Thread: Protect against possible memory corruption (src/backend/access/nbtree/nbtxlog.c)

On 11/07/2021 22:51, Ranier Vilela wrote:
> Hi,
> 
> While analyzing a possible use of an uninitialized variable, I checked that
> *_bt_restore_page* can lead to memory corruption,
> by not checking the maximum limit of array items which is
> MaxIndexTuplesPerPage.

> +    /* Protect against corrupted recovery file */
> +    nitems = (len / sizeof(IndexTupleData));
> +    if (nitems < 0 || nitems > MaxIndexTuplesPerPage)
> +        elog(PANIC, "_bt_restore_page: cannot restore %d items to page", nitems);
> +

That's not right. You don't get the number of items by dividing like 
that. 'len' includes the tuple data as well, not just the IndexTupleData 
header.

> @@ -73,12 +79,9 @@ _bt_restore_page(Page page, char *from, int len)
>      nitems = i;
>  
>      for (i = nitems - 1; i >= 0; i--)
> -    {
>          if (PageAddItem(page, items[i], itemsizes[i], nitems - i,
>                          false, false) == InvalidOffsetNumber)
>              elog(PANIC, "_bt_restore_page: cannot add item to page");
> -        from += itemsz;
> -    }
>  }

I agree with this change (except that I would leave the braces in 
place). The 'from' that's calculated here is plain wrong; oversight in 
commit 7e30c186da. Fortunately it's not used, so it can just be removed.

- Heikki

On 12/07/2021 02:34, Ranier Vilela wrote:
> If it is not possible, know the upper limits, before the loop.
> It is necessary to do this inside the loop.

> @@ -49,10 +47,14 @@ _bt_restore_page(Page page, char *from, int len)
>       * To get the items back in the original order, we add them to the page in
>       * reverse.  To figure out where one tuple ends and another begins, we
>       * have to scan them in forward order first.
> +     * Check the array upper limit to not overtake him.
>       */
>      i = 0;
> -    while (from < end)
> +    while (from < end && i <= MaxIndexTuplesPerPage)
>      {
> +        IndexTupleData itupdata;
> +        Size        itemsz;
> +
>          /*
>           * As we step through the items, 'from' won't always be properly
>           * aligned, so we need to use memcpy().  Further, we use Item (which

If we bother checking it, we should throw an error if the check fails, 
not just silently soldier on. Also, shouldn't it be '<', not '<='? In 
general though, we don't do much checking on WAL records, we assume that 
the contents are sane. It would be nice to add more checks and make WAL 
redo routines more robust to corrupt records, but this seems like an odd 
place to start.

I committed the removal of bogus assignment to 'from'. Thanks!

- Heikki