mm/memory.c: fix potential pte_unmap_unlock pte error

If all pte entry is none in 'non-create' case, we would break the loop with
pte unchanged.  Then the wrong pte - 1 would be passed to pte_unmap_unlock.
This is a theoretical issue which may not be a real bug. So it's not worth
cc stable.

Link: https://lkml.kernel.org/r/20210205081925.59809-1-linmiaohe@huawei.com
Fixes: aee16b3cee27 ("Add apply_to_page_range() which applies a function to a pte range")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ian Pratt <ian.pratt@xensource.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 83e1bcf..ef4e10c 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2394,18 +2394,18 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
                                     pte_fn_t fn, void *data, bool create,
                                     pgtbl_mod_mask *mask)
 {
-       pte_t *pte;
+       pte_t *pte, *mapped_pte;
        int err = 0;
        spinlock_t *ptl;
 
        if (create) {
-               pte = (mm == &init_mm) ?
+               mapped_pte = pte = (mm == &init_mm) ?
                        pte_alloc_kernel_track(pmd, addr, mask) :
                        pte_alloc_map_lock(mm, pmd, addr, &ptl);
                if (!pte)
                        return -ENOMEM;
        } else {
-               pte = (mm == &init_mm) ?
+               mapped_pte = pte = (mm == &init_mm) ?
                        pte_offset_kernel(pmd, addr) :
                        pte_offset_map_lock(mm, pmd, addr, &ptl);
        }
@@ -2428,7 +2428,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
        arch_leave_lazy_mmu_mode();
 
        if (mm != &init_mm)
-               pte_unmap_unlock(pte-1, ptl);
+               pte_unmap_unlock(mapped_pte, ptl);
        return err;
 }