linux-kernel - Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140826064952.GR25918@moon>
Date:	Tue, 26 Aug 2014 10:49:52 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Peter Feiner <pfeiner@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Pavel Emelyanov <xemul@...allels.com>,
	Jamie Liu <jamieliu@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs
 after VM_SOFTDIRTY cleared

On Mon, Aug 25, 2014 at 09:45:34PM -0700, Hugh Dickins wrote:
> > +static int clear_refs(struct mm_struct *mm, enum clear_refs_types type,
> > +                      int write)
> > +{
...
> > +
> > +	if (write)
> > +		down_write(&mm->mmap_sem);
> > +	else
> > +		down_read(&mm->mmap_sem);
> > +
> > +	if (type == CLEAR_REFS_SOFT_DIRTY)
> > +		mmu_notifier_invalidate_range_start(mm, 0, -1);
> > +
> > +	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> > +		cp.vma = vma;
> > +		if (is_vm_hugetlb_page(vma))
> > +			continue;
...
> > +		if (type == CLEAR_REFS_ANON && vma->vm_file)
> > +			continue;
> > +		if (type == CLEAR_REFS_MAPPED && !vma->vm_file)
> > +			continue;
> > +		if (type == CLEAR_REFS_SOFT_DIRTY &&
> > +		    (vma->vm_flags & VM_SOFTDIRTY)) {
> > +			if (!write) {
> > +				r = -EAGAIN;
> > +				break;
> 
> Hmm.  For a long time I thought you were fixing another important bug
> with down_write, since we "always" use down_write to modify vm_flags.
> 
> But now I'm realizing that if this is the _only_ place which modifies
> vm_flags with down_read, then it's "probably" safe.  I've a vague
> feeling that this was discussed before - is that so, Cyrill?

Well, as far as I remember we were not talking before about vm_flags
and read-lock in this function, maybe it was on some unrelated lkml thread
without me CC'ed? Until I miss something obvious using read-lock here
for vm_flags modification should be safe, since the only thing which is
important (in context of vma-softdirty) is the vma's presence. Hugh,
mind to refresh my memory, how long ago the discussion took place?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/