[Xen-devel] [PATCH] x86/paravirt: Partially revert "remove lazy mode in interrupts"

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

which has git commit b8bcfe997e46150fedcc3f5b26b846400122fdd9.

The unintended consequence of removing the flushing of MMU
updates when doing kmap_atomic (or kunmap_atomic) is that we can
hit a dereference bug when processing a "fork()" under a heavy loaded
machine. Specifically we can hit:

BUG: unable to handle kernel paging request at f573fc8c
IP: [<c01abc54>] swap_count_continued+0x104/0x180
*pdpt = 000000002a3b9027 *pde = 0000000001bed067 *pte = 0000000000000000
Oops: 0000 [#1] SMP
Modules linked in:
Pid: 1638, comm: apache2 Not tainted 3.0.4-linode37 #1
EIP: 0061:[<c01abc54>] EFLAGS: 00210246 CPU: 3
EIP is at swap_count_continued+0x104/0x180
.. snip..
Call Trace:
 [<c01ac222>] ? __swap_duplicate+0xc2/0x160
 [<c01040f7>] ? pte_mfn_to_pfn+0x87/0xe0
 [<c01ac2e4>] ? swap_duplicate+0x14/0x40
 [<c01a0a6b>] ? copy_pte_range+0x45b/0x500
 [<c01a0ca5>] ? copy_page_range+0x195/0x200
 [<c01328c6>] ? dup_mmap+0x1c6/0x2c0
 [<c0132cf8>] ? dup_mm+0xa8/0x130
 [<c013376a>] ? copy_process+0x98a/0xb30
 [<c013395f>] ? do_fork+0x4f/0x280
 [<c01573b3>] ? getnstimeofday+0x43/0x100
 [<c010f770>] ? sys_clone+0x30/0x40
 [<c06c048d>] ? ptregs_clone+0x15/0x48
 [<c06bfb71>] ? syscall_call+0x7/0xb

The problem looks that in copy_page_range we turn lazy mode on, and then
in swap_entry_free we call swap_count_continued which ends up in:

         map = kmap_atomic(page, KM_USER0) + offset;

and then later touches *map.

Since we are running in batched mode (lazy) we don't actually set up the
PTE mappings and the kmap_atomic is not done synchronously and ends up
trying to dereference a page that has not been set.

Looking at kmap_atomic_prot_pfn, it uses 'arch_flush_lazy_mmu_mode' and
sprinkling that in kmap_atomic_prot and __kunmap_atomic makes the problem
go away.

CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
CC: stable@xxxxxxxxxx
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/mm/highmem_32.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/x86/mm/highmem_32.c b/arch/x86/mm/highmem_32.c
index b499626..f4f29b1 100644
--- a/arch/x86/mm/highmem_32.c
+++ b/arch/x86/mm/highmem_32.c
@@ -45,6 +45,7 @@ void *kmap_atomic_prot(struct page *page, pgprot_t prot)
        vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
        BUG_ON(!pte_none(*(kmap_pte-idx)));
        set_pte(kmap_pte-idx, mk_pte(page, prot));
+       arch_flush_lazy_mmu_mode();
 
        return (void *)vaddr;
 }
@@ -88,6 +89,7 @@ void __kunmap_atomic(void *kvaddr)
                 */
                kpte_clear_flush(kmap_pte-idx, vaddr);
                kmap_atomic_idx_pop();
+               arch_flush_lazy_mmu_mode();
        }
 #ifdef CONFIG_DEBUG_HIGHMEM
        else {
-- 
1.7.4.1


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