From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 80AE7C98318 for ; Thu, 24 Sep 2026 15:29:34 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 533D96B0096; Thu, 24 Sep 2026 11:29:33 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 4EA596B0098; Thu, 24 Sep 2026 11:29:33 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 3FA826B0099; Thu, 24 Sep 2026 11:29:33 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 24BDB6B0096 for ; Thu, 24 Sep 2026 11:29:33 -0400 (EDT) Received: from smtpin03.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay09.hostedemail.com (Postfix) with ESMTP id B60C480384 for ; Thu, 24 Sep 2026 15:29:32 +0000 (UTC) X-FDA: 85249040184.03.AF35422 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf28.hostedemail.com (Postfix) with ESMTP id 1A4A8C0011 for ; Thu, 24 Sep 2026 15:29:30 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="gsOK/vtj"; spf=pass (imf28.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1790263771; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=4CDD8TQ1G8oa01QCVgmfSk+fkIBvjWyhYdUITJIOzi0=; b=kKS9ThXgwYms5CtE/l6qNMypt0FLA0rMbiiBxIjHBRdVUtDHL+7nHs59k+azSrgjItgGLR sL63MD43+ieFG8kpVegw+nMBB1Tbyjboi0MB4CvHNm/xuUrDXIqcnmM96cnVYAzitqJXNG XmtKRL8D46tarC/3qqCibSkpsbTCndw= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="gsOK/vtj"; spf=pass (imf28.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790263771; b=1PVAXDpKTSzkB4YRjIQx84APMVwvkqwB5vwTznk7YE0SYq/Y41JcUcqMM3kPlKmRe+hKtl Oo+yXTb1ldnCmoBd5iT60LDUJ1DqSiA3ljCC4hI1JUmcxIexFsGbA0jmFPIqqrh9oz5aNK gGh+z+leu6jwkzEUYDbKGnc1pTx5RPM= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 3C9B143F8C; Thu, 24 Sep 2026 15:29:30 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B46B1F000FF; Thu, 24 Sep 2026 15:29:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790263770; bh=4CDD8TQ1G8oa01QCVgmfSk+fkIBvjWyhYdUITJIOzi0=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gsOK/vtjVVEK91BGtbUMNOrj3Bo+ATVmhtSfHb7BmJN9Tc4DzT57CZaDbdG7mUqfm xnM8jAXmsVmO/rcuRvwTKO/TVzMCCyUEYUFsU+Usgwz3IOyfp13fpTaOFXjg9QY7fd FLqqp8Q3/3fms65qsdaX2pqAAuBBOHn3BUnnZhQlJ0QYAIQk6H3D9QeZvSamzpiPYj S7WOOuj3WuBcphT7nN9YALKjaUp8GROewIBaQLfpSxKfMrHzYluhPF73qGgdaZDWUa 8Z8K5rwKHAMq3Z7h4qox4wOuuSi4/b9SAi24rXJdlttIxcFHaQwtEYa+TA5xn6D2kn IzcR2ULRKF1Ig== Date: Thu, 24 Sep 2026 16:29:23 +0100 From: "Lorenzo Stoakes (ARM)" To: Greg Kroah-Hartman Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Arnd Bergmann , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com Subject: Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly Message-ID: References: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> <2026092406-irritably-maggot-9512@gregkh> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2026092406-irritably-maggot-9512@gregkh> X-Stat-Signature: ecrn8j4emfyfsossi6y9j1egrphgkxn6 X-Rspamd-Queue-Id: 1A4A8C0011 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790263770-220719 X-HE-Meta: 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 As1FzeoP NRW56G9BR1TdGvFH5QxMhqFmkHTPBj8s+aih9bDTCyHiGmA61Pu7txAkZuoEU0KqQYaNcGEXFAKsZeOP3k6u5TYuYa1FZP/t8DQnWKrloq5cmk4M05rJnOo8CiHx+PZ4COhVcBf6V1PMRV0x0ECCFrQJ4iB61Q81mFu0edbSh84lNEh1Knd5TJ8WaV22Xy2IBsUcd6sDToYZlir+2dHhyr/AfVdhEAzxz3eHVNKEru79+0mZtCIVtJ29KNgCB6df3RDxIoC78SKMbTVnatzRSSy4Vvx3qqGNm9oepLEmXZMaUHTK6TpW6hQuR/+Zdh8Whw0c0FOtudTaZFVQ2pdkLYz2Hk7hYieufwMX0SQyObr0Syxlvup0j5DCV7L4M1LUqY8AcCWX9+kvU1FujrD7l5lOcfeyZ5gABdRdqRyNe+ka1SU/sNLbDiU8a7IrMST9jsdsasP1kzVdbw8tv+hbcMD4THvL1Zi2hFGmvbjTJScRQTirP785JR6Hmzp0XwAyYMApLU7tGAnK5HyW1mcN694c26nLx5AKAF160 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 24, 2026 at 05:08:49PM +0200, Greg Kroah-Hartman wrote: > On Thu, Sep 24, 2026 at 03:48:24PM +0100, Lorenzo Stoakes (ARM) wrote: > > Rather surprisingly, opening /dev/zero read-only then mmap()'ing it > > MAP_SHARED gets you true anonymous memory (albeit in a VMA with > > non-NULL vma->vm_file). > > > > This is a by-product of MAP_PRIVATE-/dev/zero being how anonymous memory > > was mapped in Linux's distant past. > > > > It happens because mmap_zero_prepare() gates on VMA_SHARED_BIT and when > > mapping a read-only file MAP_SHARED, do_mmap() clears VMA_SHARED_BIT and > > VMA_MAYWRITE_BIT. > > > > The gating is incorrect - the (poorly named) VMA_MAYSHARE_BIT flag exists > > explicitly to tell you if something was originally mapped MAP_SHARED. > > > > So the fix is simple - gate on this instead. > > > > This isn't exactly a common use case, but it's unexpected behaviour which > > now causes an assert if CONFIG_DEBUG_VM is set. > > > > While this bug has existed since the dawn of time for linux (or at least > > since 2.6.12), it hasn't caused issues in the past, so while it's incorrect > > behaviour, it doesn't seem necessary to backport that far. > > > > The mapping is now accounted at mmap time and can fail with -ENOMEM under > > strict overcommit, and read faults allocate folios. However this is normal > > behaviour for a read-only shmem mapping. > > > > Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE > > file-backed anon folios") is the first patch at which the debug assert > > fires, so target that instead. > > > > Fixes: 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE file-backed anon folios") > > Reported-by: syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com > > Closes: https://lore.kernel.org/linux-mm/6ab4ae75.80e1c6cc.1e8e5f.000d.GAE@google.com/ > > Signed-off-by: Lorenzo Stoakes (ARM) > > --- > > drivers/char/mem.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/char/mem.c b/drivers/char/mem.c > > index 63253d1de5d7..5b93c92c2cf1 100644 > > --- a/drivers/char/mem.c > > +++ b/drivers/char/mem.c > > @@ -503,7 +503,7 @@ static int mmap_zero_prepare(struct vm_area_desc *desc) > > #ifndef CONFIG_MMU > > return -ENOSYS; > > #endif > > - if (vma_desc_test(desc, VMA_SHARED_BIT)) > > + if (vma_desc_test(desc, VMA_MAYSHARE_BIT)) > > return shmem_zero_setup_desc(desc); > > What's the odds that something "big" actually relies on this? I think it'd be fairly crazy to, unless people really care about read-faulting a lot of zero pages which they want to somehow not account properly. Anything's possible but I think on this, right to take a (v. small) risk! > > I'm all for the change, but be prepared in about 5 years for people to > start complaining :) > > Want me to take it through my tree? Sure, thanks, though there will be a conflict against a patch in mm. Though it's really that the file is moved (to mm/), so actually should be an easy resolution? > > thanks, > > greg k-h -- Cheers, Lorenzo