Messages in this thread Patch in this message | | | From | Jianyong Wu <> | Subject | [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping | Date | Fri, 10 Dec 2021 17:54:32 +0800 |
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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
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