On Tue, Nov 28 2017 at 5:07am -0500,
Reshetova, Elena <elena.reshetova@xxxxxxxxx> wrote:
>
> > On Fri, Nov 24, 2017 at 2:36 AM, Reshetova, Elena
> > <elena.reshetova@xxxxxxxxx> wrote:
> > >> On Fri, Oct 20, 2017 at 10:37:38AM +0300, Elena Reshetova wrote:
> > >> > } else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> > >> > r = upgrade_mode(dd, mode, t->md);
> > >> > if (r)
> > >> > return r;
> > >> > + refcount_inc(&dd->count);
> > >> > }
> > >>
> > >> Missing here:
> > >>
> > >> else
> > >> refcount_inc(&dd->count);
> > >>
> > >> ?
> > >
> > > Oh, yes, thanks for catching this! I think this got unnoticed so far and patch was
> > merged, so I am going to send a followup patch now.
> >
> > I pushed this fix and will send to Linus next week:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-
> > dm.git/commit/?h=dm-4.15&id=d908af82d06cc420f9581c97c6db941cb87e4434
>
>
> I guess you mean this commit:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c2318d07ead871f058dda62e942ed7b6b1c1cfcf
>
> Unfortunately it is not correct:
>
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 88130b5..f6d32ee 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -451,15 +451,15 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> return r;
> }
>
> - refcount_set(&dd->count, 1);
> + refcount_set(&dd->count, 0);
> list_add(&dd->list, &t->devices);
>
> } else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> r = upgrade_mode(dd, mode, t->md);
> if (r)
> return r;
> - refcount_inc(&dd->count);
> }
> + refcount_inc(&dd->count);
>
> Problem will be here if you hit this refcount_inc() after the refcount_set(&dd->count, 0) earlier.
> refcount_inc() does not increment on zero value *ever* for security reasons and instead people
> should initialize refcounters to 1 always and do increments from there if needed.
include/linux/refcount.h:refcount_inc() definitely doesn't avoid
incrementing zero value.
Neither does lib/refcount.c:refcount_inc() but it does spew a WARN_ON by
assuming a zero value means use-after-free.
> This was the reason for the initial change I did, my mistake was just to forget to increment it also
> in case condition (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) fails.
>
> I have issues with my intel smpt server for sending patches (I will get it fixed tomorrow from internal network),
> so I am attaching the patch I did end of last week to this thread instead (or alternatively can properly send it tomorrow after fix).
> Sorry for the delay!
I was tempted to revert your original commits that switch DM code to
using refcount_t. Already proved more trouble than it is worth.
But I'll drop my commit and take your fix.
Mike
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