linux-kernel - Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32

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Date:	Thu, 23 Apr 2015 21:50:26 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

On Thu, Apr 23, 2015 at 8:52 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, Apr 23, 2015 at 11:24:14AM -0700, Andy Lutomirski wrote:
>> That nails it.  We really do leak segment limits to other tasks on AMD
>> chips.  I see at least two questions we should answer before fixing
>> this:
>
> Ok, WTF is going on?! Even this trivial test case causes a Bus Error:
>
> ---
> static unsigned short GDT3(int idx)
> {
>         return (idx << 3) | 3;
> }
>
> static void *threadproc(void *ctx)
> {
>         printf("Hello world\n");
>         return NULL;
> }
>
> int main()
> {
>         pthread_t thread;
>         if (pthread_create(&thread, 0, threadproc, 0) != 0)
>                 err(1, "pthread_create");
>
>         while (1) {
>                 usleep(1);
>         }
>
>         return 0;
> }
> ---
>
> $ make sysret_ss_attrs_32
> gcc -m32 -o sysret_ss_attrs_32 -O2 -g -std=gnu99 -pthread -Wall  sysret_ss_attrs.c -lrt -ldl
> sysret_ss_attrs.c:23:23: warning: ‘GDT3’ defined but not used [-Wunused-function]
>  static unsigned short GDT3(int idx)
>                        ^
> $ taskset -c 0 ./sysret_ss_attrs_32
> Hello world
> Bus error
>
> in dmesg:
>
> [  583.389368] traps: sysret_ss_attrs[2135] trap stack segment ip:f7784b87 sp:ffb640c0 error:0

I reproduced it.

I also confirm that the patch fixes it.

In fact, the simplest reproducer is

int main()
{
        while (1)
                usleep(1);
        return 0;
}

- no threads necessary. You only need to do a lot of sysret32's,
and eventually it happens. If you omit -m32, it doesn't happen.

-- 
vda
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