[PATCH 4/4] binfmt_elf: Elf executable signature verification

Vivek Goyal
2013-03-15T20:36:11Z

Do elf executable signature verification (if one is present). If signature
is present, it should be valid. Validly signed files are given a capability
CAP_SIGNED.

If file is unsigned, it can execute but it does not get the capability
CAP_SIGNED.

This is work in progress. This patch is just an RFC to show how one
can go about making use of IMA APIs for executable signature
verification.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
fs/Kconfig.binfmt | 12 ++++++++++++
fs/binfmt_elf.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 56 insertions(+), 0 deletions(-)

diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
index 0efd152..cbb1d4a 100644
--- a/fs/Kconfig.binfmt
+++ b/fs/Kconfig.binfmt
@@ -23,6 +23,18 @@ 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
+ select INTEGRITY
+ select INTEGRITY_SIGNATURE
+ select INTEGRITY_ASYMMETRIC_KEYS
+ select IMA
+ select IMA_APPRAISE
+ 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 3939829..820ceb9 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -34,6 +34,7 @@
#include <linux/utsname.h>
#include <linux/coredump.h>
#include <linux/sched.h>
+#include <linux/ima.h>
#include <asm/uaccess.h>
#include <asm/param.h>
#include <asm/page.h>
@@ -581,6 +582,11 @@ 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;
+ unsigned int siglen = 0;
+ enum evm_ima_xattr_type sig_type;
+ bool mlock_mappings = false;
+
struct {
struct elfhdr elf_ex;
struct elfhdr interp_elf_ex;
@@ -722,6 +728,29 @@ 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. Lock down in memory */
+ /* Get file signature, if any */
+ 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, &sig_type);
+ if (!retval && sig_type == EVM_IMA_XATTR_DIGSIG_ASYMMETRIC)
+ mlock_mappings = true;
+ }
+
+ if (mlock_mappings)
+ current->mm->def_flags |= VM_LOCKED;
+
+#endif
/* Do this immediately, since STACK_TOP as used in setup_arg_pages
may depend on the personality. */
SET_PERSONALITY(loc->elf_ex);
@@ -893,6 +922,18 @@ static int load_elf_binary(struct linux_binprm *bprm)
goto out_free_dentry;
}

+#ifdef CONFIG_BINFMT_ELF_SIG
+ if (mlock_mappings) {
+ retval = ima_appraise_file(bprm->file, signature, siglen);
+ if (retval) {
+ send_sig(SIGKILL, current, 0);
+ goto out_free_dentry;
+ }
+ /* Signature verification successful */
+ cap_raise(bprm->cred->cap_effective, CAP_SIGNED);
+ }
+#endif
+
if (elf_interpreter) {
unsigned long interp_map_addr = 0;

@@ -986,11 +1027,14 @@ static int load_elf_binary(struct linux_binprm *bprm)
*/
ELF_PLAT_INIT(regs, reloc_func_desc);
#endif
+ if (mlock_mappings)
+ current->mm->def_flags &= ~VM_LOCKED;

start_thread(regs, elf_entry, bprm->p);
retval = 0;
out:
kfree(loc);
+ kfree(signature);
out_ret:
return retval;

--
1.7.7.6