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SubjectRe: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping
On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote:
> 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>

I am afraid there is a problem to take a spinlock there.

node 0 deferred pages initialised in 2740ms
pgdatinit0 (176) used greatest stack depth: 59184 bytes left
devtmpfs: initialized
KASLR disabled due to lack of seed
BUG: sleeping function called from invalid context at mm/page_alloc.c:5151
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
preempt_count: 1, expected: 0
1 lock held by swapper/0/1:
#0: ffff800009ea3278 (fixmap_lock){+.+.}-{2:2}, at: __create_pgd_mapping
alloc_init_pud at /usr/src/linux-next/arch/arm64/mm/mmu.c:340 (discriminator 4)
(inlined by) __create_pgd_mapping at /usr/src/linux-next/arch/arm64/mm/mmu.c:393 (discriminator 4)
CPU: 0 PID: 1 Comm: swapper/0 Tainted: G W 5.16.0-rc5-next-20211214
Call trace:
dump_backtrace
show_stack
dump_stack_lvl
dump_stack
__might_resched
__might_sleep
__alloc_pages
alloc_page_interleave
alloc_pages
__get_free_pages
__pgd_pgtable_alloc
__create_pgd_mapping
__phys_to_pte_val at /usr/src/linux-next/./arch/arm64/include/asm/pgtable.h:77
(inlined by) __pud_populate at /usr/src/linux-next/./arch/arm64/include/asm/pgalloc.h:25
(inlined by) alloc_init_cont_pmd at /usr/src/linux-next/arch/arm64/mm/mmu.c:277
(inlined by) alloc_init_pud at /usr/src/linux-next/arch/arm64/mm/mmu.c:358
(inlined by) __create_pgd_mapping at /usr/src/linux-next/arch/arm64/mm/mmu.c:393
map_entry_trampoline
map_entry_trampoline at /usr/src/linux-next/arch/arm64/mm/mmu.c:639
do_one_initcall
kernel_init_freeable
kernel_init
ret_from_fork

> ---
> 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
>

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