On 05/02/2018 11:41 AM, Jan Beulich wrote: >>>> 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. Would creating a canary variable and using it as a base address be better? -boris
