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AJvYcCViMjMNKsG2odPQGNNecDWCHpOg/LnSE3hYSs8KUFqiJ3u5dJ1/UEXse+D0BXSgSHo+vQsLx9t8Xw==@kvack.org X-Gm-Message-State: AOJu0YyvCEXWQoKCVq+8lRWAZR9ToT3nXQp2zmxDOb0XMuilxvUj6/J0 24D4RBc46GmUG299C6unr/bmB3uB0sPUwro/SUeG3Pa8f1XN2QDOU5qBN1lErk1R2K5eyh4ED7n j0ydKPb5Nl+kwtnXcfYRwxfkA9Ks1fdicRFDa X-Gm-Gg: ASbGncs8d2cJQOesN4uMtNdDGDNeuVe90y3qZfeYDaaPdCwnaAyeKED4HyXBHXei9ZA 5mKTM42TK1YrAp77mGpYLnsdvRFn6/IaRlmAS+yXEfv+uIj6dzv/KZ2JO+cE= X-Google-Smtp-Source: AGHT+IHrO5qfRHSlmQCwNzrJObJ8F+5vL3isZu6sEndbtNy8+TOZx/SUWI/RHP6UiYngvZBnewRWzZBX4xLAvWC6WJw= X-Received: by 2002:a05:6402:1351:b0:5cf:bd9a:41ec with SMTP id 4fb4d7f45d1cf-5cfdec244d3mr70875a12.2.1732020757739; Tue, 19 Nov 2024 04:52:37 -0800 (PST) MIME-Version: 1.0 References: <20241116175922.3265872-1-pasha.tatashin@soleen.com> In-Reply-To: From: Jann Horn Date: Tue, 19 Nov 2024 13:52:00 +0100 Message-ID: Subject: Re: [RFCv1 0/6] Page Detective To: Pasha Tatashin Cc: Lorenzo Stoakes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, akpm@linux-foundation.org, corbet@lwn.net, derek.kiernan@amd.com, dragan.cvetic@amd.com, arnd@arndb.de, gregkh@linuxfoundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, tj@kernel.org, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, Liam.Howlett@oracle.com, vbabka@suse.cz, shuah@kernel.org, vegard.nossum@oracle.com, vattunuru@marvell.com, schalla@marvell.com, david@redhat.com, willy@infradead.org, osalvador@suse.de, usama.anjum@collabora.com, andrii@kernel.org, ryan.roberts@arm.com, peterx@redhat.com, oleg@redhat.com, tandersen@netflix.com, rientjes@google.com, gthelen@google.com, linux-hardening@vger.kernel.org, Kernel Hardening Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: 1944fxgm139sq6na67noxmsefzu94f5r X-Rspam-User: X-Rspamd-Queue-Id: EB94C100008 X-Rspamd-Server: rspam02 X-HE-Tag: 1732020703-810771 X-HE-Meta: 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 zTao2ht5 hmGmhw3mqVI1Zfqxg+k7328iIxckTmD50eC9temuu0KfVNE6B54VhDbnmC6+78Pa2rVhH0vzDQKY5CR9lqiI7mYp3oHfYkxu9P07u5QgkbOX75zMAkqO0QAullpH3xQHjz7MSteRzWwD1IRDIagCCcx+/Lc1DzqE754aWOGe14mCmO19R8LeaCSb734xMr2cTpA0XQMR/4JHaMp4fphS5gjbzfbb97Lo1qoOlPZAv2sPD6KGOXe8bSsEWQnsAmHe7dznB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000024, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Nov 19, 2024 at 2:30=E2=80=AFAM Pasha Tatashin wrote: > > Can you point me to where a refcounted reference to the page comes > > from when page_detective_metadata() calls dump_page_lvl()? > > I am sorry, I remembered incorrectly, we are getting reference right > after dump_page_lvl() in page_detective_memcg() -> folio_try_get(); I > will move the folio_try_get() to before dump_page_lvl(). > > > > > So I think dump_page() in its current form is not something we shou= ld > > > > expose to a userspace-reachable API. > > > > > > We use dump_page() all over WARN_ONs in MM code where pages might not > > > be locked, but this is a good point, that while even the existing > > > usage might be racy, providing a user-reachable API potentially makes > > > it worse. I will see if I could add some locking before dump_page(), > > > or make a dump_page variant that does not do dump_mapping(). > > > > To be clear, I am not that strongly opposed to racily reading data > > such that the data may not be internally consistent or such; but this > > is a case of racy use-after-free reads that might end up dumping > > entirely unrelated memory contents into dmesg. I think we should > > properly protect against that in an API that userspace can invoke. > > Otherwise, if we race, we might end up writing random memory contents > > into dmesg; and if we are particularly unlucky, those random memory > > contents could be PII or authentication tokens or such. > > > > I'm not entirely sure what the right approach is here; I guess it > > makes sense that when the kernel internally detects corruption, > > dump_page doesn't take references on pages it accesses to avoid > > corrupting things further. If you are looking at a page based on a > > userspace request, I guess you could access the page with the > > necessary locking to access its properties under the normal locking > > rules? > > I will take reference, as we already do that for memcg purpose, but > have not included dump_page(). Note that taking a reference on the page does not make all of dump_page() fine; in particular, my understanding is that folio_mapping() requires that the page is locked in order to return a stable pointer, and some of the code in dump_mapping() would probably also require some other locks - probably at least on the inode and maybe also on the dentry, I think? Otherwise the inode's dentry list can probably change concurrently, and the dentry's name pointer can change too.