lguest: Revert 1ce70c4fac3c3954bd48c035f448793867592bc0, 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 37cc8d7f963ba2deec29c9b68716944516a3244f 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>

2 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 9c27c104d83c..a104c532ff70 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -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
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index 275f23c2deb4..a7f64a9d67e0 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -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;
 }