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Subject[PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping
Date
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 <jianyong.wu@arm.com>
---

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 __maybe_unused;
static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __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,
}
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,
} 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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