netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 103 insertions, 0 deletions

diff --git a/Documentation/security/secrets/coco.rst b/Documentation/security/secrets/coco.rst
new file mode 100644
index 000000000..a1210db8e
--- /dev/null
+++ b/Documentation/security/secrets/coco.rst
@@ -0,0 +1,103 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Confidential Computing secrets
+==============================
+
+This document describes how Confidential Computing secret injection is handled
+from the firmware to the operating system, in the EFI driver and the efi_secret
+kernel module.
+
+
+Introduction
+============
+
+Confidential Computing (coco) hardware such as AMD SEV (Secure Encrypted
+Virtualization) allows guest owners to inject secrets into the VMs
+memory without the host/hypervisor being able to read them.  In SEV,
+secret injection is performed early in the VM launch process, before the
+guest starts running.
+
+The efi_secret kernel module allows userspace applications to access these
+secrets via securityfs.
+
+
+Secret data flow
+================
+
+The guest firmware may reserve a designated memory area for secret injection,
+and publish its location (base GPA and length) in the EFI configuration table
+under a ``LINUX_EFI_COCO_SECRET_AREA_GUID`` entry
+(``adf956ad-e98c-484c-ae11-b51c7d336447``).  This memory area should be marked
+by the firmware as ``EFI_RESERVED_TYPE``, and therefore the kernel should not
+be use it for its own purposes.
+
+During the VM's launch, the virtual machine manager may inject a secret to that
+area.  In AMD SEV and SEV-ES this is performed using the
+``KVM_SEV_LAUNCH_SECRET`` command (see [sev]_).  The structure of the injected
+Guest Owner secret data should be a GUIDed table of secret values; the binary
+format is described in ``drivers/virt/coco/efi_secret/efi_secret.c`` under
+"Structure of the EFI secret area".
+
+On kernel start, the kernel's EFI driver saves the location of the secret area
+(taken from the EFI configuration table) in the ``efi.coco_secret`` field.
+Later it checks if the secret area is populated: it maps the area and checks
+whether its content begins with ``EFI_SECRET_TABLE_HEADER_GUID``
+(``1e74f542-71dd-4d66-963e-ef4287ff173b``).  If the secret area is populated,
+the EFI driver will autoload the efi_secret kernel module, which exposes the
+secrets to userspace applications via securityfs.  The details of the
+efi_secret filesystem interface are in [secrets-coco-abi]_.
+
+
+Application usage example
+=========================
+
+Consider a guest performing computations on encrypted files.  The Guest Owner
+provides the decryption key (= secret) using the secret injection mechanism.
+The guest application reads the secret from the efi_secret filesystem and
+proceeds to decrypt the files into memory and then performs the needed
+computations on the content.
+
+In this example, the host can't read the files from the disk image
+because they are encrypted.  Host can't read the decryption key because
+it is passed using the secret injection mechanism (= secure channel).
+Host can't read the decrypted content from memory because it's a
+confidential (memory-encrypted) guest.
+
+Here is a simple example for usage of the efi_secret module in a guest
+to which an EFI secret area with 4 secrets was injected during launch::
+
+	# ls -la /sys/kernel/security/secrets/coco
+	total 0
+	drwxr-xr-x 2 root root 0 Jun 28 11:54 .
+	drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
+	-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
+
+	# hd /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
+	00000000  74 68 65 73 65 2d 61 72  65 2d 74 68 65 2d 6b 61  |these-are-the-ka|
+	00000010  74 61 2d 73 65 63 72 65  74 73 00 01 02 03 04 05  |ta-secrets......|
+	00000020  06 07                                             |..|
+	00000022
+
+	# rm /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
+
+	# ls -la /sys/kernel/security/secrets/coco
+	total 0
+	drwxr-xr-x 2 root root 0 Jun 28 11:55 .
+	drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
+	-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
+
+
+References
+==========
+
+See [sev-api-spec]_ for more info regarding SEV ``LAUNCH_SECRET`` operation.
+
+.. [sev] Documentation/virt/kvm/x86/amd-memory-encryption.rst
+.. [secrets-coco-abi] Documentation/ABI/testing/securityfs-secrets-coco
+.. [sev-api-spec] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf