RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping

Hi Mark,

-----Original Message-----
From: Mark Rutland <mark.rutland@arm.com>
Sent: Friday, December 17, 2021 5:31 PM
To: Jianyong Wu <redacted>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>; will@kernel.org; Anshuman
Khandual [off-list ref]; akpm@linux-foundation.org;
david@redhat.com; quic_qiancai@quicinc.com; ardb@kernel.org; linux-
kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
gshan@redhat.com; Justin He [off-list ref]; nd [off-list ref]
Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create
pud mapping

On Thu, Dec 16, 2021 at 04:28:12PM +0800, Jianyong Wu wrote:

quoted

The 'fixmap' is a global resource and is used recursively by create
pud mapping(), leading to a potential race condition in the presence
of a concurrent call to alloc_init_pud():

kernel_init thread                          virtio-mem workqueue thread
==================                          ===========================

  alloc_init_pud(...)                       alloc_init_pud(...)
  pudp = pud_set_fixmap_offset(...)         pudp = pud_set_fixmap_offset(...)
  READ_ONCE(*pudp)
  pud_clear_fixmap(...)
                                            READ_ONCE(*pudp) // CRASH!

As kernel may sleep during creating pud mapping, introduce a mutex
lock to serialise use of the fixmap entries by alloc_init_pud().

Signed-off-by: Jianyong Wu <redacted>

Since there were deadlock issues with the last version, it would be very nice
if we could check this with at least:

* CONFIG_DEBUG_ATOMIC_SLEEP
* CONFIG_PROVE_LOCKING

... so that we can be reasonably certain that we're not introducing some
livelock/deadlock scenario.

I enable these 2 configs and test for the current patch. No warning related with this change found. 

Are you able to reproduce the problem for testing, or was this found by
inspection? Do you have any instructions for reproducing the problem? e.g.
can this easily be tested with QEMU?

I test it using Cloud Hypervisor not QEMU. I find the bug when I tested the incoming feature of virtio-mem using Cloud Hypervisor.
I think we can reproduce this bug using QEMU, but as there is no virtio-mem support for the current QEMU, we can only test the ACPI-based memory hotplug. However, I think it's not easy to do and I have not tried that.

In my test: firstly, start a VM and hotplug a large size of memory using virtio-mem and reboot or kexec a new kernel. When the kernel booting, memory hotplugged by virtio-mem will be added within kernel_init. As both of kernel init and memory add thread will update page table, "alloc_pud_init" may be executed concurrently. 

I think it's not easy to reproduce the bug using ACPI based memory hotplug, as we must hotplug memory at the same time of kernel_init to crash with it. 

If you're able to reproduce the issue, it would be nice to have an example
backtrace of when this goes wrong.

Yes, this bug occurs when kernel init, the function execute flow is:
-------------------------
kernel_init
  kernel_init_freeable
    ...
      do_initcall
        ...
          module_init [A]

  ...
  mark_readonly
    mark_rodata_ro [B]
-------------------------
[A] can contains memory hotplug init therefore both [A] and [B] can
update page table at the same time and may lead to crash.

Thanks
Jianyong Wu

Thanks,
Mark.

quoted

---

Change log:

from v2 to v3:
     change spin lock to mutex lock as kernel may sleep when create
pud map.

 arch/arm64/mm/mmu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index

acfae9b41cc8..e680a6a8ca40 100644

--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD]

__page_aligned_bss

quoted

__maybe_unused;  static pud_t bm_pud[PTRS_PER_PUD]

__page_aligned_bss

quoted

__maybe_unused;

 static DEFINE_SPINLOCK(swapper_pgdir_lock);
+static DEFINE_MUTEX(fixmap_lock);

 void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)  { @@ -329,6 +330,11 @@
static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long

end,

quoted

 	}
 	BUG_ON(p4d_bad(p4d));

+	/*
+	 * We only have one fixmap entry per page-table level, so take
+	 * the fixmap lock until we're done.
+	 */
+	mutex_lock(&fixmap_lock);
 	pudp = pud_set_fixmap_offset(p4dp, addr);
 	do {
 		pud_t old_pud = READ_ONCE(*pudp);

@@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned

long addr, unsigned long end,

quoted

 	} while (pudp++, addr = next, addr != end);

 	pud_clear_fixmap();
+	mutex_unlock(&fixmap_lock);
 }

 static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
--
2.17.1

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