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

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

Hi Dan,

I have adjusted the patch as suggested by moving the check
on nd_reserved2 to acpi_nfit_ctl(). The new patch can be found
at https://marc.info/?l=linux-kernel&m=150453930712916&w=2

Best Regards,
Meng

On 08/31/2017 06:42 PM, 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.

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