[PATCH 08/16] binfmt_elf: Elf executable signature verification

Vivek Goyal
2013-09-10T21:45:11Z

Do elf executable signature verification (if one is present). If signature
is present, it should be valid. Validly signed executables are locked in
memory and a flag cred->proc_signed gets set to signify this process
executable contents are signed.

If file is unsigned, it can execute but it does not have the cred->proc_signed
set.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
fs/Kconfig.binfmt | 10 +++++++++
fs/binfmt_elf.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++-
include/linux/cred.h | 2 ++
kernel/cred.c | 2 ++
4 files changed, 76 insertions(+), 1 deletion(-)

diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
index 370b24c..25ae6d3 100644
--- a/fs/Kconfig.binfmt
+++ b/fs/Kconfig.binfmt
@@ -23,6 +23,16 @@ config BINFMT_ELF
ld.so (check the file <file:Documentation/Changes> for location and
latest version).

+config BINFMT_ELF_SIG
+ bool "ELF binary signature verification"
+ depends on BINFMT_ELF
+ depends on INTEGRITY_ASYMMETRIC_KEYS
+ depends on IMA_APPRAISE
+ depends on SYSTEM_TRUSTED_KEYRING
+ default n
+ ---help---
+ Check ELF binary signature verfication.
+
config COMPAT_BINFMT_ELF
bool
depends on COMPAT && BINFMT_ELF
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 100edcc..22a8272 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -34,6 +34,8 @@
#include <linux/utsname.h>
#include <linux/coredump.h>
#include <linux/sched.h>
+#include <linux/ima.h>
+#include <keys/system_keyring.h>
#include <asm/uaccess.h>
#include <asm/param.h>
#include <asm/page.h>
@@ -584,6 +586,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
int executable_stack = EXSTACK_DEFAULT;
unsigned long def_flags = 0;
struct pt_regs *regs = current_pt_regs();
+ char *signature = NULL;
+#ifdef CONFIG_BINFMT_ELF_SIG
+ unsigned int siglen = 0;
+ bool mlock_mappings = false;
+#endif
struct {
struct elfhdr elf_ex;
struct elfhdr interp_elf_ex;
@@ -725,6 +732,43 @@ static int load_elf_binary(struct linux_binprm *bprm)
/* OK, This is the point of no return */
current->mm->def_flags = def_flags;

+#ifdef CONFIG_BINFMT_ELF_SIG
+ /*
+ * If executable is digitally signed and ima memlock info present,
+ * Lock down in memory
+ */
+ retval = ima_file_signature_alloc(bprm->file, &signature);
+
+ /*
+ * If there is an error getting signature, bail out. Having
+ * no signature is fine though.
+ */
+ if (retval < 0 && retval != -ENODATA && retval != -EOPNOTSUPP)
+ goto out_free_dentry;
+
+ if (signature != NULL) {
+ siglen = retval;
+ retval = ima_signature_type(signature);
+ if (retval == EVM_IMA_XATTR_DIGSIG &&
+ ima_memlock_file(signature, siglen)) {
+ /*
+ * Verify signature before locking down file. We don't
+ * want to memlock executables with fake signatures
+ */
+ retval = ima_appraise_file_digsig(
+ system_trusted_keyring,
+ bprm->file, signature, siglen);
+ if (retval) {
+ send_sig(SIGKILL, current, 0);
+ goto out_free_dentry;
+ }
+
+ mlock_mappings = true;
+ current->mm->def_flags |= VM_LOCKED;
+ set_bit(MMF_VM_LOCKED, &current->mm->flags);
+ }
+ }
+#endif
/* Do this immediately, since STACK_TOP as used in setup_arg_pages
may depend on the personality. */
SET_PERSONALITY(loc->elf_ex);
@@ -895,6 +939,23 @@ static int load_elf_binary(struct linux_binprm *bprm)
goto out_free_dentry;
}

+#ifdef CONFIG_BINFMT_ELF_SIG
+ if (mlock_mappings) {
+ /*
+ * File locked down in memory. Now it is safe against any
+ * modifications on disk by raw disk writes. Verify signature.
+ */
+ retval = ima_appraise_file_digsig(system_trusted_keyring,
+ bprm->file, signature, siglen);
+ if (retval) {
+ send_sig(SIGKILL, current, 0);
+ goto out_free_dentry;
+ }
+ /* Signature verification successful */
+ bprm->cred->proc_signed = true;
+ }
+#endif
+
if (elf_interpreter) {
unsigned long interp_map_addr = 0;

@@ -988,11 +1049,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
*/
ELF_PLAT_INIT(regs, reloc_func_desc);
#endif
-
start_thread(regs, elf_entry, bprm->p);
retval = 0;
out:
kfree(loc);
+ kfree(signature);
out_ret:
return retval;

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 04421e8..1f5f418 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -136,6 +136,8 @@ struct cred {
struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
struct group_info *group_info; /* supplementary groups for euid/fsgid */
struct rcu_head rcu; /* RCU deletion hook */
+ bool proc_signed; /* Executable signature have been
+ * verified post load */
};

extern void __put_cred(struct cred *);
diff --git a/kernel/cred.c b/kernel/cred.c
index e0573a4..589f1fa 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -299,6 +299,8 @@ struct cred *prepare_exec_creds(void)
new->process_keyring = NULL;
#endif

+ /* proc_signed status will be evaluated again from executable file */
+ new->proc_signed = false;
return new;
}

--
1.8.3.1