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

[Meng Xu <mengxu.gatech@gmail.com>]

Hi Jerry,

Thank you for the question. Yes, these double copies
do seem to present an issue.

__nd_ioctl() and acpi_nfit_ctl() both use the same way
to derive `out_size`, but based on different data fetches.

A simple patch would be
memcmp(buf, in_env, in_len)
memcmp(buf + in_len, out_env, out_len)

I am not sure I captured all the subtle issues with such a
patch so please allow me some time to create and test it.

Best regards,
Meng

On 09/12/2017 06:03 PM, Jerry Hoemann wrote:
> 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@gatech.edu> wrote:
>>> From: Meng Xu <mengxu.gatech@gmail.com>
>>>
>>> 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@gmail.com>
>>> ---
>>>   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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