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Commit 8b0db9db authored by Stephen Wilson's avatar Stephen Wilson Committed by Al Viro
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proc: make check_mem_permission() return an mm_struct on success 



This change allows us to take advantage of access_remote_vm(), which in turn
eliminates a security issue with the mem_write() implementation.

The previous implementation of mem_write() was insecure since the target task
could exec a setuid-root binary between the permission check and the actual
write.  Holding a reference to the target mm_struct eliminates this
vulnerability.

Signed-off-by: default avatarStephen Wilson <wilsons@start.ca>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 18f661bc

fs/proc/base.c

+34 −24
Original line number Diff line number Diff line
@@ -191,14 +191,20 @@ static int proc_root_link(struct inode *inode, struct path *path) 
	return result;

}



static int __check_mem_permission(struct task_struct *task)

static struct mm_struct *__check_mem_permission(struct task_struct *task)

{

	struct mm_struct *mm;



	mm = get_task_mm(task);

	if (!mm)

		return ERR_PTR(-EINVAL);



	/*

	 * A task can always look at itself, in case it chooses

	 * to use system calls instead of load instructions.

	 */

	if (task == current)

		return 0;

		return mm;



	/*

	 * If current is actively ptrace'ing, and would also be
@@ -210,20 +216,23 @@ static int __check_mem_permission(struct task_struct *task) 
		match = (tracehook_tracer_task(task) == current);

		rcu_read_unlock();

		if (match && ptrace_may_access(task, PTRACE_MODE_ATTACH))

			return 0;

			return mm;

	}



	/*

	 * Noone else is allowed.

	 */

	return -EPERM;

	mmput(mm);

	return ERR_PTR(-EPERM);

}



/*

 * Return zero if current may access user memory in @task, -error if not.

 * If current may access user memory in @task return a reference to the

 * corresponding mm, otherwise ERR_PTR.

 */

static int check_mem_permission(struct task_struct *task)

static struct mm_struct *check_mem_permission(struct task_struct *task)

{

	struct mm_struct *mm;

	int err;



	/*
@@ -232,12 +241,12 @@ static int check_mem_permission(struct task_struct *task) 
	 */

	err = mutex_lock_killable(&task->signal->cred_guard_mutex);

	if (err)

		return err;

		return ERR_PTR(err);



	err = __check_mem_permission(task);

	mm = __check_mem_permission(task);

	mutex_unlock(&task->signal->cred_guard_mutex);



	return err;

	return mm;

}



struct mm_struct *mm_for_maps(struct task_struct *task)
@@ -795,18 +804,14 @@ static ssize_t mem_read(struct file * file, char __user * buf, 
	if (!task)

		goto out_no_task;



	if (check_mem_permission(task))

		goto out;



	ret = -ENOMEM;

	page = (char *)__get_free_page(GFP_TEMPORARY);

	if (!page)

		goto out;



	ret = 0;

 

	mm = get_task_mm(task);

	if (!mm)

	mm = check_mem_permission(task);

	ret = PTR_ERR(mm);

	if (IS_ERR(mm))

		goto out_free;



	ret = -EIO;
@@ -820,8 +825,8 @@ static ssize_t mem_read(struct file * file, char __user * buf, 
		int this_len, retval;



		this_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;

		retval = access_process_vm(task, src, page, this_len, 0);

		if (!retval || check_mem_permission(task)) {

		retval = access_remote_vm(mm, src, page, this_len, 0);

		if (!retval) {

			if (!ret)

				ret = -EIO;

			break;
@@ -860,22 +865,25 @@ static ssize_t mem_write(struct file * file, const char __user *buf, 
	char *page;

	struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);

	unsigned long dst = *ppos;

	struct mm_struct *mm;



	copied = -ESRCH;

	if (!task)

		goto out_no_task;



	if (check_mem_permission(task))

		goto out;

	mm = check_mem_permission(task);

	copied = PTR_ERR(mm);

	if (IS_ERR(mm))

		goto out_task;



	copied = -EIO;

	if (file->private_data != (void *)((long)current->self_exec_id))

		goto out;

		goto out_mm;



	copied = -ENOMEM;

	page = (char *)__get_free_page(GFP_TEMPORARY);

	if (!page)

		goto out;

		goto out_mm;



	copied = 0;

	while (count > 0) {
@@ -886,7 +894,7 @@ static ssize_t mem_write(struct file * file, const char __user *buf, 
			copied = -EFAULT;

			break;

		}

		retval = access_process_vm(task, dst, page, this_len, 1);

		retval = access_remote_vm(mm, dst, page, this_len, 1);

		if (!retval) {

			if (!copied)

				copied = -EIO;
@@ -899,7 +907,9 @@ static ssize_t mem_write(struct file * file, const char __user *buf, 
	}

	*ppos = dst;

	free_page((unsigned long) page);

out:

out_mm:

	mmput(mm);

out_task:

	put_task_struct(task);

out_no_task:

	return copied;