Re: [PATCH v6 12/21] x86/virt/tdx: Add placeholder to construct TDMRs to cover all TDX memory regions

[Huang, Kai]

On Thu, 2022-10-27 at 08:31 -0700, Andi Kleen wrote:
> > +/* Calculate the actual TDMR_INFO size */
> > +static inline int cal_tdmr_size(void)
> > +{
> > +	int tdmr_sz;
> > +
> > +	/*
> > +	 * The actual size of TDMR_INFO depends on the maximum number
> > +	 * of reserved areas.
> > +	 */
> > +	tdmr_sz = sizeof(struct tdmr_info);
> > +	tdmr_sz += sizeof(struct tdmr_reserved_area) *
> > +		   tdx_sysinfo.max_reserved_per_tdmr;
> 
> 
> would seem safer to have a overflow check here.
> 
> 

How about below?

--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -614,6 +614,14 @@ static inline int cal_tdmr_size(void)
        tdmr_sz += sizeof(struct tdmr_reserved_area) *
                   tdx_sysinfo.max_reserved_per_tdmr;
 
+       /*
+        * Do simple check against overflow, and return 0 (invalid)
+        * TDMR_INFO size if it happened.  Also WARN() as it should
+        * should never happen in reality.
+        */
+       if (WARN_ON_ONCE(tdmr_sz < 0))
+               return 0;
+
        /*
         * TDX requires each TDMR_INFO to be 512-byte aligned.  Always
         * round up TDMR_INFO size to the 512-byte boundary.
@@ -623,19 +631,27 @@ static inline int cal_tdmr_size(void)
 
 static struct tdmr_info *alloc_tdmr_array(int *array_sz)
 {
+       int sz;
+
        /*
         * TDX requires each TDMR_INFO to be 512-byte aligned.
         * Use alloc_pages_exact() to allocate all TDMRs at once.
         * Each TDMR_INFO will still be 512-byte aligned since
         * cal_tdmr_size() always return 512-byte aligned size.
         */
-       *array_sz = cal_tdmr_size() * tdx_sysinfo.max_tdmrs;
+       sz = cal_tdmr_size() * tdx_sysinfo.max_tdmrs;
+
+       /* Overflow */
+       if (!sz || WARN_ON_ONCE(sz < 0))
+               return NULL;
+
+       *array_sz = sz;
 
        /*
         * Zero the buffer so 'struct tdmr_info::size' can be
         * used to determine whether a TDMR is valid.
         */
-       return alloc_pages_exact(*array_sz, GFP_KERNEL | __GFP_ZERO);
+       return alloc_pages_exact(sz, GFP_KERNEL | __GFP_ZERO);
 }


Btw, should I use alloc_contig_pages() instead of alloc_pages_exact() as IIUC
the latter should fail if the size is larger than 4MB?  In reality, the entire
array only takes dozens of KBs, though.