Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

[Kirill A. Shutemov]

On Wed, Jun 15, 2022 at 11:12:35AM -0700, Dave Hansen wrote:
> On 6/14/22 05:01, Kirill A. Shutemov wrote:
> > load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
> > The unwanted loads are typically harmless. But, they might be made to
> > totally unrelated or even unmapped memory. load_unaligned_zeropad()
> > relies on exception fixup (#PF, #GP and now #VE) to recover from these
> > unwanted loads.
> > 
> > In TDX guests, the second page can be shared page and VMM may configure
> > it to trigger #VE.
> > 
> > Kernel assumes that #VE on a shared page is MMIO access and tries to
> > decode instruction to handle it. In case of load_unaligned_zeropad() it
> > may result in confusion as it is not MMIO access.
> > 
> > Fix it by detecting split page MMIO accesses and fail them.
> > load_unaligned_zeropad() will recover using exception fixups.
> > 
> > The issue was discovered by analysis. It was not triggered during the
> > testing.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/coco/tdx/tdx.c | 15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 7d6d484a6d28..3bcaf2170ede 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -333,8 +333,8 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
> >  
> >  static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  {
> > +	unsigned long *reg, val, vaddr;
> >  	char buffer[MAX_INSN_SIZE];
> > -	unsigned long *reg, val;
> >  	struct insn insn = {};
> >  	enum mmio_type mmio;
> >  	int size, extend_size;
> > @@ -360,6 +360,19 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  			return -EINVAL;
> >  	}
> >  
> > +	/*
> > +	 * Reject EPT violation #VEs that split pages.
> > +	 *
> > +	 * MMIO accesses suppose to be naturally aligned and therefore never
> > +	 * cross a page boundary. Seeing split page accesses indicates a bug
> > +	 * or load_unaligned_zeropad() that steps into unmapped shared page.
> 
> Isn't this "unmapped" thing a rather superfluous implementation detail?
> 
> For the guest, it just needs to know that it *CAN* #VE on access to MMIO
> and that it needs to be prepared.  The fact that MMIO is implemented
> with TDX shared memory *AND* that "unmapped shared pages" can cause
> #VE's seems like too much detail.

Okay, fair enough.

> Also, is this all precise?  Are literal unmapped shared pages the *ONLY*
> thing that a hypervisor can do do case a #VE?  What about, say, reserved
> bits being set in a shared EPT entry?

Right, it is analogous to page fault. So, yes, it can be triggered for
a number of reasons, not only unmapped.

> I was thinking a comment like this might be better:
> 
> >         /*
> >          * Reject EPT violation #VEs that split pages.
> >          *
> >          * MMIO accesses are supposed to be naturally aligned and therefore
> >          * never cross page boundaries. Seeing split page accesses indicates
> >          * a bug or a load_unaligned_zeropad() that stepped into an MMIO page.
> >          *
> >          * load_unaligned_zeropad() will recover using exception fixups.
> >          */

Looks good, thanks.

-- 
 Kirill A. Shutemov