Re: [RFC PATCH 2/3] Introduce arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE)

From: Andy Lutomirski <luto@kernel.org>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	 Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	 "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	 linux-arch <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	 Arnd Bergmann <arnd@arndb.de>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	 Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	 Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>,  Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	 Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	 Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	 Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	 Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>
Subject: Re: [RFC PATCH 2/3] Introduce arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE)
Date: Sat, 29 Jun 2019 16:43:18 -0700	[thread overview]
Message-ID: <CALCETrVvbbCWMPo7v5eYgTocaxRQPHerJ=CRjWscGxgb6QjOFA@mail.gmail.com> (raw)
In-Reply-To: <20190628194158.2431-2-yu-cheng.yu@intel.com>

> On Jun 28, 2019, at 12:41 PM, Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> The CET legacy code bitmap covers the whole user-mode address space and is
> located at the top of the user-mode address space.  It is allocated only
> when the first time arch_prctl(ARCH_X86_MARK_LEGACY_CODE) is called from
> an application.
>
> Introduce:
>
> arch_prctl(ARCH_X86_MARK_LEGACY_CODE, unsigned long *buf)
>    Mark an address range as IBT legacy code.

How about defining a struct for this?

The change log should discuss where the bitmap goes and how it’s allocated.

> +static int alloc_bitmap(void)
> +{
> +    unsigned long addr;
> +    u64 msr_ia32_u_cet;
> +
> +    addr = do_mmap_locked(NULL, IBT_BITMAP_ADDR, IBT_BITMAP_SIZE,
> +                  PROT_READ | PROT_WRITE,
> +                  MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE,
> +                  VM_IBT | VM_NORESERVE, NULL);
> +
> +    if (IS_ERR((void *)addr))
> +        return addr;
> +
> +    current->thread.cet.ibt_bitmap_addr = addr;

addr is a constant. Why are you storing it?  If it ends up not being
constant, you should wire up mremap like the vDSO does.

> +static int set_user_bits(unsigned long __user *buf, unsigned long buf_size,
> +             unsigned long start_bit, unsigned long end_bit, unsigned long set)
> +{
> +    unsigned long start_ul, end_ul, total_ul;
> +    int i, j, r;
> +
> +    if (round_up(end_bit, BITS_PER_BYTE) / BITS_PER_BYTE > buf_size)
> +        end_bit = buf_size * BITS_PER_BYTE - 1;
> +
> +    start_ul = start_bit / BITS_PER_LONG;
> +    end_ul = end_bit / BITS_PER_LONG;
> +    total_ul = (end_ul - start_ul + 1);
> +
> +    i = start_bit % BITS_PER_LONG;
> +    j = end_bit % BITS_PER_LONG;
> +
> +    r = 0;
> +    put_user_try {

put_user_try is obsolete.  Just use get_user(), etc.

Also, I must be missing something fundamental, because this series
claims that user code can't write directly to the bitmap.  This means
that this entire function shouldn't work at all.