On Wed, Nov 15, 2023 at 08:55:05AM +0200, José Pekkarinen wrote:
> Documentation of __pte_offset_map_lock suggest there is situations where
You should have cc'd Hugh who changed all this code recently.
> a pmd may not have a corresponding page table, in which case it should
> return NULL without changing ptlp. Syzbot found its ways to produce a
> NULL dereference in the function showing this case. This patch will
> provide the exit path suggested if this unlikely situation turns up. The
> output of the kasan null-ptr-report follows:
There's no need to include all this nonsense in the changelog.
> spin_lock include/linux/spinlock.h:351 [inline]
> __pte_offset_map_lock+0x154/0x360 mm/pgtable-generic.c:373
> pte_offset_map_lock include/linux/mm.h:2939 [inline]
> filemap_map_pages+0x698/0x11f0 mm/filemap.c:3582
This was the only interesting part.
> +++ b/include/linux/mm.h
> @@ -2854,7 +2854,7 @@ void ptlock_free(struct ptdesc *ptdesc);
>
> static inline spinlock_t *ptlock_ptr(struct ptdesc *ptdesc)
> {
> - return ptdesc->ptl;
> + return (likely(ptdesc)) ? ptdesc->ptl : NULL;
> }
I don't think we should be changing ptlock_ptr().
> +++ b/mm/pgtable-generic.c
> @@ -370,6 +370,8 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> if (unlikely(!pte))
> return pte;
> ptl = pte_lockptr(mm, &pmdval);
> + if (unlikely(!ptl))
> + return NULL;
> spin_lock(ptl);
I don't understand how this could possibly solve the problem. If there's
no PTE level, then __pte_offset_map() should return NULL and we'd already
return due to the check for !pte.
