[PATCH v2 4/4] virt: Add sev_secret module to expose confidential computing secrets
Dov Murik
dovmurik at linux.ibm.com
Thu Oct 7 06:18:38 UTC 2021
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 at linux.ibm.com>
---
.../ABI/testing/securityfs-coco-sev_secret | 49 +++
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 | 320 ++++++++++++++++++
6 files changed, 386 insertions(+)
create mode 100644 Documentation/ABI/testing/securityfs-coco-sev_secret
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/Documentation/ABI/testing/securityfs-coco-sev_secret b/Documentation/ABI/testing/securityfs-coco-sev_secret
new file mode 100644
index 000000000000..3db1a66ff5c4
--- /dev/null
+++ b/Documentation/ABI/testing/securityfs-coco-sev_secret
@@ -0,0 +1,49 @@
+What: security/coco/sev_secret
+Date: October 2021
+Contact: Dov Murik <dovmurik at linux.ibm.com>
+Description:
+ Exposes SEV confidential computing (coco) secrets to
+ userspace via securityfs.
+
+ AMD SEV and SEV-ES allow the Guest Owner to inject secrets
+ during VM's launch. The secrets are encrypted by the Guest
+ Owner and decrypted by the AMD-SP (secure processor), and
+ therefore are not readable by the untrusted host.
+
+ The sev_secret module exposes the secrets to userspace. Each
+ secret appears as a file under <securityfs>/coco/sev_secret,
+ where the filename is the GUID of the entry in the secrets
+ table.
+
+ Two operations are supported for the files: read and unlink.
+ Reading the file returns the content of secret entry.
+ Unlinking the file overwrites the secret data with zeroes and
+ removes the entry from the filesystem. A secret cannot be read
+ after it has been unlinked.
+
+ For example, listing the available secrets::
+
+ # modprobe sev_secret
+ # ls -l /sys/kernel/security/coco/sev_secret
+ -r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
+ -r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
+ -r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
+ -r--r----- 1 root root 0 Jun 28 11:54 e6f5a162-d67f-4750-a67c-5d065f2a9910
+
+ Reading the secret data by reading a file::
+
+ # cat /sys/kernel/security/coco/sev_secret/e6f5a162-d67f-4750-a67c-5d065f2a9910
+ the-content-of-the-secret-data
+
+ Wiping a secret by unlinking a file::
+
+ # rm /sys/kernel/security/coco/sev_secret/e6f5a162-d67f-4750-a67c-5d065f2a9910
+ # ls -l /sys/kernel/security/coco/sev_secret
+ -r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
+ -r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
+ -r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
+
+ Note: The binary format of the secrets table injected by the
+ Guest Owner is described in
+ drivers/virt/coco/sev_secret/sev_secret.c under "Structure of
+ the SEV secret area".
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..3c83aa80f981
--- /dev/null
+++ b/drivers/virt/coco/sev_secret/sev_secret.c
@@ -0,0 +1,320 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * sev_secret module
+ *
+ * Copyright (C) 2021 IBM Corporation
+ * Author: Dov Murik <dovmurik at linux.ibm.com>
+ */
+
+/**
+ * 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>
+#include <asm/cacheflush.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)
+ *
+ * The GUID of each secret entry designates the usage of the secret data.
+ */
+
+/**
+ * struct secret_header - Header of entire secret area; this should be followed
+ * by instances of struct secret_entry.
+ * @guid: Must be EFI_SEVSECRET_TABLE_HEADER_GUID
+ * @len: Length in bytes of entire secret area, including header
+ */
+struct secret_header {
+ efi_guid_t guid;
+ u32 len;
+} __attribute((packed));
+
+/**
+ * struct secret_entry - Holds one secret entry
+ * @guid: Secret-specific GUID (or NULL_GUID if this secret entry was deleted)
+ * @len: Length of secret entry, including its guid and len fields
+ * @data: The secret data (full of zeros if this secret entry was deleted)
+ */
+struct secret_entry {
+ efi_guid_t guid;
+ u32 len;
+ u8 data[];
+} __attribute((packed));
+
+static size_t secret_entry_data_len(struct secret_entry *e)
+{
+ return e->len - sizeof(*e);
+}
+
+static struct sev_secret the_sev_secret;
+
+static inline struct sev_secret *sev_secret_get(void)
+{
+ return &the_sev_secret;
+}
+
+static int sev_secret_bin_file_show(struct seq_file *file, void *data)
+{
+ struct secret_entry *e = file->private;
+
+ if (e)
+ seq_write(file, e->data, secret_entry_data_len(e));
+
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(sev_secret_bin_file);
+
+static void wipe_memory(void *addr, size_t size)
+{
+ memzero_explicit(addr, size);
+ clean_cache_range(addr, size);
+}
+
+static int sev_secret_unlink(struct inode *dir, struct dentry *dentry)
+{
+ struct sev_secret *s = sev_secret_get();
+ struct inode *inode = d_inode(dentry);
+ struct secret_entry *e = (struct secret_entry *)inode->i_private;
+ int i;
+
+ if (e) {
+ /* Zero out the secret data */
+ wipe_memory(e->data, secret_entry_data_len(e));
+ e->guid = NULL_GUID;
+ }
+
+ inode->i_private = NULL;
+
+ for (i = 0; i < SEV_SECRET_NUM_FILES; i++)
+ if (s->fs_files[i] == dentry)
+ s->fs_files[i] = NULL;
+
+ /*
+ * securityfs_remove tries to lock the directory's inode, but we reach
+ * the unlink callback when it's already locked
+ */
+ inode_unlock(dir);
+ securityfs_remove(dentry);
+ inode_lock(dir);
+
+ return 0;
+}
+
+static const struct inode_operations sev_secret_dir_inode_operations = {
+ .lookup = simple_lookup,
+ .unlink = sev_secret_unlink,
+};
+
+static int sev_secret_map_area(void)
+{
+ struct sev_secret *s = sev_secret_get();
+ struct linux_efi_coco_secret_area *secret_area;
+ u32 secret_area_size;
+
+ if (efi.coco_secret == EFI_INVALID_TABLE_ADDR) {
+ pr_err("Secret area address is not available\n");
+ return -EINVAL;
+ }
+
+ secret_area = memremap(efi.coco_secret, sizeof(*secret_area), MEMREMAP_WB);
+ if (secret_area == NULL) {
+ pr_err("Could not map secret area header\n");
+ return -ENOMEM;
+ }
+
+ secret_area_size = sizeof(*secret_area) + secret_area->size;
+ memunmap(secret_area);
+
+ secret_area = memremap(efi.coco_secret, secret_area_size, MEMREMAP_WB);
+ if (secret_area == NULL) {
+ pr_err("Could not map secret area\n");
+ return -ENOMEM;
+ }
+
+ s->secret_area = secret_area;
+ return 0;
+}
+
+static void sev_secret_securityfs_teardown(void)
+{
+ struct sev_secret *s = sev_secret_get();
+ int i;
+
+ for (i = (SEV_SECRET_NUM_FILES - 1); i >= 0; i--) {
+ securityfs_remove(s->fs_files[i]);
+ s->fs_files[i] = NULL;
+ }
+
+ securityfs_remove(s->fs_dir);
+ s->fs_dir = NULL;
+
+ securityfs_remove(s->coco_dir);
+ s->coco_dir = NULL;
+
+ pr_debug("Removed sev_secret securityfs entries\n");
+}
+
+static int sev_secret_securityfs_setup(void)
+{
+ efi_guid_t tableheader_guid = EFI_SEVSECRET_TABLE_HEADER_GUID;
+ struct sev_secret *s = sev_secret_get();
+ int ret = 0, i = 0, bytes_left;
+ unsigned char *ptr;
+ struct secret_header *h;
+ struct secret_entry *e;
+ struct dentry *dent;
+ char guid_str[EFI_VARIABLE_GUID_LEN + 1];
+
+ s->coco_dir = NULL;
+ s->fs_dir = NULL;
+ memset(s->fs_files, 0, sizeof(s->fs_files));
+
+ dent = securityfs_create_dir("coco", NULL);
+ if (IS_ERR(dent)) {
+ pr_err("Error creating coco securityfs directory entry err=%ld\n", PTR_ERR(dent));
+ return PTR_ERR(dent);
+ }
+ s->coco_dir = dent;
+
+ dent = securityfs_create_dir("sev_secret", s->coco_dir);
+ if (IS_ERR(dent)) {
+ pr_err("Error creating SEV secret securityfs directory entry err=%ld\n",
+ PTR_ERR(dent));
+ return PTR_ERR(dent);
+ }
+ d_inode(dent)->i_op = &sev_secret_dir_inode_operations;
+ s->fs_dir = dent;
+
+ ptr = s->secret_area->area;
+ h = (struct secret_header *)ptr;
+ if (memcmp(&h->guid, &tableheader_guid, sizeof(h->guid))) {
+ pr_err("SEV secret area does not start with correct GUID\n");
+ ret = -EINVAL;
+ goto err_cleanup;
+ }
+ if (h->len < sizeof(*h)) {
+ pr_err("SEV secret area reported length is too small\n");
+ ret = -EINVAL;
+ goto err_cleanup;
+ }
+
+ bytes_left = h->len - sizeof(*h);
+ ptr += sizeof(*h);
+ while (bytes_left >= (int)sizeof(*e) && i < SEV_SECRET_NUM_FILES) {
+ e = (struct secret_entry *)ptr;
+ if (e->len < sizeof(*e) || e->len > (unsigned int)bytes_left) {
+ pr_err("SEV secret area is corrupted\n");
+ ret = -EINVAL;
+ goto err_cleanup;
+ }
+
+ /* Skip deleted entries (which will have NULL_GUID) */
+ if (efi_guidcmp(e->guid, NULL_GUID)) {
+ efi_guid_to_str(&e->guid, guid_str);
+
+ dent = securityfs_create_file(guid_str, 0440, s->fs_dir, (void *)e,
+ &sev_secret_bin_file_fops);
+ if (IS_ERR(dent)) {
+ pr_err("Error creating SEV secret securityfs entry\n");
+ ret = PTR_ERR(dent);
+ goto err_cleanup;
+ }
+
+ s->fs_files[i++] = dent;
+ }
+ ptr += e->len;
+ bytes_left -= e->len;
+ }
+
+ pr_debug("Created %d entries in sev_secret securityfs\n", i);
+ return 0;
+
+err_cleanup:
+ sev_secret_securityfs_teardown();
+ return ret;
+}
+
+static void sev_secret_unmap_area(void)
+{
+ struct sev_secret *s = sev_secret_get();
+
+ if (s->secret_area) {
+ memunmap(s->secret_area);
+ s->secret_area = NULL;
+ }
+}
+
+static int __init sev_secret_init(void)
+{
+ int ret;
+
+ ret = sev_secret_map_area();
+ if (ret)
+ return ret;
+
+ ret = sev_secret_securityfs_setup();
+ if (ret)
+ goto err_unmap;
+
+ return ret;
+
+err_unmap:
+ sev_secret_unmap_area();
+ return ret;
+}
+
+static void __exit sev_secret_exit(void)
+{
+ sev_secret_securityfs_teardown();
+ sev_secret_unmap_area();
+}
+
+module_init(sev_secret_init);
+module_exit(sev_secret_exit);
+
+MODULE_DESCRIPTION("AMD SEV confidential computing secret area access");
+MODULE_AUTHOR("IBM");
+MODULE_LICENSE("GPL");
--
2.25.1
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