lguest: Revert 1ce70c4f, fix real problem.
Ahmed managed to crash the Host in release_pgd(), which cannot be a Guest bug, and indeed it wasn't. The bug was that handing a 0 as the address of the toplevel page table being manipulated can cause the lookup code in find_pgdir() to return an uninitialized cache entry (we shadow up to 4 top level page tables for each Guest). Commit 37cc8d7f introduced this behaviour in the Guest, uncovering the bug. The patch which he submitted (which removed the /4 from the index calculation) simply ensured that these high-indexed entries hit the early exit path of guest_set_pmd(). But you get lots of segfaults in guest userspace as the PMDs aren't being updated. Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
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