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 B65A6409266 for ; Thu, 24 Sep 2026 15:51:37 +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=1790265099; cv=none; b=FqRon4XqqZZSNEzzvba1Pb6BZwFllvD9S8XI3qJUAfaDIcuw7HHY0UJVXIg6ZabpuL9labSFPE8Cnke3/rCwyI16K+I4DGJdqJFYRgIMVBoCQ0phmrXgf0DzxdODGLD1hYACSquMTB/ux9ITOz97rxk7kPYBYa2R1SgpF5GCVrw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790265099; c=relaxed/simple; bh=aaLN0Tp58ClV6eLTUeZnNxWyO00enm3cqXjj6dwZpyw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=h2NUlrKsGLlRXz+NICIcUqsaYo2uPbH8apVeFEhBHzz10BxGMX5vh1jmeP9+zsR7RJp4zhxgBu3VC8JwpxH9nsHDyPxouNQBq6tNHkyPjgi0NQwIwYk/mm+mHnejDff1a4zwohXwfuX/EKas7OJOdgTWaX/bTdxoScl/1Q6ebOI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=I2TLD0XV; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="I2TLD0XV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 585F11F00893; Thu, 24 Sep 2026 15:51:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790265097; bh=I1SyC9FPSl1SxWObKcKKHtxFd39BYxlX2zBFyTHCl90=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=I2TLD0XVn7SQ741gpQ1gNMfipLrznoSoXQo3p1M9Vtf6mj66Nzyg1wy4x3diSNy1K CbybYAHyfSMXesoua+5B+LM3eDBXvTTAEFljsDSs1x30/sIEfF3YQHLAE2MdJWI+EZ wciMGkvDJb3QgDaxmqm4QZN5l9aB6//t0WJowSWA= Date: Thu, 24 Sep 2026 17:42:08 +0200 From: Greg Kroah-Hartman To: "Lorenzo Stoakes (ARM)" 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: <2026092448-urgency-landless-f79c@gregkh> 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: On Thu, Sep 24, 2026 at 04:29:23PM +0100, Lorenzo Stoakes (ARM) wrote: > 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? No problem, you can take it if that makes it easier: Signed-off-by: Greg Kroah-Hartman