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SubjectRe: [PATCH] mm: Fix possible off-by-one in walk_pte_range()
Johannes Weiner writes:
> After the loop in walk_pte_range() pte might point to the first address
> after the pmd it walks. The pte_unmap() is then applied to something
> bad.
>
> Spotted by Roel Kluin and Andreas Schwab.
>
> Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
> CC: Roel Kluin <12o3l@tiscali.nl>
> CC: Andreas Schwab <schwab@suse.de>
> CC: Matt Mackall <mpm@selenic.com>
> CC: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> A bug is unlikely, though. kunmap_atomic() looks up the kmap entry by
> map-type instead of the address the pte points. So the worst thing I
> could find with a quick grep was that a wrong TLB entry is being
> flushed. Still, the code is wrong :)
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 1cf1417..cf3c004 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -13,7 +13,7 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> err = walk->pte_entry(pte, addr, addr + PAGE_SIZE, private);
> if (err)
> break;
> - } while (pte++, addr += PAGE_SIZE, addr != end);
> + } while (addr += PAGE_SIZE, addr != end && pte++);

Instead of obfuscating the code by putting "&& pte++" in the
condition (it will always be true in valid C), you should IMO
rewrite the do-while as a for loop + break, like this:

for (;;) {
// same body as before
addr += PAGE_SIZE;
if (addr == end)
break;
pte++;
}

This makes the ordering constraints very clear.


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