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(). 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 <[email protected]>
---
Change log:
from v3 to v4:
conditionally enable lock as we doesn't need lock when early boot.
from v2 to v3:
change spin lock to mutex lock as kernel may sleep when create pud
map.
---
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 acfae9b41cc8..1681430ecab7 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 +330,12 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
}
BUG_ON(p4d_bad(p4d));
+ /*
+ * 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);
@@ -359,6 +366,8 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
} while (pudp++, addr = next, addr != end);
pud_clear_fixmap();
+ if (system_state != SYSTEM_BOOTING)
+ mutex_unlock(&fixmap_lock);
}
static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
--
2.17.1
On Tue, Feb 01, 2022 at 07:44:00PM +0800, Jianyong Wu wrote:
> 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(). 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 <[email protected]>
This looks fine to me but I'd rather leave it in -next for a bit given
that we attempted to fix it a couple of times and got it wrong.
Reviewed-by: Catalin Marinas <[email protected]>
On Tue, 1 Feb 2022 19:44:00 +0800, Jianyong Wu wrote:
> 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
> ================== ===========================
>
> [...]
Applied to arm64 (for-next/mm), thanks!
[1/1] arm64/mm: avoid fixmap race condition when create pud mapping
https://git.kernel.org/arm64/c/ee017ee35350
Cheers,
--
Will
https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev