Re: [PATCH v2 5/5] x86/tdx: Implement movs for MMIO

[Alexey Gladkov]

On Thu, Aug 08, 2024 at 08:48:26AM -0500, Tom Lendacky wrote:
> On 8/5/24 08:29, Alexey Gladkov (Intel) wrote:
> > Add emulation of the MOVS instruction on MMIO regions. MOVS emulation
> > consists of dividing it into a series of read and write operations,
> > which in turn will be validated separately.
> > 
> > Signed-off-by: Alexey Gladkov (Intel) <legion@xxxxxxxxxx>
> > ---
> >  arch/x86/coco/tdx/tdx.c | 57 ++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 53 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 4e2fb9bf83a1..8573cb23837e 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -509,6 +509,54 @@ static int decode_insn_struct(struct insn *insn, struct pt_regs *regs)
> >  	return 0;
> >  }
> >  
> > +static int handle_mmio_movs(struct insn *insn, struct pt_regs *regs, int size, struct ve_info *ve)
> > +{
> > +	unsigned long ds_base, es_base;
> > +	unsigned char *src, *dst;
> > +	unsigned char buffer[8];
> > +	int off, ret;
> > +	bool rep;
> > +
> > +	/*
> > +	 * The in-kernel code must use a special API that does not use MOVS.
> > +	 * If the MOVS instruction is received from in-kernel, then something
> > +	 * is broken.
> > +	 */
> > +	if (WARN_ON_ONCE(!user_mode(regs)))
> > +		return -EFAULT;
> > +
> > +	ds_base = insn_get_seg_base(regs, INAT_SEG_REG_DS);
> > +	es_base = insn_get_seg_base(regs, INAT_SEG_REG_ES);
> > +
> > +	if (ds_base == -1L || es_base == -1L)
> > +		return -EINVAL;
> > +
> > +	rep = insn_has_rep_prefix(insn);
> > +
> > +	do {
> > +		src = ds_base + (unsigned char *) regs->si;
> > +		dst = es_base + (unsigned char *) regs->di;
> > +
> > +		ret = __get_iomem(src, buffer, size);
> > +		if (ret)
> > +			return ret;
> > +
> > +		ret = __put_iomem(dst, buffer, size);
> > +		if (ret)
> > +			return ret;
> > +
> > +		off = (regs->flags & X86_EFLAGS_DF) ? -size : size;
> > +
> > +		regs->si += off;
> > +		regs->di += off;
> > +
> > +		if (rep)
> > +			regs->cx -= 1;
> > +	} while (rep || regs->cx > 0);
> > +
> > +	return insn->length;
> > +}
> > +
> >  static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int size,
> >  			     struct pt_regs *regs, struct ve_info *ve)
> >  {
> > @@ -530,9 +578,8 @@ static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int si
> >  		return insn->length;
> >  	case INSN_MMIO_MOVS:
> >  		/*
> > -		 * MMIO was accessed with an instruction that could not be
> > -		 * decoded or handled properly. It was likely not using io.h
> > -		 * helpers or accessed MMIO accidentally.
> > +		 * MOVS is processed through higher level emulation which breaks
> > +		 * this instruction into a sequence of reads and writes.
> >  		 */
> >  		return -EINVAL;
> >  	default:
> > @@ -602,6 +649,9 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  	if (WARN_ON_ONCE(mmio == INSN_MMIO_DECODE_FAILED))
> >  		return -EINVAL;
> >  
> > +	if (mmio == INSN_MMIO_MOVS)
> > +		return handle_mmio_movs(&insn, regs, size, ve);
> 
> You check the address in the non-MOVS case using valid_vaddr(), but you
> don't seem to be doing that in the MOVS case, was that intentional?

Ah. You mean that read/write operations will be performed in kernel space
context and no checks will be made.

I guess you're right.

> Thanks,
> Tom
> 
> > +
> >  	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
> >  
> >  	if (user_mode(regs)) {
> > @@ -630,7 +680,6 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  	switch (mmio) {
> >  	case INSN_MMIO_WRITE:
> >  	case INSN_MMIO_WRITE_IMM:
> > -	case INSN_MMIO_MOVS:
> >  		ret = handle_mmio_write(&insn, mmio, size, regs, ve);
> >  		break;
> >  	case INSN_MMIO_READ:
> 

-- 
Rgrds, legion