linux-kernel - Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Wed, 19 Aug 2020 09:39:10 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

On Wed, Aug 19, 2020 at 7:50 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> +/*
> + * Sanitize a uaccess pointer such that it becomes NULL if it's not a valid
> + * user pointer.  This blocks speculative dereferences of user-controlled
> + * pointers.
> + */
> +#define uaccess_mask_ptr(ptr) \
> +       (__typeof__(ptr)) array_index_nospec((__force unsigned long)ptr, user_addr_max())
> +

If I dug through all the macros correctly, this is generating a fairly
complex pile of math to account for the fact that user_addr_max() is
variable and that it's a nasty number.

But I don't think there's any particular need to use the real maximum
user address here.  Allowing a mis-speculated user access to a
non-canonical address or to the top guard page of the lower canonical
region is harmless.  With current kernels, a sequence like:

if (likely((long)addr > 0) {
  masked_addr = addr & 0x7FFFFFFFFFFFFFFFUL;
} else {
  if (kernel fs) {
    masked_addr = addr;
  } else {
    EFAULT;
  }
}

could plausibly be better.  But Christoph's series fixes this whole
mess, and I think that this should be:

#define uaccess_mask_ptr(ptr) ((__typeof___(ptr)) (__force unsigned
long)ptr & USER_ADDR_MASK))

where USER_ADDR_MASK is the appropriate value for 32-bit or 64-bit.