integrity: prevent deadlock during digsig verification.

This patch aimed to prevent deadlock during digsig verification.The point
of issue - user space utility modprobe and/or it's dependencies (ld-*.so,
libz.so.*, libc-*.so and /lib/modules/ files) that could be used for
kernel modules load during digsig verification and could be signed by
digsig in the same time.

First at all, look at crypto_alloc_tfm() work algorithm:
crypto_alloc_tfm() will first attempt to locate an already loaded
algorithm. If that fails and the kernel supports dynamically loadable
modules, it will then attempt to load a module of the same name or alias.
If that fails it will send a query to any loaded crypto manager to
construct an algorithm on the fly.

We have situation, when public_key_verify_signature() in case of RSA
algorithm use alg_name to store internal information in order to construct
an algorithm on the fly, but crypto_larval_lookup() will try to use
alg_name in order to load kernel module with same name.

1) we can't do anything with crypto module work, since it designed to work
exactly in this way;
2) we can't globally filter module requests for modprobe, since it
designed to work with any requests.

In this patch, I propose add an exception for "crypto-pkcs1pad(rsa,*)"
module requests only in case of enabled integrity asymmetric keys support.
Since we don't have any real "crypto-pkcs1pad(rsa,*)" kernel modules for
sure, we are safe to fail such module request from crypto_larval_lookup().
In this way we prevent modprobe execution during digsig verification and
avoid possible deadlock if modprobe and/or it's dependencies also signed
with digsig.

Requested "crypto-pkcs1pad(rsa,*)" kernel module name formed by:
1) "pkcs1pad(rsa,%s)" in public_key_verify_signature();
2) "crypto-%s" / "crypto-%s-all" in crypto_larval_lookup().
"crypto-pkcs1pad(rsa," part of request is a constant and unique and could
be used as filter.

Signed-off-by: Mikhail Kurinnoi <viewizard@viewizard.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

 include/linux/integrity.h              | 13 +++++++++++++
 security/integrity/digsig_asymmetric.c | 23 +++++++++++++++++++++++
 security/security.c                    |  7 ++++++-
 3 files changed, 42 insertions(+), 1 deletion(-)

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/integrity.h b/include/linux/integrity.h
index 858d3f4a2241..54c853ec2fd1 100644
--- a/include/linux/integrity.h
+++ b/include/linux/integrity.h
@@ -44,4 +44,17 @@ static inline void integrity_load_keys(void)
 }
 #endif /* CONFIG_INTEGRITY */
 
+#ifdef CONFIG_INTEGRITY_ASYMMETRIC_KEYS
+
+extern int integrity_kernel_module_request(char *kmod_name);
+
+#else
+
+static inline int integrity_kernel_module_request(char *kmod_name)
+{
+	return 0;
+}
+
+#endif /* CONFIG_INTEGRITY_ASYMMETRIC_KEYS */
+
 #endif /* _LINUX_INTEGRITY_H */