lkml.org 
[lkml]   [2008]   [Jul]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] loop: prevent get_user pages call from kernel thread(v2)
From
Date
Andi Kleen <andi@firstfloor.org> writes:

> Dmitri Monakhov <dmonakhov@openvz.org> writes:
>
>> Yes... everybody know that it is bad to write from kernel thread, and it is
>> madness to do it with O_DIRECT. But occasionly file with O_DIRECT flag
>> may be passed to loop device via LOOP_SET_FD. So if file-system has't
>> address_space ops, or simply hide it like GFS, it is possible to kill kernel
>> via two lines program. In fact we can't effectively guard kernel space by
>> deny O_DIRECT in loop's code, because user space can set it via
>> fcntl(,F_SETFL,). Let's simply add sanity check mm related logic.
>
> Wouldn't it be better if loop simply dup()ed the file descriptor
> and then checked the flag? Presumably other fd flags could
> do bad things inside loop too.
Off course this can't work because both fd refer to the same struct file.
man fcntl:
File status flags
Each open file description has certain associated status flags, ini-
tialized by open(2) and possibly modified by fcntl(2). Duplicated
file descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.)
refer to the same open file description, and thus share the same
file status flags.
>
> -Andi


\
 
 \ /
  Last update: 2008-07-07 10:05    [W:1.418 / U:2.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site