Hi Jerry and Dan,
Sorry for the late reply. I looked at this issue again and found
that simple patches like memcmp(buf, in_env, in_len) &&
memcmp(buf + in_len, out_env, out_len) will only work
in the case of (cmd == ND_CMD_CALL) and does not apply
to other cmd.
In fact, I fail to find a patch that is small enough (i.e., within
20 lines of modifications) to fix this problem. This is largely
because there are too many factors that can affect the in_size
and out_size (i.e., the if-else branches in nd_cmd_in_size()
and nd_cmd_out_size()).
One option maybe is to split the loops for processing input
and output envelopes early, like this:
if (nvdimm && cmd == ND_CMD_SET_CONFIG_DATA) {
/* process an input envelope */
for (i = 0; i < desc->in_num; i++) {}
……
/* process an output envelope */
for (i = 0; i < desc->out_num; i++) {}
} else if (nvdimm && cmd == ND_CMD_VENDOR {
/* process an input envelope */
for (i = 0; i < desc->in_num; i++) {}
……
/* process an output envelope */
for (i = 0; i < desc->out_num; i++) {}
} else if (cmd == ND_CMD_CALL) {
/* process an input envelope */
for (i = 0; i < desc->in_num; i++) {}
……
/* process an output envelope */
for (i = 0; i < desc->out_num; i++) {}
}
But I guess this will require some major refactoring of the
code, which I am not sure is a good idea or is in my capability.
Please let me know your thoughts on this matter. Thanks.
Best Regards,
Meng
On Sep 12, 2017, at 6:49 PM, Meng Xu <mengxu.gatech(a)gmail.com>
wrote:
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(a)gatech.edu> wrote:
>>> From: Meng Xu <mengxu.gatech(a)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(a)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?
>
>