[PATCH v2 2/8] exec: Factor security_bprm_creds_for_exec out of security_bprm_set_creds

From: Eric W. Biederman
Date: Mon May 18 2020 - 20:33:55 EST


Today security_bprm_set_creds has several implementations:
apparmor_bprm_set_creds, cap_bprm_set_creds, selinux_bprm_set_creds,
smack_bprm_set_creds, and tomoyo_bprm_set_creds.

Except for cap_bprm_set_creds they all test bprm->called_set_creds and
return immediately if it is true. The function cap_bprm_set_creds
ignores bprm->calld_sed_creds entirely.

Create a new LSM hook security_bprm_creds_for_exec that is called just
before prepare_binprm in __do_execve_file, resulting in a LSM hook
that is called exactly once for the entire of exec. Modify the bits
of security_bprm_set_creds that only want to be called once per exec
into security_bprm_creds_for_exec, leaving only cap_bprm_set_creds
behind.

Remove bprm->called_set_creds all of it's former users have been moved
to security_bprm_creds_for_exec.

Add or upate comments a appropriate to bring them up to date and
to reflect this change.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
fs/exec.c | 6 +++-
include/linux/binfmts.h | 18 +++--------
include/linux/lsm_hook_defs.h | 1 +
include/linux/lsm_hooks.h | 50 +++++++++++++++++-------------
include/linux/security.h | 6 ++++
security/apparmor/domain.c | 7 ++---
security/apparmor/include/domain.h | 2 +-
security/apparmor/lsm.c | 2 +-
security/security.c | 5 +++
security/selinux/hooks.c | 8 ++---
security/smack/smack_lsm.c | 9 ++----
security/tomoyo/tomoyo.c | 12 ++-----
12 files changed, 63 insertions(+), 63 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 14b786158aa9..9e70da47f8d9 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1640,7 +1640,6 @@ int prepare_binprm(struct linux_binprm *bprm)
retval = security_bprm_set_creds(bprm);
if (retval)
return retval;
- bprm->called_set_creds = 1;

memset(bprm->buf, 0, BINPRM_BUF_SIZE);
return kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE, &pos);
@@ -1855,6 +1854,11 @@ static int __do_execve_file(int fd, struct filename *filename,
if (retval < 0)
goto out;

+ /* Set the unchanging part of bprm->cred */
+ retval = security_bprm_creds_for_exec(bprm);
+ if (retval)
+ goto out;
+
retval = prepare_binprm(bprm);
if (retval < 0)
goto out;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 1b48e2154766..d1217fcdedea 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -27,22 +27,14 @@ struct linux_binprm {
unsigned long argmin; /* rlimit marker for copy_strings() */
unsigned int
/*
- * True after the bprm_set_creds hook has been called once
- * (multiple calls can be made via prepare_binprm() for
- * binfmt_script/misc).
- */
- called_set_creds:1,
- /*
- * True if most recent call to the commoncaps bprm_set_creds
- * hook (due to multiple prepare_binprm() calls from the
- * binfmt_script/misc handlers) resulted in elevated
- * privileges.
+ * True if most recent call to cap_bprm_set_creds
+ * resulted in elevated privileges.
*/
cap_elevated:1,
/*
- * Set by bprm_set_creds hook to indicate a privilege-gaining
- * exec has happened. Used to sanitize execution environment
- * and to set AT_SECURE auxv for glibc.
+ * Set by bprm_creds_for_exec hook to indicate a
+ * privilege-gaining exec has happened. Used to set
+ * AT_SECURE auxv for glibc.
*/
secureexec:1,
/*
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 9cd4455528e5..aab0695f41df 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -49,6 +49,7 @@ LSM_HOOK(int, 0, syslog, int type)
LSM_HOOK(int, 0, settime, const struct timespec64 *ts,
const struct timezone *tz)
LSM_HOOK(int, 0, vm_enough_memory, struct mm_struct *mm, long pages)
+LSM_HOOK(int, 0, bprm_creds_for_exec, struct linux_binprm *bprm)
LSM_HOOK(int, 0, bprm_set_creds, struct linux_binprm *bprm)
LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 988ca0df7824..c719af37df20 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -34,40 +34,46 @@
*
* Security hooks for program execution operations.
*
+ * @bprm_creds_for_exec:
+ * If the setup in prepare_exec_creds did not setup @bprm->cred->security
+ * properly for executing @bprm->file, update the LSM's portion of
+ * @bprm->cred->security to be what commit_creds needs to install for the
+ * new program. This hook may also optionally check permissions
+ * (e.g. for transitions between security domains).
+ * The hook must set @bprm->secureexec to 1 if AT_SECURE should be set to
+ * request libc enable secure mode.
+ * @bprm contains the linux_binprm structure.
+ * Return 0 if the hook is successful and permission is granted.
* @bprm_set_creds:
- * Save security information in the bprm->security field, typically based
- * on information about the bprm->file, for later use by the apply_creds
- * hook. This hook may also optionally check permissions (e.g. for
+ * Assuming that the relevant bits of @bprm->cred->security have been
+ * previously set, examine @bprm->file and regenerate them. This is
+ * so that the credentials derived from the interpreter the code is
+ * actually going to run are used rather than credentials derived
+ * from a script. This done because the interpreter binary needs to
+ * reopen script, and may end up opening something completely different.
+ * This hook may also optionally check permissions (e.g. for
* transitions between security domains).
- * This hook may be called multiple times during a single execve, e.g. for
- * interpreters. The hook can tell whether it has already been called by
- * checking to see if @bprm->security is non-NULL. If so, then the hook
- * may decide either to retain the security information saved earlier or
- * to replace it. The hook must set @bprm->secureexec to 1 if a "secure
- * exec" has happened as a result of this hook call. The flag is used to
- * indicate the need for a sanitized execution environment, and is also
- * passed in the ELF auxiliary table on the initial stack to indicate
- * whether libc should enable secure mode.
+ * The hook must set @bprm->cap_elevated to 1 if AT_SECURE should be set to
+ * request libc enable secure mode.
* @bprm contains the linux_binprm structure.
* Return 0 if the hook is successful and permission is granted.
* @bprm_check_security:
* This hook mediates the point when a search for a binary handler will
- * begin. It allows a check the @bprm->security value which is set in the
- * preceding set_creds call. The primary difference from set_creds is
- * that the argv list and envp list are reliably available in @bprm. This
- * hook may be called multiple times during a single execve; and in each
- * pass set_creds is called first.
+ * begin. It allows a check against the @bprm->cred->security value
+ * which was set in the preceding creds_for_exec call. The argv list and
+ * envp list are reliably available in @bprm. This hook may be called
+ * multiple times during a single execve.
* @bprm contains the linux_binprm structure.
* Return 0 if the hook is successful and permission is granted.
* @bprm_committing_creds:
* Prepare to install the new security attributes of a process being
* transformed by an execve operation, based on the old credentials
* pointed to by @current->cred and the information set in @bprm->cred by
- * the bprm_set_creds hook. @bprm points to the linux_binprm structure.
- * This hook is a good place to perform state changes on the process such
- * as closing open file descriptors to which access will no longer be
- * granted when the attributes are changed. This is called immediately
- * before commit_creds().
+ * the bprm_creds_for_exec hook. @bprm points to the linux_binprm
+ * structure. This hook is a good place to perform state changes on the
+ * process such as closing open file descriptors to which access will no
+ * longer be granted when the attributes are changed. This is called
+ * immediately before commit_creds().
* @bprm_committed_creds:
* Tidy up after the installation of the new security attributes of a
* process being transformed by an execve operation. The new credentials
diff --git a/include/linux/security.h b/include/linux/security.h
index a8d9310472df..1bd7a6582775 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -276,6 +276,7 @@ int security_quota_on(struct dentry *dentry);
int security_syslog(int type);
int security_settime64(const struct timespec64 *ts, const struct timezone *tz);
int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
+int security_bprm_creds_for_exec(struct linux_binprm *bprm);
int security_bprm_set_creds(struct linux_binprm *bprm);
int security_bprm_check(struct linux_binprm *bprm);
void security_bprm_committing_creds(struct linux_binprm *bprm);
@@ -569,6 +570,11 @@ static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
return __vm_enough_memory(mm, pages, cap_vm_enough_memory(mm, pages));
}

+static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
+{
+ return 0;
+}
+
static inline int security_bprm_set_creds(struct linux_binprm *bprm)
{
return cap_bprm_set_creds(bprm);
diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
index 6ceb74e0f789..0b870a647488 100644
--- a/security/apparmor/domain.c
+++ b/security/apparmor/domain.c
@@ -854,14 +854,14 @@ static struct aa_label *handle_onexec(struct aa_label *label,
}

/**
- * apparmor_bprm_set_creds - set the new creds on the bprm struct
+ * apparmor_bprm_creds_for_exec - Update the new creds on the bprm struct
* @bprm: binprm for the exec (NOT NULL)
*
* Returns: %0 or error on failure
*
* TODO: once the other paths are done see if we can't refactor into a fn
*/
-int apparmor_bprm_set_creds(struct linux_binprm *bprm)
+int apparmor_bprm_creds_for_exec(struct linux_binprm *bprm)
{
struct aa_task_ctx *ctx;
struct aa_label *label, *new = NULL;
@@ -875,9 +875,6 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
file_inode(bprm->file)->i_mode
};

- if (bprm->called_set_creds)
- return 0;
-
ctx = task_ctx(current);
AA_BUG(!cred_label(bprm->cred));
AA_BUG(!ctx);
diff --git a/security/apparmor/include/domain.h b/security/apparmor/include/domain.h
index 21b875fe2d37..d14928fe1c6f 100644
--- a/security/apparmor/include/domain.h
+++ b/security/apparmor/include/domain.h
@@ -30,7 +30,7 @@ struct aa_domain {
struct aa_label *x_table_lookup(struct aa_profile *profile, u32 xindex,
const char **name);

-int apparmor_bprm_set_creds(struct linux_binprm *bprm);
+int apparmor_bprm_creds_for_exec(struct linux_binprm *bprm);

void aa_free_domain_entries(struct aa_domain *domain);
int aa_change_hat(const char *hats[], int count, u64 token, int flags);
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index b621ad74f54a..3623ab08279d 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1232,7 +1232,7 @@ static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(cred_prepare, apparmor_cred_prepare),
LSM_HOOK_INIT(cred_transfer, apparmor_cred_transfer),

- LSM_HOOK_INIT(bprm_set_creds, apparmor_bprm_set_creds),
+ LSM_HOOK_INIT(bprm_creds_for_exec, apparmor_bprm_creds_for_exec),
LSM_HOOK_INIT(bprm_committing_creds, apparmor_bprm_committing_creds),
LSM_HOOK_INIT(bprm_committed_creds, apparmor_bprm_committed_creds),

diff --git a/security/security.c b/security/security.c
index 7fed24b9d57e..4ee76a729f73 100644
--- a/security/security.c
+++ b/security/security.c
@@ -823,6 +823,11 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
return __vm_enough_memory(mm, pages, cap_sys_admin);
}

+int security_bprm_creds_for_exec(struct linux_binprm *bprm)
+{
+ return call_int_hook(bprm_creds_for_exec, 0, bprm);
+}
+
int security_bprm_set_creds(struct linux_binprm *bprm)
{
return call_int_hook(bprm_set_creds, 0, bprm);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 0b4e32161b77..718345dd76bb 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2286,7 +2286,7 @@ static int check_nnp_nosuid(const struct linux_binprm *bprm,
return -EACCES;
}

-static int selinux_bprm_set_creds(struct linux_binprm *bprm)
+static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
{
const struct task_security_struct *old_tsec;
struct task_security_struct *new_tsec;
@@ -2297,8 +2297,6 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)

/* SELinux context only depends on initial program or script and not
* the script interpreter */
- if (bprm->called_set_creds)
- return 0;

old_tsec = selinux_cred(current_cred());
new_tsec = selinux_cred(bprm->cred);
@@ -6385,7 +6383,7 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
/* Permission checking based on the specified context is
performed during the actual operation (execve,
open/mkdir/...), when we know the full context of the
- operation. See selinux_bprm_set_creds for the execve
+ operation. See selinux_bprm_creds_for_exec for the execve
checks and may_create for the file creation checks. The
operation will then fail if the context is not permitted. */
tsec = selinux_cred(new);
@@ -6914,7 +6912,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {

LSM_HOOK_INIT(netlink_send, selinux_netlink_send),

- LSM_HOOK_INIT(bprm_set_creds, selinux_bprm_set_creds),
+ LSM_HOOK_INIT(bprm_creds_for_exec, selinux_bprm_creds_for_exec),
LSM_HOOK_INIT(bprm_committing_creds, selinux_bprm_committing_creds),
LSM_HOOK_INIT(bprm_committed_creds, selinux_bprm_committed_creds),

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 8c61d175e195..0ac8f4518d07 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -891,12 +891,12 @@ static int smack_sb_statfs(struct dentry *dentry)
*/

/**
- * smack_bprm_set_creds - set creds for exec
+ * smack_bprm_creds_for_exec - Update bprm->cred if needed for exec
* @bprm: the exec information
*
* Returns 0 if it gets a blob, -EPERM if exec forbidden and -ENOMEM otherwise
*/
-static int smack_bprm_set_creds(struct linux_binprm *bprm)
+static int smack_bprm_creds_for_exec(struct linux_binprm *bprm)
{
struct inode *inode = file_inode(bprm->file);
struct task_smack *bsp = smack_cred(bprm->cred);
@@ -904,9 +904,6 @@ static int smack_bprm_set_creds(struct linux_binprm *bprm)
struct superblock_smack *sbsp;
int rc;

- if (bprm->called_set_creds)
- return 0;
-
isp = smack_inode(inode);
if (isp->smk_task == NULL || isp->smk_task == bsp->smk_task)
return 0;
@@ -4598,7 +4595,7 @@ static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(sb_statfs, smack_sb_statfs),
LSM_HOOK_INIT(sb_set_mnt_opts, smack_set_mnt_opts),

- LSM_HOOK_INIT(bprm_set_creds, smack_bprm_set_creds),
+ LSM_HOOK_INIT(bprm_creds_for_exec, smack_bprm_creds_for_exec),

LSM_HOOK_INIT(inode_alloc_security, smack_inode_alloc_security),
LSM_HOOK_INIT(inode_init_security, smack_inode_init_security),
diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index 716c92ec941a..f9adddc42ac8 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -63,20 +63,14 @@ static void tomoyo_bprm_committed_creds(struct linux_binprm *bprm)

#ifndef CONFIG_SECURITY_TOMOYO_OMIT_USERSPACE_LOADER
/**
- * tomoyo_bprm_set_creds - Target for security_bprm_set_creds().
+ * tomoyo_bprm_for_exec - Target for security_bprm_creds_for_exec().
*
* @bprm: Pointer to "struct linux_binprm".
*
* Returns 0.
*/
-static int tomoyo_bprm_set_creds(struct linux_binprm *bprm)
+static int tomoyo_bprm_creds_for_exec(struct linux_binprm *bprm)
{
- /*
- * Do only if this function is called for the first time of an execve
- * operation.
- */
- if (bprm->called_set_creds)
- return 0;
/*
* Load policy if /sbin/tomoyo-init exists and /sbin/init is requested
* for the first time.
@@ -539,7 +533,7 @@ static struct security_hook_list tomoyo_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(task_alloc, tomoyo_task_alloc),
LSM_HOOK_INIT(task_free, tomoyo_task_free),
#ifndef CONFIG_SECURITY_TOMOYO_OMIT_USERSPACE_LOADER
- LSM_HOOK_INIT(bprm_set_creds, tomoyo_bprm_set_creds),
+ LSM_HOOK_INIT(bprm_creds_for_exec, tomoyo_bprm_creds_for_exec),
#endif
LSM_HOOK_INIT(bprm_check_security, tomoyo_bprm_check_security),
LSM_HOOK_INIT(file_fcntl, tomoyo_file_fcntl),
--
2.25.0