From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794600059; b=WC25/w89FLBa3H8wVLIpNSQ1AziNZZvhMXAa0AhUzYcV+ARvpUWWkq0hK9CYbAWgLV/b zaZOZTtalAH7QbHH0g2T2wnn0yWufmCrkgyfPsCi2aQWbMuc3I5o4IM1Rfi8+T4P4BXbp GnhuZ6kvpaluH+Kk6QkA23pKAxFHaQxV6LT9reeHWIhxDPFNaSKrpnciGOkA1Z6hpE9+b xB0UGfwLRl4rBelJYNT+xqhCtvSpwFRBjoL9ODhs5grC/Gxsa9bZzAgsfsP9+CoerqNdn mt1S1dejdqYAIp8FYhoWP1M4uy8aXjkB4yrzNtP5Ck1T8gdnqr6btaTaVYUmzzrDzZw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794600059; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=cVUaiMaz3XMN9uwKkKZrpHW2DyS8HoLv5GhhqXeNd3c=; b=HpA7dG5qM3s5sA9gayaROX2UKpq8TZ77uPpsgBJy/3d69EDWdXH0ODfYkxrHrBAD4coh q6qzPL3QPLkNPq56qwSz3sNtKuV5Agjm6X3MohOqrus+OGNIEMoZw+YxJEEmvjimvN9KR c7f8I1GOVnnTiOMsQrYwFZjh26giPcC5yyx9p7IuSZ5WiCDOvyMQOs6UW8QfnPhP2+yxy MW/Wt+1lvpmK0uHA24v9ZtT3PJA9SkFoKYsfVvk0K4J2Vk/yBZLKTm7WxtXFZXupuZAjl L6MR9u+1LZap2ccuT8SwcnHIT2eZKumOmN8azCPpvftbdn3M/+d67x/afkjHpq9V5jA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OyUCD7Uy; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9D789606A89F; Fri, 13 Nov 2026 20:00:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OyUCD7Uy Received: by smtp.kernel.org (Postfix) with ESMTPSA id E49AC1F000C4; Fri, 13 Nov 2026 20:00:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794600059; bh=cVUaiMaz3XMN9uwKkKZrpHW2DyS8HoLv5GhhqXeNd3c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=OyUCD7UyGbgQkY37EnEigD1L/mrS7PBtAgr+4NmGTZUelqoLujjnkQSl5X8ARX9MI +N/R6NXrTN7PcrCbmG/pn27YFyv29KKY0XQoMMAUfuHO8AlBL7bBkgke2RbJvLRPau gqlZKfWqdyCRLwVmQBvbVLOkIeDne2b24kKHimyfVWs7mXPP0YasHlDFJjkbRUClJ0 B/2YPA2lbPXMYhcPkl6S0YHYVWhH4pYxJz/ZIFporIOHnC79l8MpxhfGy+VNxSydkm o5/gV4LFTaPbY5q4iab0I9lC5gCZrC/A7fMlWILAEbgb7ZUXgDiS+PGC1UtJqmTk9S k8m6JJ8gBDoYA== Message-ID: <6630ec25-3648-4d60-beb7-61572053318b@kernel.org> Date: Fri, 13 Nov 2026 21:00:42 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/9] mm: limit pagecache folio refcount when mapping pages To: Matthew Wilcox Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jan Kara , Andrew Morton , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Miklos Szeredi , Alexander Viro , Christian Brauner , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Arnd Bergmann , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard , "Jose A. Perez de Azpillaga" , Josef Bacik , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Neal Cardwell , Kuniyuki Iwashima , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, linux-arch@vger.kernel.org, iommu@lists.linux.dev, netdev@vger.kernel.org, Chris Mason , stable@vger.kernel.org, Pedro Falcato References: <20261112-refcount_overflow_stable-v4-0-ad3deef0bd23@kernel.org> <20261112-refcount_overflow_stable-v4-1-ad3deef0bd23@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/13/26 20:45, Matthew Wilcox wrote: > On Thu, Nov 12, 2026 at 11:53:20PM +0100, David Hildenbrand (Arm) wrote: >> Being able to overflow the refcount is not really big news, but it's about >> time that we do something about it because some modern technology might >> find ways to abuse it :) Hi Willy, thanks for the review! Be aware that I want to get this fixed now asap. So I will push back against any bigger changes on this for now unless there are real issues to fix. I want to get this sent early next week to Linus. With future changes I am planning, mappings will no longer be an issue. I will then drop admin reserves (as the chance for DOS will go down drastically) and we can use the negative refcount as our limit. > > Yes. My primary worry is actually it finding a path where we forget to > put the refcount on a folio and that gets exploited into a UAF. I'm not > sure the "reserve for root" aspect really helps recover from that, but > I'm not too opposed to it. > >> As a short-term solution, let's introduce two folio refcount limits: >> one for ordinary consumers and one for admins. Leave some headroom >> before the refcount goes negative, which we really don't want (having >> the mapcount go negative is also not expected). > > I don't think a negative refcount or mapcount is really a problem. What > matters is that we don't get to zero from below (for the refcount) > or that we don't get to 0xf000'0000 (for mapcount). They are a problems in current code with both negative refcount and negative mapcounts. Nothing I want to sort out in this patch set. So if we reach a negative mapcount of refcount today, we messed up. > >> +/* Leave separate reserves for administrators and concurrent reference gains. */ >> +#define FOLIO_REF_COUNT_LIMIT (INT_MAX - (1U << 26)) >> +#define FOLIO_REF_COUNT_ADMIN_LIMIT (INT_MAX - (1U << 25)) > > I think these specific values will generate bad code. Again, we can worry about fine tuning once we have a proper fix in place. > >> +static inline bool folio_ref_count_over_limit(const struct folio *folio) >> +{ >> + const unsigned int ref_count = folio_ref_count(folio); >> + >> + if (likely(ref_count < FOLIO_REF_COUNT_LIMIT)) >> + return false; > > I would replace this with a simple signed comparison against 0. As per above, this is something for the future. > >> static inline void folio_get(struct folio *folio) >> { >> - VM_BUG_ON_FOLIO(folio_ref_zero_or_close_to_overflow(folio), folio); >> + VM_WARN_ON_FOLIO(folio_ref_count(folio) <= 0, folio); >> folio_ref_inc(folio); >> } > > And I would drop this hunk. Similarly, for the futue. > >> diff --git a/mm/util.c b/mm/util.c >> index 80f6c714844f2..c9c3dc4b490fa 100644 >> --- a/mm/util.c >> +++ b/mm/util.c > > Now we have mm/folio.c, I think it should live in there. I can move it. > >> @@ -791,6 +791,32 @@ int folio_mc_copy(struct folio *dst, struct folio *src) >> } >> EXPORT_SYMBOL(folio_mc_copy); >> >> +bool folio_ref_count_over_limit_slowpath(const struct folio *folio, >> + unsigned int ref_count) >> +{ >> + if (ref_count >= FOLIO_REF_COUNT_ADMIN_LIMIT) >> + goto over_limit; > > Maybe let this go up to 0x8100'0000? > >> + /* Background threads must not consume admin reserves. */ >> + if (current->flags & PF_KTHREAD) >> + goto over_limit; >> + /* Neither must interrupt contexts on behalf of the interrupted task. */ >> + if (!in_task()) >> + goto over_limit; >> + /* Capability checks are not safe with IRQs disabled. */ >> + if (irqs_disabled()) >> + goto over_limit; >> + /* Use the task's credentials, which filesystem overrides cannot change. */ >> + if (has_capability_noaudit(current, CAP_SYS_RESOURCE)) >> + return false; >> + >> +over_limit: >> + pr_warn_ratelimited("folio:%p refcount:%d mapcount:%d mapping:%p reached refcount limit\n", >> + folio, (int)ref_count, folio_mapcount(folio), >> + folio_mapping(folio)); > > If it's worth doing folio->mapping, maybe also print folio->index? > In fact, I can do. > > pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n", > folio_ref_count(folio), mapcount, mapping, > folio->index + idx, pfn); > > might be worth pulling out into a mini helper from __dump_folio()? I want a pr_warn_ratelimited though. Again, thanks for the review, I want to get this fixed asap now. -- Cheers, David