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

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Feb 17, 2017 at 6:13 AM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> 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. It’s 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)
> +#define TASK_SIZE_MAX  ((1UL << __VIRTUAL_MASK_SHIFT) - PAGE_SIZE)

This should be in the

> -#define STACK_TOP              TASK_SIZE
> +#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
is_compat_syscall().


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

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