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lguest: Revert 1ce70c4fac, fix real problem.

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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 37cc8d7f96 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>
This commit is contained in:
Rusty Russell 2008-03-11 09:35:57 -05:00
parent 3fabc55f34
commit 4357bd9453
2 changed files with 2 additions and 2 deletions
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2
arch/x86/lguest/boot.c
View File

@ -480,7 +480,7 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
{ {
*pmdp = pmdval; *pmdp = pmdval;
lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK, 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 /* There are a couple of legacy places where the kernel sets a PTE, but we

2
drivers/lguest/page_tables.c
View File

@ -391,7 +391,7 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
{ {
unsigned int i; unsigned int i;
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); 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; break;
return i; return i;
} }