On Thu, Jan 25, 2018 at 01:12:14PM -0800, Andy Lutomirski wrote:
> Neil Berrington reported a double-fault on a VM with 768GB of RAM that
> uses large amounts of vmalloc space with PTI enabled.
>
> The cause is that load_new_mm_cr3() was never fixed to take the
> 5-level pgd folding code into account, so, on a 4-level kernel, the
> pgd synchronization logic compiles away to exactly nothing.
Ouch. Sorry for this.
>
> Interestingly, the problem doesn't trigger with nopti. I assume this
> is because the kernel is mapped with global pages if we boot with
> nopti. The sequence of operations when we create a new task is that
> we first load its mm while still running on the old stack (which
> crashes if the old stack is unmapped in the new mm unless the TLB
> saves us), then we call prepare_switch_to(), and then we switch to the
> new stack. prepare_switch_to() pokes the new stack directly, which
> will populate the mapping through vmalloc_fault(). I assume that
> we're getting lucky on non-PTI systems -- the old stack's TLB entry
> stays alive long enough to make it all the way through
> prepare_switch_to() and switch_to() so that we make it to a valid
> stack.
>
> Fixes: b50858ce3e2a ("x86/mm/vmalloc: Add 5-level paging support")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-and-tested-by: Neil Berrington <neil.berrington@xxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> arch/x86/mm/tlb.c | 34 +++++++++++++++++++++++++++++-----
> 1 file changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index a1561957dccb..5bfe61a5e8e3 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -151,6 +151,34 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> local_irq_restore(flags);
> }
>
> +static void sync_current_stack_to_mm(struct mm_struct *mm)
> +{
> + unsigned long sp = current_stack_pointer;
> + pgd_t *pgd = pgd_offset(mm, sp);
> +
> + if (CONFIG_PGTABLE_LEVELS > 4) {
Can we have
if (PTRS_PER_P4D > 1)
here instead? This way I wouldn't need to touch the code again for
boot-time switching support.
--
Kirill A. Shutemov