Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

Borislav Petkov
2015-04-23T18:52:17Z

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

This is insane.

> 1. Do we consider this to be enough of a security issue that we want
> to fix it for 64-bit userspace as well?
>
> 2. Do we fix it at sysret time (at the cost of an ss read even in the
> best case on AMD chips) or at context switch time (with the risk of
> more ss writes than necessary)?
>
> I slightly favor fixing it at sysret time for both the 32-bit and
> 64-bit paths., but I'm not really convinced.

Yeah, a "call amd_fixup_ss" which gets NOPped out on Intel with
alternatives sounds nice and clean to me.

Pending we have an explanation WTH is going on...

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
--