linux-kernel - Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

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Message-ID: <86d7cc82-8ff9-769b-f80f-ff18fe28f44d@amd.com>
Date:   Thu, 16 Feb 2023 15:21:21 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Arnd Bergmann <arnd@...db.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>, tabba@...gle.com,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        wei.w.wang@...el.com
Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to
 create restricted user memory


> +static struct file *restrictedmem_file_create(struct file *memfd)
> +{
> +	struct restrictedmem_data *data;
> +	struct address_space *mapping;
> +	struct inode *inode;
> +	struct file *file;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return ERR_PTR(-ENOMEM);
> +
> +	data->memfd = memfd;
> +	mutex_init(&data->lock);
> +	INIT_LIST_HEAD(&data->notifiers);
> +
> +	inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
> +	if (IS_ERR(inode)) {
> +		kfree(data);
> +		return ERR_CAST(inode);
> +	}

alloc_anon_inode() uses new_pseudo_inode() to get the inode. As per the comment, new inode 
is not added to the superblock s_inodes list.

/**
 *	new_inode_pseudo 	- obtain an inode
 *	@sb: superblock
 *
 *	Allocates a new inode for given superblock.
 *	Inode wont be chained in superblock s_inodes list
 *	This means :
 *	- fs can't be unmount
 *	- quotas, fsnotify, writeback can't work
 */

So the restrictedmem_error_page will not find the inode as it was never added to the s_inodes list.

We might need to add the inode after allocating.

	inode_sb_list_add(inode);

> +void restrictedmem_error_page(struct page *page, struct address_space *mapping)
> +{
> +	struct super_block *sb = restrictedmem_mnt->mnt_sb;
> +	struct inode *inode, *next;
> +
> +	if (!shmem_mapping(mapping))
> +		return;
> +
> +	spin_lock(&sb->s_inode_list_lock);
> +	list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
> +		struct restrictedmem_data *data = inode->i_mapping->private_data;
> +		struct file *memfd = data->memfd;
> +
> +		if (memfd->f_mapping == mapping) {
> +			pgoff_t start, end;
> +
> +			spin_unlock(&sb->s_inode_list_lock);
> +
> +			start = page->index;
> +			end = start + thp_nr_pages(page);
> +			restrictedmem_notifier_error(data, start, end);
> +			return;
> +		}
> +	}
> +	spin_unlock(&sb->s_inode_list_lock);
> +}

Regards
Nikunj