>>> On 02.05.18 at 17:22, <boris.ostrovsky@xxxxxxxxxx> wrote: > On 05/02/2018 11:01 AM, Jan Beulich wrote: >>>>> On 02.05.18 at 17:00, <boris.ostrovsky@xxxxxxxxxx> wrote: >>> On 05/02/2018 04:16 AM, Jan Beulich wrote: >>>>>>> On 30.04.18 at 18:23, <boris.ostrovsky@xxxxxxxxxx> wrote: >>>>> --- a/arch/x86/xen/xen-pvh.S >>>>> +++ b/arch/x86/xen/xen-pvh.S >>>>> @@ -54,6 +54,9 @@ >>>>> * charge of setting up it's own stack, GDT and IDT. >>>>> */ >>>>> >>>>> +#define PVH_GDT_ENTRY_CANARY 4 >>>>> +#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8) >>>> I can only advise against doing it this way: There's no safeguard against >>>> someone changing asm/segment.h without changing this value (in fact >>>> this applies to all of the GDT selectors populated in this file). At the >>> very >>>> least tie this to GDT_ENTRY_BOOT_TSS / __BOOT_TSS? >>>> >>>>> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen) >>>>> mov %eax,%es >>>>> mov %eax,%ss >>>>> >>>>> + mov $(PVH_CANARY_SEL),%eax >>>>> + mov %eax,%gs >>>>> + >>>>> /* Stash hvm_start_info. */ >>>>> mov $_pa(pvh_start_info), %edi >>>>> mov %ebx, %esi >>>>> @@ -150,6 +156,7 @@ gdt_start: >>>>> .quad 0x00cf9a000000ffff /* __BOOT_CS */ >>>>> #endif >>>>> .quad 0x00cf92000000ffff /* __BOOT_DS */ >>>>> + .quad 0x0040900000000018 /* PVH_CANARY_SEL */ >>>> Without any further code before loading the selector, this points at >>>> physical address 0. Don't you need to add in the base address of >>>> the per-CPU stack_canary? >>> This GDT is gone soon after we jump into generic x86 startup code.That >>> code will load its own GDT (and then set up per-cpu segments and all that). >> All understood, but why would you set up the per-CPU segment here if >> what you load into the segment register is not usable for the intended >> purpose (until that other code sets up things and reloads the segment >> registers)? > > The intended purpose here is to allow stack protector access not to > fail. At this point it doesn't really matter that GS is later used for > per-cpu segment, this code (and this GDT) will not be used when other > CPUs come up. But the place the canary would live this way is completely wrong. Anyway, you're the maintainer of this code, so I guess I better shut up now. Jan
