Re: [PATCH 3/3] virt: Add sev_secret module to expose confidential computing secrets

From: Dov Murik
Date: Thu Sep 02 2021 - 14:14:59 EST



On 02/09/2021 15:59, Greg KH wrote:
> On Mon, Aug 09, 2021 at 07:01:57PM +0000, Dov Murik wrote:
>> The new sev_secret module exposes the confidential computing (coco)
>> secret area via securityfs interface.
>>
>> When the module is loaded (and securityfs is mounted, typically under
>> /sys/kernel/security), a "coco/sev_secret" directory is created in
>> securityfs. In it, a file is created for each secret entry. The name
>> of each such file is the GUID of the secret entry, and its content is
>> the secret data.
>>
>> This allows applications running in a confidential computing setting to
>> read secrets provided by the guest owner via a secure secret injection
>> mechanism (such as AMD SEV's LAUNCH_SECRET command).
>>
>> Removing (unlinking) files in the "coco/sev_secret" directory will zero
>> out the secret in memory, and remove the filesystem entry. If the
>> module is removed and loaded again, that secret will not appear in the
>> filesystem.
>>
>> Signed-off-by: Dov Murik <dovmurik@xxxxxxxxxxxxx>
>> ---
>> drivers/virt/Kconfig | 3 +
>> drivers/virt/Makefile | 1 +
>> drivers/virt/coco/sev_secret/Kconfig | 11 +
>> drivers/virt/coco/sev_secret/Makefile | 2 +
>> drivers/virt/coco/sev_secret/sev_secret.c | 313 ++++++++++++++++++++++
>> 5 files changed, 330 insertions(+)
>> create mode 100644 drivers/virt/coco/sev_secret/Kconfig
>> create mode 100644 drivers/virt/coco/sev_secret/Makefile
>> create mode 100644 drivers/virt/coco/sev_secret/sev_secret.c
>>
>> diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
>> index 8061e8ef449f..6f73672f593f 100644
>> --- a/drivers/virt/Kconfig
>> +++ b/drivers/virt/Kconfig
>> @@ -36,4 +36,7 @@ source "drivers/virt/vboxguest/Kconfig"
>> source "drivers/virt/nitro_enclaves/Kconfig"
>>
>> source "drivers/virt/acrn/Kconfig"
>> +
>> +source "drivers/virt/coco/sev_secret/Kconfig"
>> +
>> endif
>> diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
>> index 3e272ea60cd9..2a7d472478bd 100644
>> --- a/drivers/virt/Makefile
>> +++ b/drivers/virt/Makefile
>> @@ -8,3 +8,4 @@ obj-y += vboxguest/
>>
>> obj-$(CONFIG_NITRO_ENCLAVES) += nitro_enclaves/
>> obj-$(CONFIG_ACRN_HSM) += acrn/
>> +obj-$(CONFIG_AMD_SEV_SECRET) += coco/sev_secret/
>> diff --git a/drivers/virt/coco/sev_secret/Kconfig b/drivers/virt/coco/sev_secret/Kconfig
>> new file mode 100644
>> index 000000000000..76cfb4f405e0
>> --- /dev/null
>> +++ b/drivers/virt/coco/sev_secret/Kconfig
>> @@ -0,0 +1,11 @@
>> +# SPDX-License-Identifier: GPL-2.0-only
>> +config AMD_SEV_SECRET
>> + tristate "AMD SEV secret area securityfs support"
>> + depends on AMD_MEM_ENCRYPT && EFI
>> + select SECURITYFS
>> + help
>> + This is a driver for accessing the AMD SEV secret area via
>> + securityfs.
>> +
>> + To compile this driver as a module, choose M here.
>> + The module will be called sev_secret.
>> diff --git a/drivers/virt/coco/sev_secret/Makefile b/drivers/virt/coco/sev_secret/Makefile
>> new file mode 100644
>> index 000000000000..dca0ed3f8f94
>> --- /dev/null
>> +++ b/drivers/virt/coco/sev_secret/Makefile
>> @@ -0,0 +1,2 @@
>> +# SPDX-License-Identifier: GPL-2.0-only
>> +obj-$(CONFIG_AMD_SEV_SECRET) += sev_secret.o
>> diff --git a/drivers/virt/coco/sev_secret/sev_secret.c b/drivers/virt/coco/sev_secret/sev_secret.c
>> new file mode 100644
>> index 000000000000..d9a60166b142
>> --- /dev/null
>> +++ b/drivers/virt/coco/sev_secret/sev_secret.c
>> @@ -0,0 +1,313 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * sev_secret module
>> + *
>> + * Copyright (C) 2021 IBM Corporation
>> + * Author: Dov Murik <dovmurik@xxxxxxxxxxxxx>
>> + */
>> +
>> +/**
>> + * DOC: sev_secret: Allow reading confidential computing (coco) secret area via
>> + * securityfs interface.
>> + *
>> + * When the module is loaded (and securityfs is mounted, typically under
>> + * /sys/kernel/security), a "coco/sev_secret" directory is created in
>> + * securityfs. In it, a file is created for each secret entry. The name of
>> + * each such file is the GUID of the secret entry, and its content is the
>> + * secret data.
>> + */
>> +
>> +#include <linux/seq_file.h>
>> +#include <linux/fs.h>
>> +#include <linux/kernel.h>
>> +#include <linux/init.h>
>> +#include <linux/module.h>
>> +#include <linux/io.h>
>> +#include <linux/security.h>
>> +#include <linux/efi.h>
>> +
>> +#define SEV_SECRET_NUM_FILES 64
>> +
>> +#define EFI_SEVSECRET_TABLE_HEADER_GUID \
>> + EFI_GUID(0x1e74f542, 0x71dd, 0x4d66, 0x96, 0x3e, 0xef, 0x42, 0x87, 0xff, 0x17, 0x3b)
>> +
>> +struct sev_secret {
>> + struct dentry *coco_dir;
>> + struct dentry *fs_dir;
>> + struct dentry *fs_files[SEV_SECRET_NUM_FILES];
>> + struct linux_efi_coco_secret_area *secret_area;
>> +};
>> +
>> +/*
>> + * Structure of the SEV secret area
>> + *
>> + * Offset Length
>> + * (bytes) (bytes) Usage
>> + * ------- ------- -----
>> + * 0 16 Secret table header GUID (must be 1e74f542-71dd-4d66-963e-ef4287ff173b)
>> + * 16 4 Length of bytes of the entire secret area
>> + *
>> + * 20 16 First secret entry's GUID
>> + * 36 4 First secret entry's length in bytes (= 16 + 4 + x)
>> + * 40 x First secret entry's data
>> + *
>> + * 40+x 16 Second secret entry's GUID
>> + * 56+x 4 Second secret entry's length in bytes (= 16 + 4 + y)
>> + * 60+x y Second secret entry's data
>> + *
>> + * (... and so on for additional entries)
>
> Why isn't all of this documented in Documentation/ABI/ which is needed
> for any new user/kernel api that you come up with like this. We have to
> have it documented somewhere, otherwise how will you know how to use
> these files?

Yes, you're right, I'll add such documentation.

Note that the ABI (for userspace programs) is the filesystem paths and
usage (read + unlink), and not the GUIDed table explained above your
comment. That GUIDed table is passed from the Guest Owner via SEV
secret injection into OVMF and from there to the kernel memory (patches
1+2 in this series). So userspace doesn't see this GUIDed table
structure at all.

I should probably add this story to this file's header comment, or some
other place which will document this module (suggestions welcome).

-Dov