linux-kernel - Re: [PATCH v2 09/21] x86/virt/tdx: Get information about TDX module and convertible memory

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Message-ID: <f211441a6d23321e22517684159e2c28c8492b86.camel@intel.com>
Date:   Mon, 28 Mar 2022 14:30:05 +1300
From:   Kai Huang <kai.huang@...el.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dave.hansen@...el.com, seanjc@...gle.com, pbonzini@...hat.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, peterz@...radead.org,
        tony.luck@...el.com, ak@...ux.intel.com, dan.j.williams@...el.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v2 09/21] x86/virt/tdx: Get information about TDX module
 and convertible memory


> > +
> > +static int sanitize_cmrs(struct cmr_info *cmr_array, int cmr_num)
> > +{
> > +	int i, j;
> > +
> > +	/*
> > +	 * Intel TDX module spec, 20.7.3 CMR_INFO:
> > +	 *
> > +	 *   TDH.SYS.INFO leaf function returns a MAX_CMRS (32) entry
> > +	 *   array of CMR_INFO entries. The CMRs are sorted from the
> > +	 *   lowest base address to the highest base address, and they
> > +	 *   are non-overlapping.
> > +	 *
> > +	 * This implies that BIOS may generate invalid empty entries
> > +	 * if total CMRs are less than 32.  Skip them manually.
> > +	 */
> > +	for (i = 0; i < cmr_num; i++) {
> > +		struct cmr_info *cmr = &cmr_array[i];
> > +		struct cmr_info *prev_cmr = NULL;
> > +
> > +		/* Skip further invalid CMRs */
> > +		if (!cmr_valid(cmr))
> > +			break;
> > +
> > +		if (i > 0)
> > +			prev_cmr = &cmr_array[i - 1];
> > +
> > +		/*
> > +		 * It is a TDX firmware bug if CMRs are not
> > +		 * in address ascending order.
> > +		 */
> > +		if (prev_cmr && ((prev_cmr->base + prev_cmr->size) >
> > +					cmr->base)) {
> > +			pr_err("Firmware bug: CMRs not in address ascending order.\n");
> > +			return -EFAULT;
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Also a sane BIOS should never generate invalid CMR(s) between
> > +	 * two valid CMRs.  Sanity check this and simply return error in
> > +	 * this case.
> > +	 */
> > +	for (j = i; j < cmr_num; j++)
> > +		if (cmr_valid(&cmr_array[j])) {
> > +			pr_err("Firmware bug: invalid CMR(s) among valid CMRs.\n");
> > +			return -EFAULT;
> > +		}
> 
> This check doesn't make sense because above i-for loop has break.

The break in above i-for loop will hit at the first invalid CMR entry.  Yes "j =
i" will make double check on this invalid CMR entry, but it should have no
problem.  Or we can change to "j = i + 1" to skip the first invalid CMR entry.

Does this make sense?

> 
> > +
> > +	/*
> > +	 * Trim all tail invalid empty CMRs.  BIOS should generate at
> > +	 * least one valid CMR, otherwise it's a TDX firmware bug.
> > +	 */
> > +	tdx_cmr_num = i;
> > +	if (!tdx_cmr_num) {
> > +		pr_err("Firmware bug: No valid CMR.\n");
> > +		return -EFAULT;
> > +	}
> 
> Something strange.
> Probably we'd like to check it by decrementing.
> for (i = cmr_num; i >= 0; i--)
>   if (!cmr_valid()) // if last invalid cmr
>      tdx_cmr_num
>   // more check. overlapping
> 

I don't know how does this look strange to you? As replied above, "i" is the
index to the first invalid CMR entry (or cmr_num, which is array size), so
"tdx_cmr_num = i" initializes the total valid CMR number, correct?