Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

From: Andy Lutomirski
Date: Fri Feb 17 2017 - 11:51:05 EST

On Fri, Feb 17, 2017 at 6:13 AM, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> This patch introduces two new prctl(2) handles to manage maximum virtual
> address available to userspace to map.
> On x86, 5-level paging enables 56-bit userspace virtual address space.
> Not all user space is ready to handle wide addresses. It's known that
> at least some JIT compilers use higher bits in pointers to encode their
> information. It collides with valid pointers with 5-level paging and
> leads to crashes.
> The patch aims to address this compatibility issue.
> MM would use the address as upper limit of virtual address available to
> map by userspace, instead of TASK_SIZE.
> The limit will be equal to TASK_SIZE everywhere, but the machine
> with 5-level paging enabled. In this case, the default limit would be
> (1UL << 47) - PAGE_SIZE. Itâs current x86-64 TASK_SIZE_MAX with 4-level
> paging which known to be safe.

I think this patch need to be split up. In particular, the addition
and use of mmap_max_addr() should be its own patch that doesn't change
any semantics.

> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 306c7e12af55..50bdfd6ab866 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -117,6 +117,7 @@ static inline int init_new_context(struct task_struct *tsk,
> }
> #endif
> init_new_context_ldt(tsk, mm);
> + mm->context.max_vaddr = MAX_VADDR_DEFAULT;

Is this actually correct for 32-bit binaries? Although, given the
stuff Dmitry is working on, it might pay to separately track the
32-bit and 64-bit limits per mm. If you haven't been following it,
Dmitry is trying to fix a bug in which an explicit 32-bit syscall
(int80 or similar) in an otherwise 64-bit process can allocate a VMA
above 4GB that gets truncated.

Also, why the macro? Why not just put the number in here?

> -#define TASK_SIZE_MAX ((1UL << 47) - PAGE_SIZE)

This should be in the

> +#define STACK_TOP mmap_max_addr()

Off the top of my head, this looks wrong. The 32-bit check got lost, I think.

> +unsigned long set_max_vaddr(unsigned long addr)
> +{

Perhaps this function could set a different field depending on

Anyway, can you and Dmitry try to reconcile your patches?