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Date:   Sat, 29 Apr 2023 02:43:32 +0300
From:   "Kirill A . Shutemov" <kirill@shutemov.name>
To:     David Hildenbrand <david@redhat.com>
Cc:     Peter Xu <peterx@redhat.com>, Lorenzo Stoakes <lstoakes@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
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        Nelson Escobar <neescoba@cisco.com>,
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        Ingo Molnar <mingo@redhat.com>,
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        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
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        Namhyung Kim <namhyung@kernel.org>,
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        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
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        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
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        Mika Penttila <mpenttil@redhat.com>,
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        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
Message-ID: <20230428234332.2vhprztuotlqir4x@box.shutemov.name>
References: <077c4b21-8806-455f-be98-d7052a584259@lucifer.local>
 <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com>
 <6ddc7ac4-4091-632a-7b2c-df2005438ec4@redhat.com>
 <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name>
 <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com>
 <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name>
 <ZEv2196tk5yWvgW5@x1n>
 <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com>
 <20230428165623.pqchgi5gtfhxd5b5@box.shutemov.name>
 <1039c830-acec-d99b-b315-c2a6e26c34ca@redhat.com>
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On Fri, Apr 28, 2023 at 07:02:22PM +0200, David Hildenbrand wrote:
> On 28.04.23 18:56, Kirill A . Shutemov wrote:
> > On Fri, Apr 28, 2023 at 06:51:46PM +0200, David Hildenbrand wrote:
> > > On 28.04.23 18:39, Peter Xu wrote:
> > > > On Fri, Apr 28, 2023 at 07:22:07PM +0300, Kirill A . Shutemov wrote:
> > > > > On Fri, Apr 28, 2023 at 06:13:03PM +0200, David Hildenbrand wrote:
> > > > > > On 28.04.23 18:09, Kirill A . Shutemov wrote:
> > > > > > > On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote:
> > > > > > > > On 28.04.23 17:34, David Hildenbrand wrote:
> > > > > > > > > On 28.04.23 17:33, Lorenzo Stoakes wrote:
> > > > > > > > > > On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote:
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Security is the primary case where we have historically closed uAPI
> > > > > > > > > > > > > items.
> > > > > > > > > > > > 
> > > > > > > > > > > > As this patch
> > > > > > > > > > > > 
> > > > > > > > > > > > 1) Does not tackle GUP-fast
> > > > > > > > > > > > 2) Does not take care of !FOLL_LONGTERM
> > > > > > > > > > > > 
> > > > > > > > > > > > I am not convinced by the security argument in regard to this patch.
> > > > > > > > > > > > 
> > > > > > > > > > > > 
> > > > > > > > > > > > If we want to sells this as a security thing, we have to block it
> > > > > > > > > > > > *completely* and then CC stable.
> > > > > > > > > > > 
> > > > > > > > > > > Regarding GUP-fast, to fix the issue there as well, I guess we could do
> > > > > > > > > > > something similar as I did in gup_must_unshare():
> > > > > > > > > > > 
> > > > > > > > > > > If we're in GUP-fast (no VMA), and want to pin a !anon page writable,
> > > > > > > > > > > fallback to ordinary GUP. IOW, if we don't know, better be safe.
> > > > > > > > > > 
> > > > > > > > > > How do we determine it's non-anon in the first place? The check is on the
> > > > > > > > > > VMA. We could do it by following page tables down to folio and checking
> > > > > > > > > > folio->mapping for PAGE_MAPPING_ANON I suppose?
> > > > > > > > > 
> > > > > > > > > PageAnon(page) can be called from GUP-fast after grabbing a reference.
> > > > > > > > > See gup_must_unshare().
> > > > > > > > 
> > > > > > > > IIRC, PageHuge() can also be called from GUP-fast and could special-case
> > > > > > > > hugetlb eventually, as it's table while we hold a (temporary) reference.
> > > > > > > > Shmem might be not so easy ...
> > > > > > > 
> > > > > > > page->mapping->a_ops should be enough to whitelist whatever fs you want.
> > > > > > > 
> > > > > > 
> > > > > > The issue is how to stabilize that from GUP-fast, such that we can safely
> > > > > > dereference the mapping. Any idea?
> > > > > > 
> > > > > > At least for anon page I know that page->mapping only gets cleared when
> > > > > > freeing the page, and we don't dereference the mapping but only check a
> > > > > > single flag stored alongside the mapping. Therefore, PageAnon() is fine in
> > > > > > GUP-fast context.
> > > > > 
> > > > > What codepath you are worry about that clears ->mapping on pages with
> > > > > non-zero refcount?
> > > > > 
> > > > > I can only think of truncate (and punch hole). READ_ONCE(page->mapping)
> > > > > and fail GUP_fast if it is NULL should be fine, no?
> > > > > 
> > > > > I guess we should consider if the inode can be freed from under us and the
> > > > > mapping pointer becomes dangling. But I think we should be fine here too:
> > > > > VMA pins inode and VMA cannot go away from under GUP.
> > > > 
> > > > Can vma still go away if during a fast-gup?
> > > > 
> > > 
> > > So, after we grabbed the page and made sure the the PTE didn't change (IOW,
> > > the PTE was stable while we processed it), the page can get unmapped (but
> > > not freed, because we hold a reference) and the VMA can theoretically go
> > > away (and as far as I understand, nothing stops the file from getting
> > > deleted, truncated etc).
> > > 
> > > So we might be looking at folio->mapping and the VMA is no longer there.
> > > Maybe even the file is no longer there.
> > 
> > No. VMA cannot get away before PTEs are unmapped and TLB is flushed. And
> > TLB flushing is serialized against GUP_fast().
> 
> 
> The whole CONFIG_MMU_GATHER_RCU_TABLE_FREE handling makes the situation more
> complicated.

I think I found relevant snippet of code that solves similar issue.
get_futex_key() uses RCU to stabilize page->mapping after GUP_fast:


		/*
		 * The associated futex object in this case is the inode and
		 * the page->mapping must be traversed. Ordinarily this should
		 * be stabilised under page lock but it's not strictly
		 * necessary in this case as we just want to pin the inode, not
		 * update the radix tree or anything like that.
		 *
		 * The RCU read lock is taken as the inode is finally freed
		 * under RCU. If the mapping still matches expectations then the
		 * mapping->host can be safely accessed as being a valid inode.
		 */
		rcu_read_lock();

		if (READ_ONCE(page->mapping) != mapping) {
			rcu_read_unlock();
			put_page(page);

			goto again;
		}

		inode = READ_ONCE(mapping->host);
		if (!inode) {
			rcu_read_unlock();
			put_page(page);

			goto again;
		}

I think something similar can be used inside GUP_fast too.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov