Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  andrew.cooper3@citrix.com) identity=pra;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="andrew.cooper3@citrix.com";
  x-conformance=sidf_compatible
Received-SPF: Pass (esa6.hc3370-68.iphmx.com: domain of
  Andrew.Cooper3@citrix.com designates 162.221.158.21 as
  permitted sender) identity=mailfrom;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="Andrew.Cooper3@citrix.com";
  x-conformance=sidf_compatible; x-record-type="v=spf1";
  x-record-text="v=spf1 ip4:209.167.231.154 ip4:178.63.86.133
  ip4:195.66.111.40/30 ip4:85.115.9.32/28 ip4:199.102.83.4
  ip4:192.28.146.160 ip4:192.28.146.107 ip4:216.52.6.88
  ip4:216.52.6.188 ip4:162.221.158.21 ip4:162.221.156.83
  ip4:168.245.78.127 ~all"
Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  postmaster@mail.citrix.com) identity=helo;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="postmaster@mail.citrix.com";
  x-conformance=sidf_compatible
IronPort-SDR: T07NPnfs91txdCmkBzYw7URfU7gbHGeWsimmexUlwAFk+7pISy9lknhEjUONEtzYabCua/c4FC
 9xsOYdZ0SQy/Gwjegmq58XM0TvJxdGeI/Myp573HIzABBdGd9nE24Y9UdBelg6kPJPqYTaLB/B
 YJV8/jr22AjwTBz0Vv1aOV3AY5WR2cwPZrMroh40RBuUywuTLipfHMnbKKZvU/zfmM2Qjiz7cF
 pRWIYnqfnsHf1qtRLi4TFCz2Z1lbGN5ViHiptcRXdLVHas++pQGRi4r9zl35yvxHMwpZaCq976
 n28=
Subject: Re: [PATCH 0/4] x86/module: Out-of-tree module decode and sanitize
To:     Peter Zijlstra <peterz@infradead.org>
CC:     <tglx@linutronix.de>, <linux-kernel@vger.kernel.org>,
        <hch@infradead.org>, <sean.j.christopherson@intel.com>,
        <mingo@redhat.com>, <bp@alien8.de>, <hpa@zytor.com>,
        <x86@kernel.org>, <kenny@panix.com>, <jeyu@kernel.org>,
        <rasmus.villemoes@prevas.dk>, <pbonzini@redhat.com>,
        <fenghua.yu@intel.com>, <xiaoyao.li@intel.com>,
        <nadav.amit@gmail.com>, <thellstrom@vmware.com>,
        <tony.luck@intel.com>, <rostedt@goodmis.org>,
        <gregkh@linuxfoundation.org>, <jannh@google.com>,
        <keescook@chromium.org>, <David.Laight@aculab.com>,
        <dcovelli@vmware.com>, <mhiramat@kernel.org>
References: <20200407110236.930134290@infradead.org>
 <a53a01b9-2907-4eb3-a9fd-16e6e8029028@citrix.com>
 <20200407194112.GQ2452@worktop.programming.kicks-ass.net>
 <e6d9f83e-e88c-c079-50b3-ff8ad8682074@citrix.com>
 <20200407204530.GR2452@worktop.programming.kicks-ass.net>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <34f2a325-45c4-44e8-595a-187d0d37afa6@citrix.com>
Date:   Tue, 7 Apr 2020 22:21:02 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.1
MIME-Version: 1.0
In-Reply-To: <20200407204530.GR2452@worktop.programming.kicks-ass.net>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Content-Language: en-GB
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 07/04/2020 21:45, Peter Zijlstra wrote:
>> POPF: Don't really want someone being able to set IOPL3.  However, this
>> might quite easily show up as a false positive depending on how the
>> irqsafe infrastructure gets inlined.
> local_irq_restore() will be a POPF :/

Ok.  Something to consider in an orthogonal direction.  A while ago, I
put this into Xen as a security fix:

iret_exit_to_guest:
        andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
        orl   $X86_EFLAGS_IF,24(%rsp)
        addq  $8,%rsp
.Lft0:  iretq

which unconditionally fixes up the unsafe flags even if something
manages to slips through (e.g. local_irq_restore() against stack
rubble).  It turns out that it has saved us several CVEs in the
intervening time.

Is this the kind of things the hardening folk would be interested in?

> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -282,6 +282,68 @@ static bool insn_is_mov_DRn(struct insn
>  	return false;
>  }
>
> +static bool insn_is_GDT_modifier(struct insn *insn)
> +{
> +	u8 modrm = insn->modrm.bytes[0];
> +	u8 modrm_mod = X86_MODRM_MOD(modrm);
> +	u8 modrm_reg = X86_MODRM_REG(modrm);
> +
> +	if (insn->opcode.bytes[0] != 0x0f)
> +		return false;
> +
> +	switch (insn->opcode.bytes[1]) {
> +	case 0x00: /* Grp6 */
> +		switch (modrm_reg) {
> +		/* case 0x0: SLDT */
> +		case 0x2: /* LLDT */
> +		case 0x3: /* LTR */
> +			return true;

Come to think of it, if you include the Sxxx variants, a sufficiently
clever compiler can collapse this entire switch statement into a single
"and $~3, modrm_reg" instruction, rather than being forced to use "and
$~1, modrm_reg; cmp $2, ...".

Probably on the extreme end of micro-optimising however.

~Andrew