lkml.org 
[lkml]   [2022]   [Oct]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] x86/mm: Do not verify W^X at boot up
On Mon, 24 Oct 2022 09:14:45 -0700
Dave Hansen <dave.hansen@intel.com> wrote:

> On 10/24/22 08:45, Steven Rostedt wrote:
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -587,6 +587,10 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
> > {
> > unsigned long end;
> >
> > + /* Kernel text is rw at boot up */
> > + if (system_state == SYSTEM_BOOTING)
> > + return new;
>
> Hi Steven,
>
> Thanks for the report and the patch. That seems reasonable, but I'm a
> bit worried that it opens up a big hole (boot time) when a W+X mapping
> could be created *anywhere*.
>
> Could we restrict this bypass to *only* kernel text addresses during
> boot? Maybe something like this:
>
> if ((system_state == SYSTEM_BOOTING) &&
> __kernel_text_address(start))
> return new;

Actually, that brings back the warning, as ftrace creates a trampoline, but
text_poke() will still use memcpy on it at early boot up.

The trampolines are set to ro at the end of boot up by:

59566b0b622e3 ("x86/ftrace: Have ftrace trampolines turn read-only at the end of system boot up")

Which was added because of text_poke() doing the memcpy().

>
> That would be safe because we know that kernel_text_address() addresses
> will be made read-only by the time userspace shows up and that
> is_kernel_inittext() addresses will be freed.
>
> Long-term, I wonder if we could teach the early patching code that it
> can't just use memcpy().
>

Maybe.

-- Steve

\
 
 \ /
  Last update: 2022-10-24 21:52    [W:0.208 / U:1.692 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site