Re: Local DoS attack on i386 (was: new kernel bug)

[Nuno Monteiro]

On 2004.06.14 14:58, Gianni Tedesco wrote:
> On Sat, 2004-06-12 at 11:59 +0200, Manuel Arostegui Ramirez wrote:
> > Somebody know a patch to solved this new bug?
> > http://reviewed.homelinux.org/news/2004-06-11_kernel_crash/index.html.en
> > Affected versions:
> >     * Linux 2.6.x
> >           o Linux 2.6.7-rc2
> >           o Linux 2.6.6 (all versions)
> >           o Linux 2.6.6 SMP (verified by riven)
> >           o Linux 2.6.5-gentoo (verified by RatiX)
> >           o Linux 2.6.5-mm6 - (verified by Mariux) 
> >     * Linux 2.4.2x
> >           o Linux 2.4.26 vanilla
> >           o Linux 2.4.26-rc1 vanilla
> >           o Linux 2.4.26-gentoo-r1
> >           o Linux 2.4.22 
> 
> Seems to be a scheduler race or something?
> 

This was already fixed in 2.6, see http://linux.bkbits.net:8080/linux-2.5/diffs/include/asm-i386/i387.h@xxxx?nav=index.html|src/.|src/include|src/include/asm-i386|hist/include/asm-i386/i387.h


The same fix should be applied to 2.4. I'm running locally a very
hacked version of 2.4.22 with it and it survives that crash.c program.

Here's the diff. Marcelo, please merge.


--- linux-2.4.27-pre5/include/asm-i386/i387.h~fix-x86-clear_fpu-macro	2004-06-14 15:12:13.909059344 +0100
+++ linux-2.4.27-pre5/include/asm-i386/i387.h	2004-06-14 15:12:45.970185312 +0100
@@ -34,7 +34,7 @@ extern void kernel_fpu_begin(void);
 
 #define clear_fpu( tsk ) do { \
 	if ( tsk->flags & PF_USEDFPU ) { \
-		asm volatile("fwait"); \
+		asm volatile("fnclex ; fwait"); \
 		tsk->flags &= ~PF_USEDFPU; \
 		stts(); \
 	} \
-
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