linux-kernel - Re: [PATCH 1/2] ras: fix an off-by-one error in __find

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Date:   Tue, 16 Apr 2019 16:28:33 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>, linux-edac@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] ras: fix an off-by-one error in __find_elem()

On Tue, Apr 16, 2019 at 04:18:57PM -0700, Cong Wang wrote:
> > The problem case occurs when we've seen enough distinct
> > errors that we have filled every entry, then we try to
> > look up a pfn that is larger that any seen before.
> >
> > The loop:
> >
> >         while (min < max) {
> >                 ...
> >         }
> >
> > will terminate with "min" set to MAX_ELEMS. Then we
> > execute:
> >
> >         this_pfn = PFN(ca->array[min]);
> >
> > which references beyond the end of the space allocated
> > for ca->array.
> 
> Exactly.

Hmmm. But can we ever really have this happen?  The call
sequence to get here looks like:


        mutex_lock(&ce_mutex);

        if (ca->n == MAX_ELEMS)
                WARN_ON(!del_lru_elem_unlocked(ca));

        ret = find_elem(ca, pfn, &to);

I.e. if the array was all the way full, we delete one element
before calling find_elem().  So when we get here:

static int __find_elem(struct ce_array *ca, u64 pfn, unsigned int *to)
{
        u64 this_pfn;
        int min = 0, max = ca->n;

The biggest value "max" can have is MAX_ELEMS-1


-Tony