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proc: make check_mem_permission() return an mm_struct on success

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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: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Stephen Wilson 2011-03-13 15:49:23 -04:00 committed by Al Viro
parent 18f661bcf8
commit 8b0db9db19
1 changed files with 34 additions and 24 deletions
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58
fs/proc/base.c
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@ -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;