[PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

Jianyong Wu
2021-12-10T09:54:44Z

fixmap is a global resource and is used recursively in create pud mapping.
It may lead to race condition when alloc_init_pud is called concurrently.

Fox example:
alloc_init_pud is called when kernel_init. If memory hotplug
thread, which will also call alloc_init_pud, happens during
kernel_init, the race for fixmap occurs.

The race condition flow can be:

*************** begin **************

kerenl_init thread virtio-mem workqueue thread
================== ======== ==================
alloc_init_pud(...)
pudp = pud_set_fixmap_offset(..) alloc_init_pud(...)
... ...
READ_ONCE(*pudp) //OK! pudp = pud_set_fixmap_offset(
... ...
pud_clear_fixmap() //fixmap break
READ_ONCE(*pudp) //CRASH!

**************** end ***************

Hence, a spin lock is introduced to protect the fixmap during create pdg
mapping.

Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
---
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..98ac09ae9588 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 __maybe_unused;
static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;

static DEFINE_SPINLOCK(swapper_pgdir_lock);
+static DEFINE_SPINLOCK(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,
}
BUG_ON(p4d_bad(p4d));

+ /*
+ * fixmap is global resource, thus it needs to be protected by a lock
+ * in case of race condition.
+ */
+ spin_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,
} while (pudp++, addr = next, addr != end);

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

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