[v3,11/19] LSM: Introduce kernel_post_load_data() hook
diff mbox series

Message ID 20200724213640.389191-12-keescook@chromium.org
State New
Headers show
Series
  • Introduce partial kernel_read_file() support
Related show

Commit Message

Kees Cook July 24, 2020, 9:36 p.m. UTC
There are a few places in the kernel where LSMs would like to have
visibility into the contents of a kernel buffer that has been loaded or
read. While security_kernel_post_read_file() (which includes the
buffer) exists as a pairing for security_kernel_read_file(), no such
hook exists to pair with security_kernel_load_data().

Earlier proposals for just using security_kernel_post_read_file() with a
NULL file argument were rejected (i.e. "file" should always be valid for
the security_..._file hooks, but it appears at least one case was
left in the kernel during earlier refactoring. (This will be fixed in
a subsequent patch.)

Since not all cases of security_kernel_load_data() can have a single
contiguous buffer made available to the LSM hook (e.g. kexec image
segments are separately loaded), there needs to be a way for the LSM to
reason about its expectations of the hook coverage. In order to handle
this, add a "contents" argument to the "kernel_load_data" hook that
indicates if the newly added "kernel_post_load_data" hook will be called
with the full contents once loaded. That way, LSMs requiring full contents
can choose to unilaterally reject "kernel_load_data" with contents=false
(which is effectively the existing hook coverage), but when contents=true
they can allow it and later evaluate the "kernel_post_load_data" hook
once the buffer is loaded.

With this change, LSMs can gain coverage over non-file-backed data loads
(e.g. init_module(2) and firmware userspace helper), which will happen
in subsequent patches.

Additionally prepare IMA to start processing these cases.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/base/firmware_loader/fallback.c       |  2 +-
 .../base/firmware_loader/fallback_platform.c  |  2 +-
 include/linux/ima.h                           | 12 +++++++++--
 include/linux/lsm_hook_defs.h                 |  4 +++-
 include/linux/lsm_hooks.h                     |  9 ++++++++
 include/linux/security.h                      | 12 +++++++++--
 kernel/kexec.c                                |  2 +-
 kernel/module.c                               |  2 +-
 security/integrity/ima/ima_main.c             | 21 ++++++++++++++++++-
 security/loadpin/loadpin.c                    |  2 +-
 security/security.c                           | 18 +++++++++++++---
 security/selinux/hooks.c                      |  2 +-
 12 files changed, 73 insertions(+), 15 deletions(-)

Comments

Mimi Zohar July 27, 2020, 10:49 a.m. UTC | #1
On Fri, 2020-07-24 at 14:36 -0700, Kees Cook wrote:
> There are a few places in the kernel where LSMs would like to have
> visibility into the contents of a kernel buffer that has been loaded or
> read. While security_kernel_post_read_file() (which includes the
> buffer) exists as a pairing for security_kernel_read_file(), no such
> hook exists to pair with security_kernel_load_data().
> 
> Earlier proposals for just using security_kernel_post_read_file() with a
> NULL file argument were rejected (i.e. "file" should always be valid for
> the security_..._file hooks, but it appears at least one case was
> left in the kernel during earlier refactoring. (This will be fixed in
> a subsequent patch.)
> 
> Since not all cases of security_kernel_load_data() can have a single
> contiguous buffer made available to the LSM hook (e.g. kexec image
> segments are separately loaded), there needs to be a way for the LSM to
> reason about its expectations of the hook coverage. In order to handle
> this, add a "contents" argument to the "kernel_load_data" hook that
> indicates if the newly added "kernel_post_load_data" hook will be called
> with the full contents once loaded. That way, LSMs requiring full contents
> can choose to unilaterally reject "kernel_load_data" with contents=false
> (which is effectively the existing hook coverage), but when contents=true
> they can allow it and later evaluate the "kernel_post_load_data" hook
> once the buffer is loaded.
> 
> With this change, LSMs can gain coverage over non-file-backed data loads
> (e.g. init_module(2) and firmware userspace helper), which will happen
> in subsequent patches.
> 
> Additionally prepare IMA to start processing these cases.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>

At least from an IMA perspective, the original
security_kernel_load_data() hook was defined in order to prevent
certain syscalls - init_module, kexec_load - and loading firmware via
sysfs.  The resulting error messages were generic.
  
Unlike security_kernel_load_data(), security_kernel_post_load_data()
is meant to be used, but without a file desciptor specific
information, like the filename associated with the buffer, is missing.
 Having the filename isn't actually necessary for verifying the
appended signature, but it is needed for auditing signature
verification failures and including in the IMA measurement list.

Mimi
Kees Cook July 28, 2020, 7:41 p.m. UTC | #2
On Mon, Jul 27, 2020 at 06:49:11AM -0400, Mimi Zohar wrote:
> On Fri, 2020-07-24 at 14:36 -0700, Kees Cook wrote:
> > There are a few places in the kernel where LSMs would like to have
> > visibility into the contents of a kernel buffer that has been loaded or
> > read. While security_kernel_post_read_file() (which includes the
> > buffer) exists as a pairing for security_kernel_read_file(), no such
> > hook exists to pair with security_kernel_load_data().
> > 
> > Earlier proposals for just using security_kernel_post_read_file() with a
> > NULL file argument were rejected (i.e. "file" should always be valid for
> > the security_..._file hooks, but it appears at least one case was
> > left in the kernel during earlier refactoring. (This will be fixed in
> > a subsequent patch.)
> > 
> > Since not all cases of security_kernel_load_data() can have a single
> > contiguous buffer made available to the LSM hook (e.g. kexec image
> > segments are separately loaded), there needs to be a way for the LSM to
> > reason about its expectations of the hook coverage. In order to handle
> > this, add a "contents" argument to the "kernel_load_data" hook that
> > indicates if the newly added "kernel_post_load_data" hook will be called
> > with the full contents once loaded. That way, LSMs requiring full contents
> > can choose to unilaterally reject "kernel_load_data" with contents=false
> > (which is effectively the existing hook coverage), but when contents=true
> > they can allow it and later evaluate the "kernel_post_load_data" hook
> > once the buffer is loaded.
> > 
> > With this change, LSMs can gain coverage over non-file-backed data loads
> > (e.g. init_module(2) and firmware userspace helper), which will happen
> > in subsequent patches.
> > 
> > Additionally prepare IMA to start processing these cases.
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> 
> At least from an IMA perspective, the original
> security_kernel_load_data() hook was defined in order to prevent
> certain syscalls - init_module, kexec_load - and loading firmware via
> sysfs.  The resulting error messages were generic.
>   
> Unlike security_kernel_load_data(), security_kernel_post_load_data()
> is meant to be used, but without a file desciptor specific
> information, like the filename associated with the buffer, is missing.
>  Having the filename isn't actually necessary for verifying the
> appended signature, but it is needed for auditing signature
> verification failures and including in the IMA measurement list.

Right -- I'm open to ideas on this, but as it stands, other LSMs (e.g.
BPF LSM) can benefit from the security_kernel_post_load_data() to
examine the contents, etc.

Is there anything that needs to change in this patch?

Patch
diff mbox series

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index 5327bfc6ba71..a196aacce22c 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -613,7 +613,7 @@  static bool fw_run_sysfs_fallback(u32 opt_flags)
 		return false;
 
 	/* Also permit LSMs and IMA to fail firmware sysfs fallback */
-	ret = security_kernel_load_data(LOADING_FIRMWARE);
+	ret = security_kernel_load_data(LOADING_FIRMWARE, false);
 	if (ret < 0)
 		return false;
 
diff --git a/drivers/base/firmware_loader/fallback_platform.c b/drivers/base/firmware_loader/fallback_platform.c
index 6958ab1a8059..a12c79d47efc 100644
--- a/drivers/base/firmware_loader/fallback_platform.c
+++ b/drivers/base/firmware_loader/fallback_platform.c
@@ -17,7 +17,7 @@  int firmware_fallback_platform(struct fw_priv *fw_priv, u32 opt_flags)
 	if (!(opt_flags & FW_OPT_FALLBACK_PLATFORM))
 		return -ENOENT;
 
-	rc = security_kernel_load_data(LOADING_FIRMWARE);
+	rc = security_kernel_load_data(LOADING_FIRMWARE, false);
 	if (rc)
 		return rc;
 
diff --git a/include/linux/ima.h b/include/linux/ima.h
index 148636bfcc8f..502e36ad7804 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h
@@ -20,7 +20,9 @@  extern void ima_post_create_tmpfile(struct inode *inode);
 extern void ima_file_free(struct file *file);
 extern int ima_file_mmap(struct file *file, unsigned long prot);
 extern int ima_file_mprotect(struct vm_area_struct *vma, unsigned long prot);
-extern int ima_load_data(enum kernel_load_data_id id);
+extern int ima_load_data(enum kernel_load_data_id id, bool contents);
+extern int ima_post_load_data(char *buf, loff_t size,
+			      enum kernel_load_data_id id);
 extern int ima_read_file(struct file *file, enum kernel_read_file_id id);
 extern int ima_post_read_file(struct file *file, void *buf, loff_t size,
 			      enum kernel_read_file_id id);
@@ -78,7 +80,13 @@  static inline int ima_file_mprotect(struct vm_area_struct *vma,
 	return 0;
 }
 
-static inline int ima_load_data(enum kernel_load_data_id id)
+static inline int ima_load_data(enum kernel_load_data_id id, bool contents)
+{
+	return 0;
+}
+
+static inline int ima_post_load_data(char *buf, loff_t size,
+				     enum kernel_load_data_id id)
 {
 	return 0;
 }
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 6791813cd439..aaa2916bbae7 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -184,7 +184,9 @@  LSM_HOOK(void, LSM_RET_VOID, cred_getsecid, const struct cred *c, u32 *secid)
 LSM_HOOK(int, 0, kernel_act_as, struct cred *new, u32 secid)
 LSM_HOOK(int, 0, kernel_create_files_as, struct cred *new, struct inode *inode)
 LSM_HOOK(int, 0, kernel_module_request, char *kmod_name)
-LSM_HOOK(int, 0, kernel_load_data, enum kernel_load_data_id id)
+LSM_HOOK(int, 0, kernel_load_data, enum kernel_load_data_id id, bool contents)
+LSM_HOOK(int, 0, kernel_post_load_data, char *buf, loff_t size,
+	 enum kernel_read_file_id id)
 LSM_HOOK(int, 0, kernel_read_file, struct file *file,
 	 enum kernel_read_file_id id)
 LSM_HOOK(int, 0, kernel_post_read_file, struct file *file, char *buf,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 95b7c1d32062..812d626195fc 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -635,7 +635,16 @@ 
  * @kernel_load_data:
  *	Load data provided by userspace.
  *	@id kernel load data identifier
+ *	@contents if a subsequent @kernel_post_load_data will be called.
  *	Return 0 if permission is granted.
+ * @kernel_post_load_data:
+ *	Load data provided by a non-file source (usually userspace buffer).
+ *	@buf pointer to buffer containing the data contents.
+ *	@size length of the data contents.
+ *	@id kernel load data identifier
+ *	Return 0 if permission is granted.
+ *	This must be paired with a prior @kernel_load_data call that had
+ *	@contents set to true.
  * @kernel_read_file:
  *	Read a file specified by userspace.
  *	@file contains the file structure pointing to the file being read
diff --git a/include/linux/security.h b/include/linux/security.h
index 42df0d9b4c37..e748974c707b 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -387,7 +387,9 @@  void security_cred_getsecid(const struct cred *c, u32 *secid);
 int security_kernel_act_as(struct cred *new, u32 secid);
 int security_kernel_create_files_as(struct cred *new, struct inode *inode);
 int security_kernel_module_request(char *kmod_name);
-int security_kernel_load_data(enum kernel_load_data_id id);
+int security_kernel_load_data(enum kernel_load_data_id id, bool contents);
+int security_kernel_post_load_data(char *buf, loff_t size,
+				   enum kernel_load_data_id id);
 int security_kernel_read_file(struct file *file, enum kernel_read_file_id id);
 int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
 				   enum kernel_read_file_id id);
@@ -1014,7 +1016,13 @@  static inline int security_kernel_module_request(char *kmod_name)
 	return 0;
 }
 
-static inline int security_kernel_load_data(enum kernel_load_data_id id)
+static inline int security_kernel_load_data(enum kernel_load_data_id id, bool contents)
+{
+	return 0;
+}
+
+static inline int security_kernel_post_load_data(char *buf, loff_t size,
+						 enum kernel_load_data_id id)
 {
 	return 0;
 }
diff --git a/kernel/kexec.c b/kernel/kexec.c
index f977786fe498..c82c6c06f051 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -205,7 +205,7 @@  static inline int kexec_load_check(unsigned long nr_segments,
 		return -EPERM;
 
 	/* Permit LSMs and IMA to fail the kexec */
-	result = security_kernel_load_data(LOADING_KEXEC_IMAGE);
+	result = security_kernel_load_data(LOADING_KEXEC_IMAGE, false);
 	if (result < 0)
 		return result;
 
diff --git a/kernel/module.c b/kernel/module.c
index 860d713dd910..d56cb34d9a2f 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2967,7 +2967,7 @@  static int copy_module_from_user(const void __user *umod, unsigned long len,
 	if (info->len < sizeof(*(info->hdr)))
 		return -ENOEXEC;
 
-	err = security_kernel_load_data(LOADING_MODULE);
+	err = security_kernel_load_data(LOADING_MODULE, false);
 	if (err)
 		return err;
 
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index dab4a13221cf..85000dc8595c 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -676,6 +676,8 @@  int ima_post_read_file(struct file *file, void *buf, loff_t size,
 /**
  * ima_load_data - appraise decision based on policy
  * @id: kernel load data caller identifier
+ * @contents: whether the full contents will be available in a later
+ *	      call to ima_post_load_data().
  *
  * Callers of this LSM hook can not measure, appraise, or audit the
  * data provided by userspace.  Enforce policy rules requring a file
@@ -683,7 +685,7 @@  int ima_post_read_file(struct file *file, void *buf, loff_t size,
  *
  * For permission return 0, otherwise return -EACCES.
  */
-int ima_load_data(enum kernel_load_data_id id)
+int ima_load_data(enum kernel_load_data_id id, bool contents)
 {
 	bool ima_enforce, sig_enforce;
 
@@ -723,6 +725,23 @@  int ima_load_data(enum kernel_load_data_id id)
 	return 0;
 }
 
+/**
+ * ima_post_load_data - appraise decision based on policy
+ * @buf: pointer to in memory file contents
+ * @size: size of in memory file contents
+ * @id: kernel load data caller identifier
+ *
+ * Measure/appraise/audit in memory buffer based on policy.  Policy rules
+ * are written in terms of a policy identifier.
+ *
+ * On success return 0.  On integrity appraisal error, assuming the file
+ * is in policy and IMA-appraisal is in enforcing mode, return -EACCES.
+ */
+int ima_post_load_data(char *buf, loff_t size, enum kernel_load_data_id load_id)
+{
+	return 0;
+}
+
 /*
  * process_buffer_measurement - Measure the buffer to ima log.
  * @buf: pointer to the buffer that needs to be added to the log.
diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index 81bc95127f92..db320a43f42e 100644
--- a/security/loadpin/loadpin.c
+++ b/security/loadpin/loadpin.c
@@ -176,7 +176,7 @@  static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
 	return 0;
 }
 
-static int loadpin_load_data(enum kernel_load_data_id id)
+static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 {
 	return loadpin_read_file(NULL, (enum kernel_read_file_id) id);
 }
diff --git a/security/security.c b/security/security.c
index f5920115a325..090674f1197a 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1680,17 +1680,29 @@  int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
 }
 EXPORT_SYMBOL_GPL(security_kernel_post_read_file);
 
-int security_kernel_load_data(enum kernel_load_data_id id)
+int security_kernel_load_data(enum kernel_load_data_id id, bool contents)
 {
 	int ret;
 
-	ret = call_int_hook(kernel_load_data, 0, id);
+	ret = call_int_hook(kernel_load_data, 0, id, contents);
 	if (ret)
 		return ret;
-	return ima_load_data(id);
+	return ima_load_data(id, contents);
 }
 EXPORT_SYMBOL_GPL(security_kernel_load_data);
 
+int security_kernel_post_load_data(char *buf, loff_t size,
+				   enum kernel_load_data_id id)
+{
+	int ret;
+
+	ret = call_int_hook(kernel_post_load_data, 0, buf, size, id);
+	if (ret)
+		return ret;
+	return ima_post_load_data(buf, size, id);
+}
+EXPORT_SYMBOL_GPL(security_kernel_post_load_data);
+
 int security_task_fix_setuid(struct cred *new, const struct cred *old,
 			     int flags)
 {
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 5de45010fb1a..1a5c68196faf 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4019,7 +4019,7 @@  static int selinux_kernel_read_file(struct file *file,
 	return rc;
 }
 
-static int selinux_kernel_load_data(enum kernel_load_data_id id)
+static int selinux_kernel_load_data(enum kernel_load_data_id id, bool contents)
 {
 	int rc = 0;