arm64/mm: avoid fixmap race condition when create pud mapping

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]>
Reviewed-by: Catalin Marinas <[email protected]>
Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index acfae9b41cc8c983ae6c4a5bc639a5f6d03a139a..1681430ecab73813b1e81cc7ac8942c84d560e79 100644 (file)
--- 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,