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. -boris
