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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Randy Dunlap , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, stable@vger.kernel.org, Chris Mason Subject: Re: [PATCH 1/2] mm/memfd: handle GUP pins more reliably when write-sealing Message-ID: References: <20261103-fixes_core_2-v1-0-f332954f1df4@kernel.org> <20261103-fixes_core_2-v1-1-f332954f1df4@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261103-fixes_core_2-v1-1-f332954f1df4@kernel.org> X-Stat-Signature: 8dnaoji5mqxgtu4gx67ctafeubyz38bm X-Rspam-User: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 6951F100009 X-HE-Tag: 1793783714-989806 X-HE-Meta: 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 eEOW/hO1 1yeQsDW4+0FOuEj7WKcsaFF5XYVZlQC20lEGDnJFqVVDwiDIM9RlhLLK2tD9R2SArV+AyKHJdJ73gWuHt4neKx1O/KHcD68wW5KTF6kdBesMW+mIQlrZ0cNJ/x0AeZNOI5FTxNnx0UpuB/gZutwwY4Ym3iNX5MWT5u9awghudmFII3ot6q0jm3iSrUIwPF/TDLopy4/oHIjg+2uXR3GWUCYmRHcuRVzw+mbuHDpR4lGCYKnOydw6bk55I+tezF4ow2av0QqNipVoZ+/WaglseW6nKBQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Nov 03, 2026 at 07:31:44PM +0100, David Hildenbrand (Arm) wrote: > memfd_wait_for_pins() detects GUP references by comparing a folio's > actual refcount with its expected refcount. As spelled out by > folio_expected_ref_count(), obtaining the expected refcount when there > can be concurrent folio (un)mapping is racy. > > So it is possible that a changing folio mapcount can race with our > check for GUP references, and F_SEAL_WRITE might succeed even if there > are some, allowing file contents to be modified after sealing. > > For folios with a reliable pincount, detect FOLL_PIN references using > folio_maybe_dma_pinned() before falling back to the refcount comparison. > For small folios without that, the 1024 pinning BIAS makes it very hard > (although not impossible) to misguide the refcount comparison. > > This improves FOLL_PIN detection for folios with a pincount. Handling > FOLL_GET and small folios requires a more fundamental change. > > Fortunately, most interfaces that allow for GUP'ing such pages use > FOLL_PIN, in particular interfaces using FOLL_LONGTERM. However, we > still have some FOLL_GET leftovers ... > > Fixes: 05f65b5c7090 ("shm: wait for pins to be released when sealing") > Cc: stable@vger.kernel.org > Cc: Chris Mason > Signed-off-by: David Hildenbrand (Arm) All very delicate stuff, I wonder if we could somehow make the logic here generalised in a follow-up? Anyway seems reasonable to me so: Reviewed-by: Lorenzo Stoakes (ARM) > --- > mm/memfd.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/memfd.c b/mm/memfd.c > index 0f6fff004f5e4..9eda4c3eeaca5 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -32,6 +32,13 @@ > > static bool memfd_folio_has_extra_refs(struct folio *folio) > { > + /* > + * Use the precise pincount where available to avoid racy refcount > + * comparisons. > + */ > + if (folio_has_pincount(folio) && folio_maybe_dma_pinned(folio)) > + return true; > + > return folio_ref_count(folio) != folio_expected_ref_count(folio); > } > > > -- > 2.43.0 > -- Cheers, Lorenzo