[PATCH 5.15 204/913] arm64/mm: avoid fixmap race condition when create pud mapping

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 10:59:20 EST

From: Jianyong Wu <jianyong.wu@xxxxxxx>

[ Upstream commit ee017ee353506fcec58e481673e4331ff198a80e ]

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) // CRASH!

As kernel may sleep during creating pud mapping, introduce a mutex lock to
serialise use of the fixmap entries by alloc_init_pud(). However, there is
no need for locking in early boot stage and it doesn't work well with
KASLR enabled when early boot. So, enable lock when system_state doesn't
equal to "SYSTEM_BOOTING".

Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Link: https://lore.kernel.org/r/20220201114400.56885-1-jianyong.wu@xxxxxxx
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/arm64/mm/mmu.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9d0380631690..03aa6bee7dae 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)
@@ -328,6 +329,12 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,

+ /*
+ * No need for locking during early boot. And it doesn't work as
+ * expected with KASLR enabled.
+ */
+ if (system_state != SYSTEM_BOOTING)
+ mutex_lock(&fixmap_lock);
pudp = pud_set_fixmap_offset(p4dp, addr);
do {
pud_t old_pud = READ_ONCE(*pudp);
@@ -358,6 +365,8 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
} while (pudp++, addr = next, addr != end);

+ if (system_state != SYSTEM_BOOTING)
+ mutex_unlock(&fixmap_lock);

static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,