Re: [PATCH v3 02/10] x86/tdx: Add validation of userspace MMIO instructions

[Alexey Gladkov <legion@kernel.org>]

On Mon, Aug 19, 2024 at 03:07:50PM +0300, Kirill A. Shutemov wrote:
> On Mon, Aug 19, 2024 at 01:48:16PM +0200, Alexey Gladkov wrote:
> > On Mon, Aug 19, 2024 at 01:39:17PM +0300, Kirill A. Shutemov wrote:
> > > On Fri, Aug 16, 2024 at 03:43:52PM +0200, Alexey Gladkov wrote:
> > > > From: "Alexey Gladkov (Intel)" <legion@kernel.org>
> > > > 
> > > > Instructions from kernel space are considered trusted. If the MMIO
> > > > instruction is from userspace it must be checked.
> > > > 
> > > > For userspace instructions, it is need to check that the INSN has not
> > > > changed at the time of #VE and before the execution of the instruction.
> > > 
> > > Well, we cannot really check if the instruction changed under us. We can
> > > only check if the parsed instruction does an MMIO operation that is
> > > allowed for the process.
> > 
> > We also check that the memory access (read/write) type matches. Yes, we
> > can't check the instruction itself, but we check the arguments.
> > 
> > > > 
> > > > Once the userspace instruction parsed is enforced that the address
> > > > points to mapped memory of current process and that address does not
> > > > point to private memory.
> > > > 
> > > > After parsing the userspace instruction, it is necessary to ensure that:
> > > > 
> > > > 1. the operation direction (read/write) corresponds to #VE info;
> > > > 2. the address still points to mapped memory of current process;
> > > > 3. the address does not point to private memory.
> > > 
> > > I don't see where you check 3.
> > > 
> > > I guess you can add pte_decrypted(pte) check to get_phys_addr().
> > > 
> > > But I'm not sure it is strictly needed.
> > 
> > (ve->gpa != cc_mkdec(phys_addr)
> > 
> > The ve->gpa was checked in the virt_exception_user/kernel().
> 
> phys_addr doesn't have shared bit. It is masked out on pte_pfn(). That's
> the reason you use cc_mkdec() to compare with ve->gpa. Otherwise it would
> fail.

Ok. I think I've confused myself. I will add pte_decrypted(). 

> 
> > 
> > > 
> > > > Signed-off-by: Alexey Gladkov (Intel) <legion@kernel.org>
> > > > ---
> > > >  arch/x86/coco/tdx/tdx.c | 128 ++++++++++++++++++++++++++++++++++++----
> > > >  1 file changed, 115 insertions(+), 13 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > > > index af0b6c1cacf7..86c22fec97fb 100644
> > > > --- a/arch/x86/coco/tdx/tdx.c
> > > > +++ b/arch/x86/coco/tdx/tdx.c
> > > > @@ -8,6 +8,7 @@
> > > >  #include <linux/export.h>
> > > >  #include <linux/io.h>
> > > >  #include <linux/kexec.h>
> > > > +#include <linux/mm.h>
> > > >  #include <asm/coco.h>
> > > >  #include <asm/tdx.h>
> > > >  #include <asm/vmx.h>
> > > > @@ -405,6 +406,84 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
> > > >  			       EPT_WRITE, addr, val);
> > > >  }
> > > >  
> > > > +static inline bool is_private_gpa(u64 gpa)
> > > > +{
> > > > +	return gpa == cc_mkenc(gpa);
> > > > +}
> > > > +
> > > > +static int get_phys_addr(unsigned long addr, phys_addr_t *phys_addr, bool *writable)
> > > > +{
> > > > +	unsigned int level;
> > > > +	pgd_t *pgdp;
> > > > +	pte_t *ptep;
> > > > +
> > > > +	/*
> > > > +	 * Address validation only makes sense for a user process. The lock must
> > > > +	 * be obtained before validation can begin.
> > > > +	 */
> > > > +	mmap_assert_locked(current->mm);
> > > > +
> > > > +	pgdp = pgd_offset(current->mm, addr);
> > > > +
> > > > +	if (!pgd_none(*pgdp)) {
> > > > +		ptep = lookup_address_in_pgd(pgdp, addr, &level);
> > > > +		if (ptep) {
> > > > +			unsigned long offset;
> > > > +
> > > > +			offset = addr & ~page_level_mask(level);
> > > > +			*phys_addr = PFN_PHYS(pte_pfn(*ptep));
> > > > +			*phys_addr |= offset;
> > > > +
> > > > +			*writable = pte_write(*ptep);
> > > > +
> > > > +			return 0;
> > > > +		}
> > > > +	}
> > > > +
> > > > +	return -EFAULT;
> > > > +}
> > > > +
> > > > +static int valid_vaddr(struct ve_info *ve, enum insn_mmio_type mmio, int size,
> > > > +		       unsigned long vaddr)
> > > > +{
> > > > +	phys_addr_t phys_addr;
> > > > +	bool writable = false;
> > > > +
> > > > +	/* It's not fatal. This can happen due to swap out or page migration. */
> > > > +	if (get_phys_addr(vaddr, &phys_addr, &writable) || (ve->gpa != cc_mkdec(phys_addr)))
> > > 
> > > Too long line?
> > 
> > All patches pass checkpatch without warnings.
> 
> Checkpatch is not the ultimate authority. But I am neither. :P
> 
> > > 
> > > > +		return -EAGAIN;
> > > > +
> > > > +	/*
> > > > +	 * Re-check whether #VE info matches the instruction that was decoded.
> > > > +	 *
> > > > +	 * The ve->gpa was valid at the time ve_info was received. But this code
> > > > +	 * executed with interrupts enabled, allowing tlb shootdown and therefore
> > > > +	 * munmap() to be executed in the parallel thread.
> > > > +	 *
> > > > +	 * By the time MMIO emulation is performed, ve->gpa may be already
> > > > +	 * unmapped from the process, the device it belongs to removed from
> > > > +	 * system and something else could be plugged in its place.
> > > > +	 */
> > > > +	switch (mmio) {
> > > > +	case INSN_MMIO_WRITE:
> > > > +	case INSN_MMIO_WRITE_IMM:
> > > > +		if (!writable || !(ve->exit_qual & EPT_VIOLATION_ACC_WRITE))
> > > > +			return -EFAULT;
> > > > +		break;
> > > > +	case INSN_MMIO_READ:
> > > > +	case INSN_MMIO_READ_ZERO_EXTEND:
> > > > +	case INSN_MMIO_READ_SIGN_EXTEND:
> > > > +		if (!(ve->exit_qual & EPT_VIOLATION_ACC_READ))
> > > > +			return -EFAULT;
> > > > +		break;
> > > > +	default:
> > > > +		WARN_ONCE(1, "Unsupported mmio instruction: %d", mmio);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > >  static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int size,
> > > >  			     struct pt_regs *regs, struct ve_info *ve)
> > > >  {
> > > > @@ -489,7 +568,7 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> > > >  	enum insn_mmio_type mmio;
> > > >  	struct insn insn = {};
> > > >  	unsigned long vaddr;
> > > > -	int size;
> > > > +	int size, ret;
> > > >  
> > > >  	/* Only in-kernel MMIO is supported */
> > > >  	if (WARN_ON_ONCE(user_mode(regs)))
> > > > @@ -505,6 +584,17 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> > > >  	if (WARN_ON_ONCE(mmio == INSN_MMIO_DECODE_FAILED))
> > > >  		return -EINVAL;
> > > >  
> > > > +	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
> > > > +
> > > > +	if (current->mm) {
> > > 
> > > Hm. This path will be taken for any MMIO if it is done in context of a
> > > process, even in-kernel only. I don't think we want it. It is useless
> > > overhead.
> > 
> > The kthread do not have a current->mm.
> 
> I am not talking about kthread. I am talking about initiating MMIO from
> kernel, but within a process context. Like, you call an ioctl() on a
> device fd and it triggers MMIO in kernel. This scenario would have
> current->mm, but it is not userspace MMIO.

Ok. I will use user_mode here and in the movs patch I will add a special
flag to perform checks in case of nested exceptions.

> > > Use user_mode(regs) instead.
> > 
> > I can't use this. When nested exception happens in the handle_mmio_movs()
> > the regs will be not in the user mode.
> > 
> > I can make a flag that will be set either for user_mode or if we have a
> > nested exception.
> 
> Hm. Yeah. This is ugly. Let me think about it.

Yes, it's not very good.

-- 
Rgrds, legion