Re: [PATCH 3/3] tomoyo: robustify handling of mm->exe_file

Davidlohr Bueso
2015-02-20T16:28:27Z

On Fri, 2015-02-20 at 07:11 +0900, Tetsuo Handa wrote:
> Davidlohr Bueso wrote:
> > On Thu, 2015-02-19 at 20:07 +0900, Tetsuo Handa wrote:
> > > Why do we need to let the caller call path_put() ?
> > > There is no need to do like proc_exe_link() does, for
> > > tomoyo_get_exe() returns pathname as "char *".
> >
> > Having the pathname doesn't guarantee anything later, and thus doesn't
> > seem very robust in the manager call if it can be dropped during the
> > call... or can this never occur in this context?
> >
> tomoyo_get_exe() returns the pathname of executable of current thread.
> The executable of current thread cannot be changed while current thread
> is inside the manager call. Although the pathname of executable of
> current thread could be changed by other threads via namespace manipulation
> like pivot_root(), holding a reference guarantees nothing. Your patch helps
> for avoiding memory allocation with mmap_sem held, but does not robustify
> handling of mm->exe_file for tomoyo.

Fair enough, I won't argue. This is beyond the scope if what I'm trying
to accomplish here anyway. Are you ok with this instead?

8<--------------------------------------------------------------------
Subject: [PATCH v2 3/3] tomoyo: reduce mmap_sem hold for mm->exe_file

The mm->exe_file is currently serialized with mmap_sem (shared) in order
to both safely (1) read the file and (2) compute the realpath by calling
tomoyo_realpath_from_path, making it an absolute overkill. Good users will,
on the other hand, make use of the more standard get_mm_exe_file(), requiring
only holding the mmap_sem to read the value, and relying on reference
counting to make sure that the exe_file won't disappear underneath us.

While at it, do some very minor cleanups around tomoyo_get_exe(), such as
make it local to common.c

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
security/tomoyo/common.c | 33 +++++++++++++++++++++++++++++++--
security/tomoyo/common.h | 1 -
security/tomoyo/util.c | 22 ----------------------
3 files changed, 31 insertions(+), 25 deletions(-)

diff --git a/security/tomoyo/common.c b/security/tomoyo/common.c
index e0fb750..73ce629 100644
--- a/security/tomoyo/common.c
+++ b/security/tomoyo/common.c
@@ -908,6 +908,31 @@ static void tomoyo_read_manager(struct tomoyo_io_buffer *head)
}

/**
+ * tomoyo_get_exe - Get tomoyo_realpath() of current process.
+ *
+ * Returns the tomoyo_realpath() of current process on success, NULL otherwise.
+ *
+ * This function uses kzalloc(), so the caller must call kfree()
+ * if this function didn't return NULL.
+ */
+static const char *tomoyo_get_exe(void)
+{
+ struct file *exe_file;
+ const char *cp = NULL;
+ struct mm_struct *mm = current->mm;
+
+ if (!mm)
+ return NULL;
+ exe_file = get_mm_exe_file(mm);
+ if (!exe_file)
+ return NULL;
+
+ cp = tomoyo_realpath_from_path(&exe_file->f_path);
+ fput(exe_file);
+ return cp;
+}
+
+/**
* tomoyo_manager - Check whether the current process is a policy manager.
*
* Returns true if the current process is permitted to modify policy
@@ -920,9 +945,11 @@ static bool tomoyo_manager(void)
struct tomoyo_manager *ptr;
const char *exe;
const struct task_struct *task = current;
- const struct tomoyo_path_info *domainname = tomoyo_domain()->domainname;
+ const struct tomoyo_path_info *domainname;
bool found = false;

+ domainname = tomoyo_domain()->domainname;
+
if (!tomoyo_policy_loaded)
return true;
if (!tomoyo_manage_by_non_root &&
@@ -932,8 +959,10 @@ static bool tomoyo_manager(void)
exe = tomoyo_get_exe();
if (!exe)
return false;
+
list_for_each_entry_rcu(ptr, &tomoyo_kernel_namespace.
- policy_list[TOMOYO_ID_MANAGER], head.list) {
+ policy_list[TOMOYO_ID_MANAGER],
+ head.list) {
if (!ptr->head.is_deleted &&
(!tomoyo_pathcmp(domainname, ptr->manager) ||
!strcmp(exe, ptr->manager->name))) {
diff --git a/security/tomoyo/common.h b/security/tomoyo/common.h
index b897d48..fc89eba 100644
--- a/security/tomoyo/common.h
+++ b/security/tomoyo/common.h
@@ -947,7 +947,6 @@ char *tomoyo_init_log(struct tomoyo_request_info *r, int len, const char *fmt,
char *tomoyo_read_token(struct tomoyo_acl_param *param);
char *tomoyo_realpath_from_path(struct path *path);
char *tomoyo_realpath_nofollow(const char *pathname);
-const char *tomoyo_get_exe(void);
const char *tomoyo_yesno(const unsigned int value);
const struct tomoyo_path_info *tomoyo_compare_name_union
(const struct tomoyo_path_info *name, const struct tomoyo_name_union *ptr);
diff --git a/security/tomoyo/util.c b/security/tomoyo/util.c
index 2952ba5..7eff479 100644
--- a/security/tomoyo/util.c
+++ b/security/tomoyo/util.c
@@ -939,28 +939,6 @@ bool tomoyo_path_matches_pattern(const struct tomoyo_path_info *filename,
}

/**
- * tomoyo_get_exe - Get tomoyo_realpath() of current process.
- *
- * Returns the tomoyo_realpath() of current process on success, NULL otherwise.
- *
- * This function uses kzalloc(), so the caller must call kfree()
- * if this function didn't return NULL.
- */
-const char *tomoyo_get_exe(void)
-{
- struct mm_struct *mm = current->mm;
- const char *cp = NULL;
-
- if (!mm)
- return NULL;
- down_read(&mm->mmap_sem);
- if (mm->exe_file)
- cp = tomoyo_realpath_from_path(&mm->exe_file->f_path);
- up_read(&mm->mmap_sem);
- return cp;
-}
-
-/**
* tomoyo_get_mode - Get MAC mode.
*
* @ns: Pointer to "struct tomoyo_policy_namespace".
--
2.1.4