linux-kernel - Re: [PATCH 09/10] mm: Prevent madvise from changing shadow stack

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Message-Id: <D1A84B62-E971-4ECD-A873-2072F2692382@gmail.com>
Date:   Thu, 7 Jun 2018 14:09:05 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        linux-arch@...r.kernel.org,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 09/10] mm: Prevent madvise from changing shadow stack

Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:

> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
> mm/madvise.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4d3c922ea1a1..2a6988badd6b 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -839,6 +839,14 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
> 	if (vma && start > vma->vm_start)
> 		prev = vma;
> 
> +	/*
> +	 * Don't do anything on shadow stack.
> +	 */
> +	if (vma->vm_flags & VM_SHSTK) {
> +		error = -EINVAL;
> +		goto out_no_plug;
> +	}
> +
> 	blk_start_plug(&plug);
> 	for (;;) {
> 		/* Still start < end. */

What happens if the madvise() revolves multiple VMAs, the first one is not
VM_SHSTK, but the another one is? Shouldn’t the test be done inside the
loop, potentially in madvise_vma() ?