linux-kernel - Re: [PATCH v3 1/1] KVM: s390: fix race in gmap_make

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Date:   Fri, 28 Apr 2023 10:20:19 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, frankja@...ux.ibm.com,
        borntraeger@...ibm.com, nrb@...ux.ibm.com, nsg@...ux.ibm.com,
        seiden@...ux.ibm.com, hca@...ux.ibm.com
Subject: Re: [PATCH v3 1/1] KVM: s390: fix race in gmap_make_secure()

On Fri, Apr 28, 2023 at 11:27:53AM +0200, Claudio Imbrenda wrote:
> Fix a potential race in gmap_make_secure() and remove the last user of
> follow_page() without FOLL_GET.
> 
> The old code is locking something it doesn't have a reference to, and
> as explained by Jason and David in this discussion:
> https://lore.kernel.org/linux-mm/Y9J4P%2FRNvY1Ztn0Q@nvidia.com/
> it can lead to all kind of bad things, including the page getting
> unmapped (MADV_DONTNEED), freed, reallocated as a larger folio and the
> unlock_page() would target the wrong bit.
> There is also another race with the FOLL_WRITE, which could race
> between the follow_page() and the get_locked_pte().
> 
> The main point is to remove the last use of follow_page() without
> FOLL_GET or FOLL_PIN, removing the races can be considered a nice
> bonus.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Link: https://lore.kernel.org/linux-mm/Y9J4P%2FRNvY1Ztn0Q@nvidia.com/
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Fixes: 214d9bbcd3a6 ("s390/mm: provide memory management functions for protected KVM guests")
> ---
>  arch/s390/kernel/uv.c | 32 +++++++++++---------------------
>  1 file changed, 11 insertions(+), 21 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason