Re: [PATCH v2 11/18] LSM: Introduce kernel_post_load_data() hook

Scott Branden
2020-07-23T17:39:12Z

Patch 11-14 work for me but I have little knowledge to comment on these
patches.

On 2020-07-22 12:30 p.m., 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>
> ---
> 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(-)
>
> 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;
>