Commit 30cd8903 authored May 26, 2011 by KOSAKI Motohiro Committed by Linus Torvalds May 26, 2011

proc: put check_mem_permission after __get_free_page in mem_write



It whould be better if put check_mem_permission after __get_free_page in
mem_write, to be same as function mem_read.

Hugh Dickins explained the reason.

    check_mem_permission gets a reference to the mm.  If we __get_free_page
    after check_mem_permission, imagine what happens if the system is out
    of memory, and the mm we're looking at is selected for killing by the
    OOM killer: while we wait in __get_free_page for more memory, no memory
    is freed from the selected mm because it cannot reach exit_mmap while
    we hold that reference.

Reported-by: Jovi Zhang <bookjovi@gmail.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Stephen Wilson <wilsons@start.ca>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent a4dbf0ec

fs/proc/base.c

+9 −7

Original line number	Diff line number	Diff line
		@@ -894,20 +894,20 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
		if (!task)
		goto out_no_task;

		copied = -ENOMEM;
		page = (char *)__get_free_page(GFP_TEMPORARY);
		if (!page)
		goto out_task;

		mm = check_mem_permission(task);
		copied = PTR_ERR(mm);
		if (IS_ERR(mm))
		goto out_task;
		goto out_free;

		copied = -EIO;
		if (file->private_data != (void *)((long)current->self_exec_id))
		goto out_mm;

		copied = -ENOMEM;
		page = (char *)__get_free_page(GFP_TEMPORARY);
		if (!page)
		goto out_mm;

		copied = 0;
		while (count > 0) {
		int this_len, retval;
		@@ -929,9 +929,11 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
		count -= retval;
		}
		*ppos = dst;
		free_page((unsigned long) page);

		out_mm:
		mmput(mm);
		out_free:
		free_page((unsigned long) page);
		out_task:
		put_task_struct(task);
		out_no_task: