linux-kernel - Re: [PATCH v2 2/5] x86/tdx: Add validation of userspace MMIO instructions

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: <ZrIE4FVBflJrJagX@example.org>
Date: Tue, 6 Aug 2024 13:11:28 +0200
From: Alexey Gladkov <legion@...nel.org>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ytcoode@...il.com" <ytcoode@...il.com>,
	"Huang, Kai" <kai.huang@...el.com>,
	"Yao, Yuan" <yuan.yao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"John.Starks@...rosoft.com" <John.Starks@...rosoft.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Cui, Dexuan" <decui@...rosoft.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"bhe@...hat.com" <bhe@...hat.com>, "hpa@...or.com" <hpa@...or.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"geert@...ux-m68k.org" <geert@...ux-m68k.org>,
	"cho@...rosoft.com" <cho@...rosoft.com>
Subject: Re: [PATCH v2 2/5] x86/tdx: Add validation of userspace MMIO
 instructions

On Tue, Aug 06, 2024 at 10:18:20AM +0300, kirill.shutemov@...ux.intel.com wrote:
> On Mon, Aug 05, 2024 at 10:40:55PM +0000, Edgecombe, Rick P wrote:
> > On Mon, 2024-08-05 at 15:29 +0200, Alexey Gladkov (Intel) wrote:
> > > +       vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
> > > +
> > > +       if (user_mode(regs)) {
> > > +               if (mmap_read_lock_killable(current->mm))
> > > +                       return -EINTR;
> > > +
> > > +               ret = valid_vaddr(ve, mmio, size, vaddr);
> > > +               if (ret)
> > > +                       goto unlock;
> > > +       }
> > > +
> > 
> > In the case of user MMIO, if the user instruction + MAX_INSN_SIZE straddles a
> > page, then the "fetch" in the kernel could trigger a #VE. In this case the  
> > kernel would handle this second #VE as a !user_mode() MMIO I guess.
> > 
> > Would something prevent the same munmap() checks needing to happen for that
> > second kernel #VE? If not, I wonder if the munmap() protection logic should also
> > trigger for any userspace range ve->gpa as well.
> 
> That's an interesting scenario, but I think we are fine.
> 
> The fetch is copy_from_user() which is "REP; MOVSB" on all TDX platforms.
> Kernel rejects MOVS instruction emulation for !user_mode() with -EFAULT.

But MOVS will be used only if X86_FEATURE_FSRM feature is present.
Otherwise rep_movs_alternative will be used, which uses MOVB.

I know that X86_FEATURE_FSRM appeared since Ice Lake, but still.

-- 
Rgrds, legion