From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 76EB14A4EE4 for ; Thu, 24 Sep 2026 15:29:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790263771; cv=none; b=gH+yogb1VzcbTx1MyCIPVKJhMkgkXngQcwyJVGEnPQE9tZy9FH7bhbH2lvcg+tQ7KjdQ7Gn4qjDn6sG4YkYqySJfoHgFLEO2bPe2QRLkbSVE3X1a/W/tvI9Vxf2vqxKgzX7aj0mYBGuc+utfKpZytsUGn/G2SGVXudxvy6VRG4A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790263771; c=relaxed/simple; bh=Kfhzpb3IDath8WiZROoZA7Fe4Lrl6ZBBQ5kG/oDRVHQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BwzUSSw1vk0kyQ0Ol684Kd/HwAqJO7zd21fwD7RVfJcm4ekKbiqulnGHE7kd0hNG9/3y3SMKI+rFsTYiOFn/1UjsmzDX2g5ZoHUPMDKVzOn6Rwh9DJe9VIDiv6tEbdWiJDLUovaEuciBHAnjl0JcV5DaPg75KvOyseJNKGd+1Is= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gsOK/vtj; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gsOK/vtj" 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2026092406-irritably-maggot-9512@gregkh> 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