lkml.org 
[lkml]   [2015]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: question about save_xstate_sig() - WHY DOES THIS WORK?
On 01/23/2015 11:34 AM, Rik van Riel wrote:
> While working on a patch series to defer FPU state loading until
> kernel -> user space transition, and be more lazy with FPU state
> while in the kernel, I came across this code in save_xstate_sig().
>
> Not only is this broken with my new code, but it looks like it may
> be broken with the current code, too...
>
> Specifically, save_user_xstate() may page fault and sleep. After
> returning from the page fault, there is no guarantee that the
> FPU state will be restored into the CPU, when the system is not
> running with eager fpu mode.
>
> In that case, what prevents us from saving random FPU register state
> to the user's stack frame? Potentially state containing data from
> other programs...
>

If the FPU state is not current, we'll have CR0.TS = 1 and the XSAVE
will cause an #NM exception, which will cause the FPU state to be
swapped in.

-hpa




\
 
 \ /
  Last update: 2015-01-23 22:21    [W:0.242 / U:0.232 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site