On Tue, Jul 15, 2014 at 8:50 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: > On 07/15/2014 08:38 AM, Konrad Rzeszutek Wilk wrote: >> >> I think just disallowing would be preferrable. >> > > So I have yet to hear anyone talk about what the Xen PV IRET does for > 16-bit stack segments. Can we get the answer for that, please? > If I'm reading the right thing, it does: 59 .Lforce_iret: 60 /* Mimic SYSRET behavior. */ 61 movq 8(%rsp),%rcx # RIP 62 movq 24(%rsp),%r11 # RFLAGS 63 ALIGN 64 /* No special register assumptions. */ 65 iret_exit_to_guest: 66 addq $8,%rsp 67 .Lft0: iretq while running on Xen's stack. IOW, it leaks bits 31:16 of the Xen stack, and there's nothing the guest can do about it short of using a user-mode trampoline or disabling 16-bit stack segments entirely. See: http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/x86_64/entry.S;h=a3ed216b390c2e87a21ff377850ee34ee7f2bc74;hb=HEAD and (search for do_iret): http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/traps.c;h=677074b4e628ed99d407b1045d859355e590d604;hb=HEAD > -hpa > > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html