[PATCH v5 11/12] S.A.R.A.: /proc/*/mem write limitation

[Salvatore Mesoraca]

Prevent a task from opening, in "write" mode, any /proc/*/mem
file that operates on the task's mm.
A process could use it to overwrite read-only memory, bypassing
S.A.R.A. restrictions.

Signed-off-by: Salvatore Mesoraca <s.mesoraca16 at gmail.com>
---
 security/sara/include/sara_data.h | 18 ++++++++++++++++-
 security/sara/sara_data.c         |  8 ++++++++
 security/sara/wxprot.c            | 41 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/security/sara/include/sara_data.h b/security/sara/include/sara_data.h
index 9216c47..ee95f74 100644
--- a/security/sara/include/sara_data.h
+++ b/security/sara/include/sara_data.h
@@ -15,6 +15,7 @@
 #define __SARA_DATA_H
 
 #include <linux/cred.h>
+#include <linux/fs.h>
 #include <linux/init.h>
 #include <linux/lsm_hooks.h>
 #include <linux/spinlock.h>
@@ -40,6 +41,10 @@ struct sara_shm_data {
 	spinlock_t	lock;
 };
 
+struct sara_inode_data {
+	struct task_struct *task;
+};
+
 
 static inline struct sara_data *get_sara_data(const struct cred *cred)
 {
@@ -79,6 +84,17 @@ static inline struct sara_shm_data *get_sara_shm_data(
 #define lock_sara_shm(X) (spin_lock(&get_sara_shm_data((X))->lock))
 #define unlock_sara_shm(X) (spin_unlock(&get_sara_shm_data((X))->lock))
 
-#endif
+
+static inline struct sara_inode_data *get_sara_inode_data(
+						const struct inode *inode)
+{
+	if (unlikely(!inode->i_security))
+		return NULL;
+	return inode->i_security + sara_blob_sizes.lbs_inode;
+}
+
+#define get_sara_inode_task(X) (get_sara_inode_data((X))->task)
+
+#endif /* CONFIG_SECURITY_SARA_WXPROT */
 
 #endif /* __SARA_H */
diff --git a/security/sara/sara_data.c b/security/sara/sara_data.c
index 9afca37..e875cf0 100644
--- a/security/sara/sara_data.c
+++ b/security/sara/sara_data.c
@@ -17,6 +17,7 @@
 #include <linux/cred.h>
 #include <linux/lsm_hooks.h>
 #include <linux/mm.h>
+#include <linux/slab.h>
 #include <linux/spinlock.h>
 
 static int sara_cred_prepare(struct cred *new, const struct cred *old,
@@ -40,15 +41,22 @@ static int sara_shm_alloc_security(struct kern_ipc_perm *shp)
 	return 0;
 }
 
+static void sara_task_to_inode(struct task_struct *t, struct inode *i)
+{
+	get_sara_inode_task(i) = t;
+}
+
 static struct security_hook_list data_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(cred_prepare, sara_cred_prepare),
 	LSM_HOOK_INIT(cred_transfer, sara_cred_transfer),
 	LSM_HOOK_INIT(shm_alloc_security, sara_shm_alloc_security),
+	LSM_HOOK_INIT(task_to_inode, sara_task_to_inode),
 };
 
 struct lsm_blob_sizes sara_blob_sizes __lsm_ro_after_init = {
 	.lbs_cred = sizeof(struct sara_data),
 	.lbs_ipc = sizeof(struct sara_shm_data),
+	.lbs_inode = sizeof(struct sara_inode_data),
 };
 
 int __init sara_data_init(void)
diff --git a/security/sara/wxprot.c b/security/sara/wxprot.c
index 773d1fd..1a8d132 100644
--- a/security/sara/wxprot.c
+++ b/security/sara/wxprot.c
@@ -22,8 +22,11 @@
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/module.h>
+#include <linux/mount.h>
 #include <linux/printk.h>
 #include <linux/ratelimit.h>
+#include <linux/sched/mm.h>
+#include <linux/sched/task.h>
 #include <linux/spinlock.h>
 #include <linux/xattr.h>
 
@@ -615,6 +618,43 @@ static int sara_file_mprotect(struct vm_area_struct *vma,
 	return 0;
 }
 
+static int sara_file_open(struct file *file)
+{
+	struct task_struct *t;
+	struct mm_struct *mm;
+	u16 sara_wxp_flags = get_current_sara_wxp_flags();
+
+	/*
+	 * Prevent write access to /proc/.../mem
+	 * if it operates on the mm_struct of the
+	 * current process: it could be used to
+	 * bypass W^X.
+	 */
+
+	if (!sara_enabled ||
+	    !wxprot_enabled ||
+	    !(sara_wxp_flags & SARA_WXP_WXORX) ||
+	    !(file->f_mode & FMODE_WRITE))
+		return 0;
+
+	t = get_sara_inode_task(file_inode(file));
+	if (unlikely(t != NULL &&
+		     strcmp(file->f_path.dentry->d_name.name,
+			    "mem") == 0)) {
+		get_task_struct(t);
+		mm = get_task_mm(t);
+		put_task_struct(t);
+		if (unlikely(mm == current->mm))
+			sara_warn_or_goto(error,
+					  "write access to /proc/*/mem");
+		mmput(mm);
+	}
+	return 0;
+error:
+	mmput(mm);
+	return -EACCES;
+}
+
 #ifdef CONFIG_SECURITY_SARA_WXPROT_EMUTRAMP
 static int sara_pagefault_handler(struct pt_regs *regs,
 				  unsigned long error_code,
@@ -778,6 +818,7 @@ static int sara_setprocattr(const char *name, void *value, size_t size)
 	LSM_HOOK_INIT(check_vmflags, sara_check_vmflags),
 	LSM_HOOK_INIT(shm_shmat, sara_shm_shmat),
 	LSM_HOOK_INIT(file_mprotect, sara_file_mprotect),
+	LSM_HOOK_INIT(file_open, sara_file_open),
 #ifdef CONFIG_SECURITY_SARA_WXPROT_EMUTRAMP
 	LSM_HOOK_INIT(pagefault_handler, sara_pagefault_handler),
 #endif
-- 
1.9.1