Re: [PATCH v3 3/3] selftests/tdx: Test GetQuote TDX attestation feature

Sathyanarayanan Kuppuswamy
2023-06-28T15:55:49Z

Hi Samuel,

On 6/28/23 8:41 AM, Samuel Ortiz wrote:
> On Mon, Jun 26, 2023 at 05:39:12PM -0700, Sathyanarayanan Kuppuswamy wrote:
>> +Atish
>>
>> Atish, any comments on this topic from RISC-v?
>
> The CoVE (RISC-V confidential computing specification) would benefit
> from the proposed abstract API. Similar to at least both TDX and SEV,
> the CoVE attestation evidence generation proposed (The spec is not
> ratified yet) interface [1] basically takes some binary blobs in (a TVM
> public key and an attestation challenge blob) and requests the TSM to
> generate an attesation evidence for the caller. The TSM will generate
> such evidence from all static and runtime measurements and sign it with
> its DICE derived key. Attestation lingo set aside, the pattern here is
> similar to SEV and TDX. Having a common API for a generic attestation
> evidence generation interface would avoid having to add yet another
> ioctl based interface specific to CoVE.

Great, this gives us more confidence about the generic ABI design.

>
> Another interface we could think about commonizing is the measurement
> extension one. I think both TDX and CoVE allow for a guest to
> dynamically extend its measurements (to dedicated, runtime PCRs).
>

Yes. I think most vendors will need similar support. We are planning
to add a generic ABI for this as well.

> Cheers,
> Samuel.
>
> [1] https://github.com/riscv-non-isa/riscv-ap-tee/blob/main/specification/sbi_cove.adoc#function-cove-guest-get-evidence-fid-8
>
>> On 6/26/23 11:57 AM, Dionna Amalie Glaze wrote:
>>> On Sun, Jun 25, 2023 at 8:06 PM Sathyanarayanan Kuppuswamy
>>> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>>>>
>>>> Hi Dan,
>>>>
>>>> On 6/23/23 3:27 PM, Dan Williams wrote:
>>>>> Dan Williams wrote:
>>>>>> [ add David, Brijesh, and Atish]
>>>>>>
>>>>>> Kuppuswamy Sathyanarayanan wrote:
>>>>>>> In TDX guest, the second stage of the attestation process is Quote
>>>>>>> generation. This process is required to convert the locally generated
>>>>>>> TDREPORT into a remotely verifiable Quote. It involves sending the
>>>>>>> TDREPORT data to a Quoting Enclave (QE) which will verify the
>>>>>>> integrity of the TDREPORT and sign it with an attestation key.
>>>>>>>
>>>>>>> Intel's TDX attestation driver exposes TDX_CMD_GET_QUOTE IOCTL to
>>>>>>> allow the user agent to get the TD Quote.
>>>>>>>
>>>>>>> Add a kernel selftest module to verify the Quote generation feature.
>>>>>>>
>>>>>>> TD Quote generation involves following steps:
>>>>>>>
>>>>>>> * Get the TDREPORT data using TDX_CMD_GET_REPORT IOCTL.
>>>>>>> * Embed the TDREPORT data in quote buffer and request for quote
>>>>>>> generation via TDX_CMD_GET_QUOTE IOCTL request.
>>>>>>> * Upon completion of the GetQuote request, check for non zero value
>>>>>>> in the status field of Quote header to make sure the generated
>>>>>>> quote is valid.
>>>>>>
>>>>>> What this cover letter does not say is that this is adding another
>>>>>> instance of the similar pattern as SNP_GET_REPORT.
>>>>>>
>>>>>> Linux is best served when multiple vendors trying to do similar
>>>>>> operations are brought together behind a common ABI. We see this in the
>>>>>> history of wrangling SCSI vendors behind common interfaces. Now multiple
>>>>>> confidential computing vendors trying to develop similar flows with
>>>>>> differentiated formats where that differentiation need not leak over the
>>>>>> ABI boundary.
>>>>> [..]
>>>>>
>>>>> Below is a rough mock up of this approach to demonstrate the direction.
>>>>> Again, the goal is to define an ABI that can support any vendor's
>>>>> arch-specific attestation method and key provisioning flows without
>>>>> leaking vendor-specific details, or confidential material over the
>>>>> user/kernel ABI.
>>>>
>>>> Thanks for working on this mock code and helping out. It gives me the
>>>> general idea about your proposal.
>>>>
>>>>>
>>>>> The observation is that there are a sufficient number of attestation
>>>>> flows available to review where Linux can define a superset ABI to
>>>>> contain them all. The other observation is that the implementations have
>>>>> features that may cross-polinate over time. For example the SEV
>>>>> privelege level consideration ("vmpl"), and the TDX RTMR (think TPM
>>>>> PCRs) mechanisms address generic Confidential Computing use cases.
>>>>
>>>>
>>>> I agree with your point about VMPL and RTMR feature cases. This observation
>>>> is valid for AMD SEV and TDX attestation flows. But I am not sure whether
>>>> it will hold true for other vendor implementations. Our sample set is not
>>>> good enough to make this conclusion. The reason for my concern is, if you
>>>> check the ABI interface used in the S390 arch attestation driver
>>>> (drivers/s390/char/uvdevice.c), you would notice that there is a significant
>>>> difference between the ABI used in that driver and SEV/TDX drivers. The S390
>>>> driver attestation request appears to accept two data blobs as input, as well
>>>> as a variety of vendor-specific header configurations.
>>>>
>>>> Maybe the s390 attestation model is a special case, but, I think we consider
>>>> this issue. Since we don't have a common spec, there is chance that any
>>>> superset ABI we define now may not meet future vendor requirements. One way to
>>>> handle it to leave enough space in the generic ABI to handle future vendor
>>>> requirements.
>>>>
>>>> I think it would be better if other vendors (like ARM or RISC) can comment and
>>>> confirm whether this proposal meets their demands.
>>>>
>>>
>>> The VMPL-based separation that will house the supervisor module known
>>> as SVSM can have protocols that implement a TPM command interface, or
>>> an RTMR-extension interface, and will also need to have an
>>> SVSM-specific protocol attestation report format to keep the secure
>>> chain of custody apparent. We'd have different formats and protocols
>>> in the kernel, at least, to speak to each technology. I'm not sure
>>> it's worth the trouble of papering over all the... 3-4 technologies
>>> with similar but still weirdly different formats and ways of doing
>>> things with an abstracted attestation ABI, especially since the output
>>> all has to be interpreted in an architecture-specific way anyway.
>>>
>>> ARM's Confidential Computing Realm Management Extensions (RME) seems
>>> to be going along the lines of a runtime measurement register model
>>> with their hardware enforced security. The number of registers isn't
>>> prescribed in the spec.
>>>
>>> +Joey Gouly +linux-coco@lists.linux.dev as far as RME is concerned, do
>>> you know who would be best to weigh in on this discussion of a unified
>>> attestation model?
>>
>>
>>>
>>>>>
>>>>> Vendor specific ioctls for all of this feels like surrender when Linux
>>>>> already has the keys subsystem which has plenty of degrees of freedom
>>>>> for tracking blobs with signatures and using those blobs to instantiate
>>>>> other blobs. It already serves as the ABI wrapping various TPM
>>>>> implementations and marshaling keys for storage encryption and other use
>>>>> cases that intersect Confidential Computing.
>>>>>
>>>>> The benefit of deprecating vendor-specific abstraction layers in
>>>>> userspace is secondary. The primary benefit is collaboration. It enables
>>>>> kernel developers from various architectures to collaborate on common
>>>>> infrastructure. If, referring back to my previous example, SEV adopts an
>>>>> RTMR-like mechanism and TDX adopts a vmpl-like mechanism it would be
>>>>> unfortunate if those efforts were siloed, duplicated, and needlessly
>>>>> differentiated to userspace. So while there are arguably a manageable
>>>>> number of basic arch attestation methods the planned expansion of those
>>>>> to build incremental functionality is where I believe we, as a
>>>>> community, will be glad that we invested in a "Linux format" for all of
>>>>> this.
>>>>>
>>>>> An example, to show what the strawman patch below enables: (req_key is
>>>>> the sample program from "man 2 request_key")
>>>>>
>>>>> # ./req_key guest_attest guest_attest:0:0-$desc $(cat user_data | base64)
>>>>> Key ID is 10e2f3a7
>>>>> # keyctl pipe 0x10e2f3a7 | hexdump -C
>>>>> 00000000 54 44 58 20 47 65 6e 65 72 61 74 65 64 20 51 75 |TDX Generated Qu|
>>>>> 00000010 6f 74 65 00 00 00 00 00 00 00 00 00 00 00 00 00 |ote.............|
>>>>> 00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
>>>>> *
>>>>> 00004000
>>>>>
>>>>> This is the kernel instantiating a TDX Quote without the TDREPORT
>>>>> implementation detail ever leaving the kernel. Now, this is only the
>>>>
>>>> IIUC, the idea here is to cache the quote data and return it to the user whenever
>>>> possible, right? If yes, I think such optimization may not be very useful for our
>>>> case. AFAIK, the quote data will change whenever there is a change in the guest
>>>> measurement data. Since the validity of the generated quote will not be long,
>>>> and the frequency of quote generation requests is expected to be less, we may not
>>>> get much benefit from caching the quote data. I think we can keep this logic simple
>>>> by directly retrieving the quote data from the quoting enclave whenever there is a
>>>> request from the user.
>>>>
>>>>> top-half of what is needed. The missing bottom half takes that material
>>>>> and uses it to instantiate derived key material like the storage
>>>>> decryption key internal to the kernel. See "The Process" in
>>>>> Documentation/security/keys/request-key.rst for how the Keys subsystem
>>>>> handles the "keys for keys" use case.
>>>>
>>>> This is only useful for key-server use case, right? Attestation can also be
>>>> used for use cases like pattern matching or uploading some secure data, etc.
>>>> Since key-server is not the only use case, does it make sense to suppport
>>>> this derived key feature?
>>>>
>>>>>
>>>>> ---
>>>>> diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
>>>>> index f79ab13a5c28..0f775847028e 100644
>>>>> --- a/drivers/virt/Kconfig
>>>>> +++ b/drivers/virt/Kconfig
>>>>> @@ -54,4 +54,8 @@ source "drivers/virt/coco/sev-guest/Kconfig"
>>>>>
>>>>> source "drivers/virt/coco/tdx-guest/Kconfig"
>>>>>
>>>>> +config GUEST_ATTEST
>>>>> + tristate
>>>>> + select KEYS
>>>>> +
>>>>> endif
>>>>> diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
>>>>> index e9aa6fc96fab..66f6b838f8f4 100644
>>>>> --- a/drivers/virt/Makefile
>>>>> +++ b/drivers/virt/Makefile
>>>>> @@ -12,3 +12,4 @@ obj-$(CONFIG_ACRN_HSM) += acrn/
>>>>> obj-$(CONFIG_EFI_SECRET) += coco/efi_secret/
>>>>> obj-$(CONFIG_SEV_GUEST) += coco/sev-guest/
>>>>> obj-$(CONFIG_INTEL_TDX_GUEST) += coco/tdx-guest/
>>>>> +obj-$(CONFIG_GUEST_ATTEST) += coco/guest-attest/
>>>>> diff --git a/drivers/virt/coco/guest-attest/Makefile b/drivers/virt/coco/guest-attest/Makefile
>>>>> new file mode 100644
>>>>> index 000000000000..5581c5a27588
>>>>> --- /dev/null
>>>>> +++ b/drivers/virt/coco/guest-attest/Makefile
>>>>> @@ -0,0 +1,2 @@
>>>>> +obj-$(CONFIG_GUEST_ATTEST) += guest_attest.o
>>>>> +guest_attest-y := key.o
>>>>> diff --git a/drivers/virt/coco/guest-attest/key.c b/drivers/virt/coco/guest-attest/key.c
>>>>> new file mode 100644
>>>>> index 000000000000..2a494b6dd7a7
>>>>> --- /dev/null
>>>>> +++ b/drivers/virt/coco/guest-attest/key.c
>>>>> @@ -0,0 +1,159 @@
>>>>> +// SPDX-License-Identifier: GPL-2.0-only
>>>>> +/* Copyright(c) 2023 Intel Corporation. All rights reserved. */
>>>>> +
>>>>> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>>>>> +#include <linux/seq_file.h>
>>>>> +#include <linux/key-type.h>
>>>>> +#include <linux/module.h>
>>>>> +#include <linux/base64.h>
>>>>> +
>>>>> +#include <keys/request_key_auth-type.h>
>>>>> +#include <keys/user-type.h>
>>>>> +
>>>>> +#include "guest-attest.h"
>>>>
>>>> Can you share you guest-attest.h?
>>>>
>>>>> +
>>>>> +static LIST_HEAD(guest_attest_list);
>>>>> +static DECLARE_RWSEM(guest_attest_rwsem);
>>>>> +
>>>>> +static struct guest_attest_ops *fetch_ops(void)
>>>>> +{
>>>>> + return list_first_entry_or_null(&guest_attest_list,
>>>>> + struct guest_attest_ops, list);
>>>>> +}
>>>>> +
>>>>> +static struct guest_attest_ops *get_ops(void)
>>>>> +{
>>>>> + down_read(&guest_attest_rwsem);
>>>>> + return fetch_ops();
>>>>> +}
>>>>> +
>>>>> +static void put_ops(void)
>>>>> +{
>>>>> + up_read(&guest_attest_rwsem);
>>>>> +}
>>>>> +
>>>>> +int register_guest_attest_ops(struct guest_attest_ops *ops)
>>>>> +{
>>>>> + struct guest_attest_ops *conflict;
>>>>> + int rc;
>>>>> +
>>>>> + down_write(&guest_attest_rwsem);
>>>>> + conflict = fetch_ops();
>>>>> + if (conflict) {
>>>>> + pr_err("\"%s\" ops already registered\n", conflict->name);
>>>>> + rc = -EEXIST;
>>>>> + goto out;
>>>>> + }
>>>>> + list_add(&ops->list, &guest_attest_list);
>>>>> + try_module_get(ops->module);
>>>>> + rc = 0;
>>>>> +out:
>>>>> + up_write(&guest_attest_rwsem);
>>>>> + return rc;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(register_guest_attest_ops);
>>>>> +
>>>>> +void unregister_guest_attest_ops(struct guest_attest_ops *ops)
>>>>> +{
>>>>> + down_write(&guest_attest_rwsem);
>>>>> + list_del(&ops->list);
>>>>> + up_write(&guest_attest_rwsem);
>>>>> + module_put(ops->module);
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(unregister_guest_attest_ops);
>>>>> +
>>>>> +static int __guest_attest_request_key(struct key *key, int level,
>>>>> + struct key *dest_keyring,
>>>>> + const char *callout_info, int callout_len,
>>>>> + struct key *authkey)
>>>>> +{
>>>>> + struct guest_attest_ops *ops;
>>>>> + void *payload = NULL;
>>>>> + int rc, payload_len;
>>>>> +
>>>>> + ops = get_ops();
>>>>> + if (!ops)
>>>>> + return -ENOKEY;
>>>>> +
>>>>> + payload = kzalloc(max(GUEST_ATTEST_DATALEN, callout_len), GFP_KERNEL);
>>>>> + if (!payload) {
>>>>> + rc = -ENOMEM;
>>>>> + goto out;
>>>>> + }
>>>>
>>>> Is the idea to get the values like vmpl part of the payload?
>>>>
>>>>> +
>>>>> + payload_len = base64_decode(callout_info, callout_len, payload);
>>>>> + if (payload_len < 0 || payload_len > GUEST_ATTEST_DATALEN) {
>>>>> + rc = -EINVAL;
>>>>> + goto out;
>>>>> + }
>>>>> +
>>>>> + rc = ops->request_attest(key, level, dest_keyring, payload, payload_len,
>>>>> + authkey);
>>>>> +out:
>>>>> + kfree(payload);
>>>>> + put_ops();
>>>>> + return rc;
>>>>> +}
>>>>> +
>>>>> +static int guest_attest_request_key(struct key *authkey, void *data)
>>>>> +{
>>>>> + struct request_key_auth *rka = get_request_key_auth(authkey);
>>>>> + struct key *key = rka->target_key;
>>>>> + unsigned long long id;
>>>>> + int rc, level;
>>>>> +
>>>>> + pr_debug("desc: %s op: %s callout: %s\n", key->description, rka->op,
>>>>> + rka->callout_info ? (char *)rka->callout_info : "\"none\"");
>>>>> +
>>>>> + if (sscanf(key->description, "guest_attest:%d:%llu", &level, &id) != 2)
>>>>> + return -EINVAL;
>>>>> +
>>>>
>>>> Can you explain some details about the id and level? It is not very clear why
>>>> we need it.
>>>>
>>>>> + if (!rka->callout_info) {
>>>>> + rc = -EINVAL;
>>>>> + goto out;
>>>>> + }
>>>>> +
>>>>> + rc = __guest_attest_request_key(key, level, rka->dest_keyring,
>>>>> + rka->callout_info, rka->callout_len,
>>>>> + authkey);
>>>>> +out:
>>>>> + complete_request_key(authkey, rc);
>>>>> + return rc;
>>>>> +}
>>>>> +
>>>>> +static int guest_attest_vet_description(const char *desc)
>>>>> +{
>>>>> + unsigned long long id;
>>>>> + int level;
>>>>> +
>>>>> + if (sscanf(desc, "guest_attest:%d:%llu", &level, &id) != 2)
>>>>> + return -EINVAL;
>>>>> + return 0;
>>>>> +}
>>>>> +
>>>>> +static struct key_type key_type_guest_attest = {
>>>>> + .name = "guest_attest",
>>>>> + .preparse = user_preparse,
>>>>> + .free_preparse = user_free_preparse,
>>>>> + .instantiate = generic_key_instantiate,
>>>>> + .revoke = user_revoke,
>>>>> + .destroy = user_destroy,
>>>>> + .describe = user_describe,
>>>>> + .read = user_read,
>>>>> + .vet_description = guest_attest_vet_description,
>>>>> + .request_key = guest_attest_request_key,
>>>>> +};
>>>>> +
>>>>> +static int __init guest_attest_init(void)
>>>>> +{
>>>>> + return register_key_type(&key_type_guest_attest);
>>>>> +}
>>>>> +
>>>>> +static void __exit guest_attest_exit(void)
>>>>> +{
>>>>> + unregister_key_type(&key_type_guest_attest);
>>>>> +}
>>>>> +
>>>>> +module_init(guest_attest_init);
>>>>> +module_exit(guest_attest_exit);
>>>>> +MODULE_LICENSE("GPL v2");
>>>>> diff --git a/drivers/virt/coco/tdx-guest/Kconfig b/drivers/virt/coco/tdx-guest/Kconfig
>>>>> index 14246fc2fb02..9a1ec85369fe 100644
>>>>> --- a/drivers/virt/coco/tdx-guest/Kconfig
>>>>> +++ b/drivers/virt/coco/tdx-guest/Kconfig
>>>>> @@ -1,6 +1,7 @@
>>>>> config TDX_GUEST_DRIVER
>>>>> tristate "TDX Guest driver"
>>>>> depends on INTEL_TDX_GUEST
>>>>> + select GUEST_ATTEST
>>>>> help
>>>>> The driver provides userspace interface to communicate with
>>>>> the TDX module to request the TDX guest details like attestation
>>>>> diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-guest/tdx-guest.c
>>>>> index 388491fa63a1..65b5aab284d9 100644
>>>>> --- a/drivers/virt/coco/tdx-guest/tdx-guest.c
>>>>> +++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
>>>>> @@ -13,11 +13,13 @@
>>>>> #include <linux/string.h>
>>>>> #include <linux/uaccess.h>
>>>>> #include <linux/set_memory.h>
>>>>> +#include <linux/key-type.h>
>>>>>
>>>>> #include <uapi/linux/tdx-guest.h>
>>>>>
>>>>> #include <asm/cpu_device_id.h>
>>>>> #include <asm/tdx.h>
>>>>> +#include "../guest-attest/guest-attest.h"
>>>>>
>>>>> /*
>>>>> * Intel's SGX QE implementation generally uses Quote size less
>>>>> @@ -229,6 +231,62 @@ static const struct x86_cpu_id tdx_guest_ids[] = {
>>>>> };
>>>>> MODULE_DEVICE_TABLE(x86cpu, tdx_guest_ids);
>>>>>
>>>>> +static int tdx_request_attest(struct key *key, int level,
>>>>> + struct key *dest_keyring, void *payload,
>>>>> + int payload_len, struct key *authkey)
>>>>> +{
>>>>> + u8 *tdreport;
>>>>> + long ret;
>>>>> +
>>>>> + tdreport = kzalloc(TDX_REPORT_LEN, GFP_KERNEL);
>>>>> + if (!tdreport)
>>>>> + return -ENOMEM;
>>>>> +
>>>>> + /* Generate TDREPORT0 using "TDG.MR.REPORT" TDCALL */
>>>>> + ret = tdx_mcall_get_report0(payload, tdreport);
>>>>> + if (ret)
>>>>> + goto out;
>>>>> +
>>>>> + mutex_lock(&quote_lock);
>>>>> +
>>>>> + memset(qentry->buf, 0, qentry->buf_len);
>>>>> + reinit_completion(&qentry->compl);
>>>>> + qentry->valid = true;
>>>>> +
>>>>> + /* Submit GetQuote Request using GetQuote hyperetall */
>>>>> + ret = tdx_hcall_get_quote(qentry->buf, qentry->buf_len);
>>>>> + if (ret) {
>>>>> + pr_err("GetQuote hyperetall failed, status:%lx\n", ret);
>>>>> + ret = -EIO;
>>>>> + goto quote_failed;
>>>>> + }
>>>>> +
>>>>> + /*
>>>>> + * Although the GHCI specification does not state explicitly that
>>>>> + * the VMM must not wait indefinitely for the Quote request to be
>>>>> + * completed, a sane VMM should always notify the guest after a
>>>>> + * certain time, regardless of whether the Quote generation is
>>>>> + * successful or not. For now just assume the VMM will do so.
>>>>> + */
>>>>> + wait_for_completion(&qentry->compl);
>>>>> +
>>>>> + ret = key_instantiate_and_link(key, qentry->buf, qentry->buf_len,
>>>>> + dest_keyring, authkey);
>>>>> +
>>>>> +quote_failed:
>>>>> + qentry->valid = false;
>>>>> + mutex_unlock(&quote_lock);
>>>>> +out:
>>>>> + kfree(tdreport);
>>>>> + return ret;
>>>>> +}
>>>>> +
>>>>> +static struct guest_attest_ops tdx_attest_ops = {
>>>>> + .name = KBUILD_MODNAME,
>>>>> + .module = THIS_MODULE,
>>>>> + .request_attest = tdx_request_attest,
>>>>> +};
>>>>> +
>>>>> static int __init tdx_guest_init(void)
>>>>> {
>>>>> int ret;
>>>>> @@ -251,8 +309,14 @@ static int __init tdx_guest_init(void)
>>>>> if (ret)
>>>>> goto free_quote;
>>>>>
>>>>> + ret = register_guest_attest_ops(&tdx_attest_ops);
>>>>> + if (ret)
>>>>> + goto free_irq;
>>>>> +
>>>>> return 0;
>>>>>
>>>>> +free_irq:
>>>>> + tdx_unregister_event_irq_cb(quote_cb_handler, qentry);
>>>>> free_quote:
>>>>> free_quote_entry(qentry);
>>>>> free_misc:
>>>>> @@ -264,6 +328,7 @@ module_init(tdx_guest_init);
>>>>>
>>>>> static void __exit tdx_guest_exit(void)
>>>>> {
>>>>> + unregister_guest_attest_ops(&tdx_attest_ops);
>>>>> tdx_unregister_event_irq_cb(quote_cb_handler, qentry);
>>>>> free_quote_entry(qentry);
>>>>> misc_deregister(&tdx_misc_dev);
>>>>
>>>> --
>>>> Sathyanarayanan Kuppuswamy
>>>> Linux Kernel Developer
>>>
>>>
>>>
>>> --
>>> -Dionna Glaze, PhD (she/her)
>>
>> --
>> Sathyanarayanan Kuppuswamy
>> Linux Kernel Developer
>>

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer