Messages in this thread | | | Date | Thu, 23 Apr 2015 20:52:02 +0200 | From | Borislav Petkov <> | Subject | Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss |
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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. --
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