Commit 4357bd94 authored Mar 11, 2008 by Rusty Russell

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: Rusty Russell <rusty@rustcorp.com.au>

parent 3fabc55f

arch/x86/lguest/boot.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -480,7 +480,7 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
		{
		*pmdp = pmdval;
		lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
		(__pa(pmdp)&(PAGE_SIZE-1)), 0);
		(__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
		}

		/* There are a couple of legacy places where the kernel sets a PTE, but we

drivers/lguest/page_tables.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -391,7 +391,7 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
		{
		unsigned int i;
		for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
		if (lg->pgdirs[i].gpgdir == pgtable)
		if (lg->pgdirs[i].pgdir && lg->pgdirs[i].gpgdir == pgtable)
		break;
		return i;
		}