Re: [patch 11/16] x86/ldt: Force access bit for CS/SS

[Andy Lutomirski <luto@kernel.org>]

On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>
> When mapping the LDT RO the hardware will typically generate write faults
> on first use. These faults can be trapped and the backing pages can be
> modified by the kernel.
>
> There is one exception; IRET will immediately load CS/SS and unrecoverably
> #GP. To avoid this issue access the LDT descriptors used by CS/SS before
> the IRET to userspace.
>
> For this use LAR, which is a safe operation in that it will happily consume
> an invalid LDT descriptor without traps. It gets the CPU to load the
> descriptor and observes the (preset) ACCESS bit.
>
> So far none of the obvious candidates like dosemu/wine/etc. do care about
> the ACCESS bit at all, so it should be rather safe to enforce it.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/entry/common.c            |    8 ++++-
>  arch/x86/include/asm/desc.h        |    2 +
>  arch/x86/include/asm/mmu_context.h |   53 +++++++++++++++++++++++--------------
>  arch/x86/include/asm/thread_info.h |    4 ++
>  arch/x86/kernel/cpu/common.c       |    4 +-
>  arch/x86/kernel/ldt.c              |   30 ++++++++++++++++++++
>  arch/x86/mm/tlb.c                  |    2 -
>  arch/x86/power/cpu.c               |    2 -
>  8 files changed, 78 insertions(+), 27 deletions(-)
>
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -30,6 +30,7 @@
>  #include <asm/vdso.h>
>  #include <linux/uaccess.h>
>  #include <asm/cpufeature.h>
> +#include <asm/mmu_context.h>
>
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/syscalls.h>
> @@ -130,8 +131,8 @@ static long syscall_trace_enter(struct p
>         return ret ?: regs->orig_ax;
>  }
>
> -#define EXIT_TO_USERMODE_LOOP_FLAGS                            \
> -       (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |   \
> +#define EXIT_TO_USERMODE_LOOP_FLAGS                                    \
> +       (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | _TIF_LDT |\
>          _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY | _TIF_PATCH_PENDING)
>
>  static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
> @@ -171,6 +172,9 @@ static void exit_to_usermode_loop(struct
>                 /* Disable IRQs and retry */
>                 local_irq_disable();
>
> +               if (cached_flags & _TIF_LDT)
> +                       ldt_exit_user(regs);

Nope.  To the extent that this code actually does anything (which it
shouldn't since you already forced the access bit), it's racy against
flush_ldt() from another thread, and that race will be exploitable for
privilege escalation.  It needs to be outside the loopy part.

--Andy