining_ncpus, numvecs)),
we should probably also check on remaining_ncpus before the division.