[Lguest] [BUG + PATCH/Bugfix] x86/lguest: fix pgdir pmd index calculation

[Rusty Russell]

On Monday 25 February 2008 02:55:15 Ahmed S. Darwish wrote:
> Hi all,
>
> Beginning from commits close to v2.6.25-rc2, running lguest always oopses
> the host kernel. Oops is at [1].

OK, whatever the guest does should not break the host.  So your patch can't be
the whole solution.

> 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))/4, 0);
> }
>
> The first hcall parameter is global pgdir which looks fine. The second
> parameter is the pmd index in the pgdir which is suspectful.
>
> AFAIK, calculating the index of pmd does not need a divisoin over four.
> Removing the division made lguest work fine again . Patch is at [2].

Each pmd is 4 bytes long, so the divide by 4 gives the index into the (top
level) page. ie. a number in the range 0 to 1023.

Indeed, here's the correct fix.  Does it work for you?

==
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 at rustcorp.com.au>

diff -r 26838579cab1 arch/x86/lguest/boot.c
--- a/arch/x86/lguest/boot.c	Tue Mar 04 22:55:11 2008 +1100
+++ b/arch/x86/lguest/boot.c	Tue Mar 04 23:43:10 2008 +1100
@@ -480,7 +480,7 @@ static void lguest_set_pmd(pmd_t *pmdp, 
 {
 	*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 -r 26838579cab1 drivers/lguest/page_tables.c
--- a/drivers/lguest/page_tables.c	Tue Mar 04 22:55:11 2008 +1100
+++ b/drivers/lguest/page_tables.c	Tue Mar 04 23:43:10 2008 +1100
@@ -391,7 +391,7 @@ static unsigned int find_pgdir(struct lg
 {
 	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;
 }