[PATCH v2 4/6] keyctl.2: document the ability to provide KDF parameters in KEYCTL_DH_COMPUTE

[Eugene Syromyatnikov <evgsyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

---
 man2/keyctl.2 | 182 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 179 insertions(+), 3 deletions(-)

diff --git a/man2/keyctl.2 b/man2/keyctl.2
index e4b4bc9..28d15a3 100644
--- a/man2/keyctl.2
+++ b/man2/keyctl.2
@@ -25,6 +25,7 @@
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" %%%LICENSE_END
 .\"
+.mso www.tmac
 .TH KEYCTL 2 2017-03-13 Linux "Linux Key Management Calls"
 .SH NAME
 keyctl \- manipulate the kernel's key management facility
@@ -1413,7 +1414,8 @@ via the function
 .TP
 .BR KEYCTL_DH_COMPUTE " (since Linux 4.7)"
 .\" commit ddbb41148724367394d0880c516bfaeed127b52e
-Compute a Diffie-Hellman shared secret or public key.
+Compute a Diffie-Hellman shared secret or public key,
+optionally applying key derivation function (KDF) to the result.
 .IP
 The
 .I arg2
@@ -1476,10 +1478,65 @@ system call was considered a good fit due to the DH algorithm's use
 for deriving shared keys;
 it also allows the type of the key to determine
 which DH implementation (software or hardware) is appropriate.
+.\" commit f1c316a3ab9d24df6022682422fe897492f2c0c8
 .IP
-The
+If the
 .I arg5
-argument is reserved and must be 0.
+argument is
+.BR NULL ,
+then the DH result itself is returned.
+Otherwise (since Linux 4.12), it is a pointer to a structure which specifies
+parameters of the KDF operation to be applied:
+.IP
+.in +4n
+.EX
+struct keyctl_kdf_params {
+    char *hashname;     /* Hash algorithm name */
+    char *otherinfo;    /* SP800-56A OtherInfo */
+    __u32 otherinfolen; /* Length of otherinfo data */
+    __u32 __spare[8];   /* Reserved */
+};
+.EE
+.in
+.IP
+The
+.B hashname
+field is a null-terminated string which specifies hash name
+(available into the kernel's crypto API; the list of the hashes available
+is rather tricky to observe; please refer to the
+.URL https://www.kernel.org/doc/html/latest/crypto/architecture.html "Kernel Crypto API Architecture"
+for the information regarding how hash names are constructed and
+to your kernel's source and configuration regarding what ciphers
+and templates with type
+.B CRYPTO_ALG_TYPE_SHASH
+are available)
+to be applied to DH result in KDF operation.
+.IP
+The
+.B otherinfo
+field is an
+.I OtherInfo
+data as described in SP800-56A section 5.8.1.2 and is algorithm-specific.
+This data is concatenated with the result of DH operation and is provided as
+an input to the KDF operation.
+Its size is provided in the
+.B otherinfolen
+field and is limited by
+.B KEYCTL_KDF_MAX_OI_LEN
+constant that defined in
+.B security/keys/internal.h
+to a value of 64.
+.IP
+The
+.B __spare
+field is currently unused.
+.\" commit 4f9dabfaf8df971f8a3b6aa324f8f817be38d538
+It was ignored until Linux 4.13 (but still should be
+user-addressable since it is copied to the kernel),
+and should contain zeroes since Linux 4.13.
+.IP
+The KDF implementation complies with SP800-56A as well
+as with SP800-108 (the counter KDF).
 .SH RETURN VALUE
 For a successful call, the return value depends on the operation:
 .TP
@@ -1535,6 +1592,12 @@ is set appropriately to indicate the error.
 .B EACCES
 The requested operation wasn't permitted.
 .TP
+.B EAGAIN
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and there was an error during crypto module initialisation.
+.TP
 .B EDEADLK
 .I operation
 was
@@ -1545,6 +1608,43 @@ and the requested link would result in a cycle.
 The key quota for the caller's user would be exceeded by creating a key or
 linking it to the keyring.
 .TP
+.B EFAULT
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and one of the following has failed:
+.RS
+.IP \(bu 3
+copying of
+.BR "struct keyctl_dh_params" ,
+provided in
+.I arg2
+argument, from the user space;
+.IP \(bu
+copying of
+.BR "struct keyctl_kdf_params" ,
+provided in non-NULL
+.I arg5
+argument, from the user space
+(in case kernel supports performing KDF operation on DH operation result);
+.IP \(bu
+copying of data pointed by
+.B hashname
+field of
+.B "struct keyctl_kdf_params"
+from the user space;
+.IP \(bu
+copying of data pointed by
+.B otherinfo
+field of
+.B struct keyctl_kdf_params
+from the user space if
+.B otherinfolen
+field was non-zero;
+.IP \(bu
+copying of the result to the user space.
+.RE
+.TP
 .B EINVAL
 .I operation
 was
@@ -1567,6 +1667,20 @@ or
 (the key description)
 exceeded the limit (32 bytes and 4096 bytes respectively).
 .TP
+.BR EINVAL " (Linux kernels before 4.12)"
+.I operation
+was
+.BR KEYCTL_DH_COMPUTE ,
+argument
+.I arg5
+was non-NULL.
+.TP
+.B EINVAL
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+And the digest size of the hashing algorithm supplied is zero.
+.TP
 .B EINVAL
 .I operation
 was
@@ -1574,6 +1688,33 @@ was
 and the buffer size provided is not enough to hold the result.
 Provide 0 as a buffer size in order to obtain the minimum buffer size.
 .TP
+.B EINVAL
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and the hash name provided in the
+.B hashname
+field of the
+.B struct keyctl_kdf_params
+pointed by
+.I arg5
+argument is too big (the limit is implementation-specific and varies between
+kernel versions, but it is deemed big enough for all valid algorithm names).
+.TP
+.B EINVAL
+.\" commit 4f9dabfaf8df971f8a3b6aa324f8f817be38d538
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and the
+.B __spare
+field of the
+.B struct keyctl_kdf_params
+provided in
+.I arg5
+argument
+contains non-zero values.
+.TP
 .B EKEYEXPIRED
 An expired key was found or specified.
 .TP
@@ -1590,6 +1731,23 @@ was
 and the requested link would cause the maximum nesting depth
 for keyrings to be exceeded.
 .TP
+.B EMSGSIZE
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and the buffer length exceeds
+.B KEYCTL_KDF_MAX_OUTPUT_LEN
+(which is 1024 currently)
+or the
+.B otherinfolen
+field of
+.B struct keyctl_kdf_parms
+passed in
+.I arg5
+exceeds
+.B KEYCTL_KDF_MAX_OI_LEN
+(which is 64 currently).
+.TP
 .BR ENFILE " (Linux kernels before 3.13)"
 .IR operation
 was
@@ -1606,6 +1764,18 @@ was
 .B KEYCTL_UNLINK
 and the key to be unlinked isn't linked to the keyring.
 .TP
+.B ENOENT
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and the hashing algorithm specified in the
+.B hashname
+field of the
+.B struct keyctl_kdf_params
+pointed by
+.I arg5
+argument hasn't been found.
+.TP
 .B ENOKEY
 No matching key was found or an invalid key was specified.
 .TP
@@ -1662,6 +1832,12 @@ the parent process is not single-thread;
 or the parent process is
 .BR init (1)
 or a kernel thread.
+.TP
+.B ETIMEDOUT
+.I operation
+was
+.B KEYCTL_DH_COMPUTE
+and the initialisation of crypto modules has timed out.
 .SH VERSIONS
 This system call first appeared in Linux 2.6.10.
 .SH CONFORMING TO
-- 
2.1.4

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