linux-kernel - Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

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]

Message-ID: <2220e2f1-2c91-e054-c107-a761fdfe7ffb@amd.com>
Date:   Fri, 31 Mar 2023 08:40:30 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        David Laight <David.Laight@...LAB.COM>
Cc:     'Wu Zongyong' <wuzongyong@...ux.alibaba.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "wutu.xq2@...ux.alibaba.com" <wutu.xq2@...ux.alibaba.com>
Subject: Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

On 3/31/23 05:06, Kirill A. Shutemov wrote:
> On Fri, Mar 31, 2023 at 08:49:48AM +0000, David Laight wrote:
>> From: Wu Zongyong
>>> Sent: 31 March 2023 03:24
>>>
>>> On Thu, Mar 30, 2023 at 03:39:51PM +0300, kirill.shutemov@...ux.intel.com wrote:
>>>> On Wed, Mar 29, 2023 at 10:59:37AM +0800, Wu Zongyong wrote:
>>>>> It seems MOVSXD which opcode is 0x63 is not handled, support
>>>>> to decode it in insn_decode_mmio().
>>>>
>>>> Do you have a particular user in mind?
>>> To be honest, I don't find a specific user which uses the MOVSXD.
>>>
>>> But both Intel and AMD's instructions reference contains MOVSXD and lots
>>> of MOVSXD instructions occur when I "objdump -S vmlinux", so I think it
>>> may be useful to support it in insn_decode_mmio().
>>>
>>> Are there some special consideration about this instruction?
>>
>> It is a sign-extending memory read (32bit to 64bit).
>> You pretty much never want to do that to a device register.
>> Also kernel code should be using readl() (etc) which do
>> unsigned reads.
>> So they should never happen for mmio.
>>
>> Of course, if you mmap() PCIe space directly into a program's
>> address space anything might happen ...
> 
> There are two users of the interface: TDX and SEV. TDX doesn't allow
> userspace MMIO. SEV *seems* allows it, but I am not sure how it is safe.
> 
> Tom?

The insn_decode_mmio() function is only called by the SEV/TDX related code 
and is specifically MMIO oriented. As David said, this instruction is 
likely not being used for that in the kernel. If we come across a case 
where this is used, we can look at how it is being used in that situation 
and it can be addressed then.

Thanks,
Tom

>