linux-kernel - Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 10 Feb 2023 17:00:05 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "bp@...en8.de" <bp@...en8.de>
CC:     "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "pavel@....cz" <pavel@....cz>, "oleg@...hat.com" <oleg@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Schimpe, Christina" <christina.schimpe@...el.com>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "john.allen@....com" <john.allen@....com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>
Subject: Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE_COW

On Fri, 2023-02-10 at 14:57 +0100, Borislav Petkov wrote:
> @@ -749,14 +740,8 @@ static inline pte_t pte_modify(pte_t pte,
> pgprot_t newprot)
>  
>         pte_result = __pte(val);
>  
> -       /*
> -        * Dirty bit is not preserved above so it can be done
> -        * in a special way for the shadow stack case, where it
> -        * may need to set _PAGE_COW. __pte_mkdirty() will do this in
> -        * the case of shadow stack.
> -        */
>         if (pte_dirty(pte))
> -               pte_result = __pte_mkdirty(pte_result, false);
> +               pte_result = pte_set_flags(pte_result, _PAGE_DIRTY);
>  
>         return pte_result;
>  }
> 

Oh, I see what you are seeing now. Did you notice that  the
__pte_mkdirty() logic got expanded in "x86/mm: Start actually marking
_PAGE_COW"? So if we don't put that logic in a usable helper, it ends
up open coded with pte_modify() looking something like this:
static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
{
	pteval_t val = pte_val(pte), oldval = val;
	pte_t pte_result;

	/*
	 * Chop off the NX bit (if present), and add the NX portion of
	 * the newprot (if present):
	 */
	val &= (_PAGE_CHG_MASK & ~_PAGE_DIRTY);
	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);

	pte_result = __pte(val);

	/*
	 * Dirty bit is not preserved above so it can be done
	 * in a special way for the shadow stack case, where it
	 * may need to set _PAGE_SAVED_DIRTY. __pte_mkdirty() will do
	 * this in the case of shadow stack.
	 */
	if (oldval & _PAGE_DIRTY)
		if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK) &&
		    !pte_write(pte_result))
			pte_set_flags(pte_result, _PAGE_SAVED_DIRTY);
		else
			pte_set_flags(pte_result, _PAGE_DIRTY);
	}

	return pte_result;
}

So the later logic of doing the _PAGE_SAVED_DIRTY (_PAGE_COW) part is
not centralized. It's ok?