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Subject: Re: [PATCH] mm/shmem.c: don't set 'seals' to 'F_SEAL_SEAL' in
 shmem_get_inode
To:     Hugh Dickins <hughd@google.com>
CC:     <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <yi.zhang@huawei.com>, <zhengbin13@huawei.com>
References: <20191127040051.39169-1-yukuai3@huawei.com>
 <alpine.LSU.2.11.1911262008330.2204@eggly.anvils>
From:   "yukuai (C)" <yukuai3@huawei.com>
Message-ID: <f4ffdbc2-6728-1838-1a82-c0de4c484a54@huawei.com>
Date:   Wed, 27 Nov 2019 14:46:55 +0800
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On 2019/11/27 12:24, Hugh Dickins Wrote:
> On Wed, 27 Nov 2019, yu kuai wrote:
> 
>> 'seals' is set to 'F_SEAL_SEAL' in shmem_get_inode, which means "prevent
>> further seals from being set", thus sealing API will be useless and many
>> code in shmem.c will never be reached. For example:
> 
> The sealing API is not useless, and that code can be reached.
> 
>>
>> shmem_setattr
>> 	if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
>> 	    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
>> 		return -EPERM;
>>
>> So, initialize 'seals' to zero is more reasonable.
>>
>> Signed-off-by: yu kuai <yukuai3@huawei.com>
> 
> NAK.
> 
> See memfd_create in mm/memfd.c (code which originated in mm/shmem.c,
> then was extended to support hugetlbfs also): sealing is for memfds,
> not for tmpfs or hugetlbfs files or SHM.  Without thinking about it too
> hard, I believe that to allow sealing on tmpfs files would introduce
> surprising new behaviors on them, which might well raise security issues;
> and also be incompatible with the guarantees intended by sealing.

Thank you for your response.
Yu Kuai