Re: [PATCH] nvdimm: fix potential double-fetch bug

[Jerry Hoemann]

On Thu, Aug 31, 2017 at 03:42:52PM -0700, Dan Williams wrote:
> [ adding Jerry ]
> 
> On Wed, Aug 23, 2017 at 2:07 PM, Meng Xu <meng.xu@xxxxxxxxxx> wrote:
> > From: Meng Xu <mengxu.gatech@xxxxxxxxx>
> >
> > While examining the kernel source code, I found a dangerous operation that
> > could turn into a double-fetch situation (a race condition bug) where
> > the same userspace memory region are fetched twice into kernel with sanity
> > checks after the first fetch while missing checks after the second fetch.
> >
> > In the case of _IOC_NR(ioctl_cmd) == ND_CMD_CALL:
> >
> > 1. The first fetch happens in line 935 copy_from_user(&pkg, p, sizeof(pkg)
> >
> > 2. subsequently `pkg.nd_reserved2` is asserted to be all zeroes
> > (line 984 to 986).
> >
> > 3. The second fetch happens in line 1022 copy_from_user(buf, p, buf_len)
> >
> > 4. Given that `p` can be fully controlled in userspace, an attacker can
> > race condition to override the header part of `p`, say,
> > `((struct nd_cmd_pkg *)p)->nd_reserved2` to arbitrary value
> > (say nine 0xFFFFFFFF for `nd_reserved2`) after the first fetch but before the
> > second fetch. The changed value will be copied to `buf`.
> >
> > 5. There is no checks on the second fetches until the use of it in
> > line 1034: nd_cmd_clear_to_send(nvdimm_bus, nvdimm, cmd, buf) and
> > line 1038: nd_desc->ndctl(nd_desc, nvdimm, cmd, buf, buf_len, &cmd_rc)
> > which means that the assumed relation, `p->nd_reserved2` are all zeroes might
> > not hold after the second fetch. And once the control goes to these functions
> > we lose the context to assert the assumed relation.
> >
> > 6. Based on my manual analysis, `p->nd_reserved2` is not used in function
> > `nd_cmd_clear_to_send` and potential implementations of `nd_desc->ndctl`
> > so there is no working exploit against it right now. However, this could
> > easily turns to an exploitable one if careless developers start to use
> > `p->nd_reserved2` later and assume that they are all zeroes.
> >
> > Proposed patch:
> >
> > The patch explicitly overrides `buf->nd_reserved2` after the second fetch with
> > the value `pkg.nd_reserved2` from the first fetch. In this way, it is assured
> > that the relation, `buf->nd_reserved2` are all zeroes, holds after the second
> > fetch.
> >
> > Signed-off-by: Meng Xu <mengxu.gatech@xxxxxxxxx>
> > ---
> >  drivers/nvdimm/bus.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c
> > index 937fafa..20c4d0f 100644
> > --- a/drivers/nvdimm/bus.c
> > +++ b/drivers/nvdimm/bus.c
> > @@ -1024,6 +1024,12 @@ static int __nd_ioctl(struct nvdimm_bus *nvdimm_bus, struct nvdimm *nvdimm,
> >                 goto out;
> >         }
> >
> > +       if (cmd == ND_CMD_CALL) {
> > +               struct nd_cmd_pkg *hdr = (struct nd_cmd_pkg *)buf;
> > +               memcpy(hdr->nd_reserved2, pkg.nd_reserved2,
> > +                               sizeof(pkg.nd_reserved2));
> > +       }
> > +
> 
> I think we're ok because the end point like acpi_nfit_ctl() is
> responsible for re-validating the buffer. So what I would rather like
> to see is deleting this loop:
> 
>                 for (i = 0; i < ARRAY_SIZE(pkg.nd_reserved2); i++)
>                         if (pkg.nd_reserved2[i])
>                                 return -EINVAL;
> 
> ...from __nd_ioctl() and move it into acpi_nfit_ctl() directly where it belongs.

Sorry for the delay, I've been away.

I'm okay with moving the test to the beginning of acpi_nfit_ctl.  If/When the reserved
fields are defined/used, we may need to tweak that.  But we can cross that
bridge when it comes.

However, I do have a question.

There are two for loops in __nd_ioctl that process desc->in_num and desc->out_num
respectively.  These loops also copy_from_user before

        buf = vmalloc(buf_len);
        if (!buf)
                return -ENOMEM;

        if (copy_from_user(buf, p, buf_len)) {
                rc = -EFAULT;
                goto out;
        }


Do these double copy instances present any problems?


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Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
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