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Subject: Re: [PATCH] xen/gntdev: fix gntdev_mmap() error exit path
To:     Luca Fancellu <luca.fancellu@arm.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org,
        =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>
References: <20210423054038.26696-1-jgross@suse.com>
 <467B8109-C829-4755-8398-196E50090898@arm.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <9cb9bd6c-8185-9741-31b9-8f6baf3848a3@suse.com>
Date:   Fri, 23 Apr 2021 09:00:26 +0200
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From: Juergen Gross <jgross@suse.com>
To: Luca Fancellu <luca.fancellu@arm.com>
Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
 Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org,
 =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= <marmarek@invisiblethingslab.com>
Message-ID: <9cb9bd6c-8185-9741-31b9-8f6baf3848a3@suse.com>
Subject: Re: [PATCH] xen/gntdev: fix gntdev_mmap() error exit path
References: <20210423054038.26696-1-jgross@suse.com>
 <467B8109-C829-4755-8398-196E50090898@arm.com>
In-Reply-To: <467B8109-C829-4755-8398-196E50090898@arm.com>

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On 23.04.21 08:55, Luca Fancellu wrote:
>=20
>=20
>> On 23 Apr 2021, at 06:40, Juergen Gross <jgross@suse.com> wrote:
>>
>> Commit d3eeb1d77c5d0af ("xen/gntdev: use mmu_interval_notifier_insert"=
)
>> introduced an error in gntdev_mmap(): in case the call of
>> mmu_interval_notifier_insert_locked() fails the exit path should not
>> call mmu_interval_notifier_remove(), as this might result in NULL
>> dereferences.
>>
>> One reason for failure is e.g. a signal pending for the running
>> process.
>>
>> Fixes: d3eeb1d77c5d0af ("xen/gntdev: use mmu_interval_notifier_insert"=
)
>> Cc: stable@vger.kernel.org
>> Reported-by: Marek Marczykowski-G=C3=B3recki <marmarek@invisiblethings=
lab.com>
>> Tested-by: Marek Marczykowski-G=C3=B3recki <marmarek@invisiblethingsla=
b.com>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> drivers/xen/gntdev.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>> index f01d58c7a042..a3e7be96527d 100644
>> --- a/drivers/xen/gntdev.c
>> +++ b/drivers/xen/gntdev.c
>> @@ -1017,8 +1017,10 @@ static int gntdev_mmap(struct file *flip, struc=
t vm_area_struct *vma)
>> 		err =3D mmu_interval_notifier_insert_locked(
>> 			&map->notifier, vma->vm_mm, vma->vm_start,
>> 			vma->vm_end - vma->vm_start, &gntdev_mmu_ops);
>> -		if (err)
>> +		if (err) {
>> +			map->vma =3D NULL;
>> 			goto out_unlock_put;
>> +		}
>> 	}
>> 	mutex_unlock(&priv->lock);
>>
>> --=20
>> 2.26.2
>>
>>
>=20
> Hi Juergen,
>=20
> I can see from the code that there is another path to out_unlock_put la=
bel some lines before:
>=20
>          [=E2=80=A6]
>          vma->vm_private_data =3D map;
> 	if (map->flags) {
> 		if ((vma->vm_flags & VM_WRITE) &&
> 				(map->flags & GNTMAP_readonly))
> 			goto out_unlock_put;
> 	} else {
> 		map->flags =3D GNTMAP_host_map;
> 		if (!(vma->vm_flags & VM_WRITE))
> 			map->flags |=3D GNTMAP_readonly;
> 	}
>          [=E2=80=A6]
>=20
> Can be the case that use_ptemod is !=3D 0 also for that path?

map->vma will be NULL in this case, so there will be no problem
resulting from that path.


Juergen

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