linux-kernel - Re: [PATCH] hugetlbfs: check for pgoff value overflow

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Date:   Thu, 8 Mar 2018 13:03:21 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Yisheng Xie <xieyisheng1@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org
Cc:     Michal Hocko <mhocko@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nic Losby <blurbdust@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlbfs: check for pgoff value overflow

On 03/07/2018 08:25 PM, Mike Kravetz wrote:
> On 03/07/2018 05:35 PM, Yisheng Xie wrote:
>> However, region_chg makes me a litter puzzle that when its return value < 0, sometime
>> adds_in_progress is added like this case, while sometime it is not. so why not just
>> change at the beginning of region_chg ?
>> 	if (f > t)
>> 		return -EINVAL;
> 
> If region_chg returns a value < 0, this indicates an error and adds_in_progress
> should not be incremented.  In the case of this bug, region_chg was passed
> values where f > t.  Of course, this should never happen.  But, because it
> assumed f <= t, it returned a negative count needed huge page reservations.
> The calling code interpreted the negative value as an error and a subsequent
> region_add or region_abort.
> 
> I am not opposed to adding the suggested "if (f > t)".  However, the
> region tracking routines are simple helpers only used by the hugetlbfs
> code and the assumption is that they are being called correctly.  As
> such, I would prefer to leave off the check.  But, this is the second
> time they have been called incorrectly due to insufficient argument
> checking.  If we do add this to region_chg, I would also add the check
> to all region_* routines for consistency.

I really did not want to add the (f > t) check to the region_* routines.
As mentioned we should never encounter this condition.  Adding the check
here says that we missed discovering an error at higher levels.  Therefore,
I went back and examined the callers of region_chg.  There are only 2:
hugetlb_reserve_pages and __vma_reservation_common.  hugetlb_reserve_pages
is called to set up a reservation for a mapping.  __vma_reservation_common
is called to check on an existing reservation, and only operates on a
single huge page.  With this in mind, a check in hugetlb_reserve_pages
would be sufficient.  Therefore, I added an explicit check to that routine
and printed a warning if ever encountered.

> I will send out a V2 of this patch tomorrow with the corrected overflow
> checking and possibly checks added to the region_* routines.

v2 will be sent shortly.  In v2 I Cc stable as this is an issue for
stable branches as well.

-- 
Mike Kravetz