From 4357bd9453b81e0a41db1dec16e06d74256b7560 Mon Sep 17 00:00:00 2001
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Tue, 11 Mar 2008 09:35:57 -0500
Subject: 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>
---
 arch/x86/lguest/boot.c       | 2 +-
 drivers/lguest/page_tables.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 9c27c104d83..a104c532ff7 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 275f23c2deb..a7f64a9d67e 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;
 }
-- 
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