diff options
Diffstat (limited to 'mm/gup.c')
| -rw-r--r-- | mm/gup.c | 44 |
1 files changed, 38 insertions, 6 deletions
@@ -382,13 +382,22 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address, } /* - * FOLL_FORCE can write to even unwritable pte's, but only - * after we've gone through a COW cycle and they are dirty. + * FOLL_FORCE or a forced COW break can write even to unwritable pte's, + * but only after we've gone through a COW cycle and they are dirty. */ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags) { - return pte_write(pte) || - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte)); + return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte)); +} + +/* + * A (separate) COW fault might break the page the other way and + * get_user_pages() would return the page from what is now the wrong + * VM. So we need to force a COW break at GUP time even for reads. + */ +static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags) +{ + return is_cow_mapping(vma->vm_flags) && (flags & (FOLL_GET | FOLL_PIN)); } static struct page *follow_page_pte(struct vm_area_struct *vma, @@ -1066,9 +1075,11 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, goto out; } if (is_vm_hugetlb_page(vma)) { + if (should_force_cow_break(vma, foll_flags)) + foll_flags |= FOLL_WRITE; i = follow_hugetlb_page(mm, vma, pages, vmas, &start, &nr_pages, i, - gup_flags, locked); + foll_flags, locked); if (locked && *locked == 0) { /* * We've got a VM_FAULT_RETRY @@ -1082,6 +1093,10 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, continue; } } + + if (should_force_cow_break(vma, foll_flags)) + foll_flags |= FOLL_WRITE; + retry: /* * If we have a pending SIGKILL, don't keep faulting pages and @@ -2674,6 +2689,10 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end) * * If the architecture does not support this function, simply return with no * pages pinned. + * + * Careful, careful! COW breaking can go either way, so a non-write + * access can get ambiguous page results. If you call this function without + * 'write' set, you'd better be sure that you're ok with that ambiguity. */ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages) @@ -2709,6 +2728,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, * * We do not adopt an rcu_read_lock(.) here as we also want to * block IPIs that come from THPs splitting. + * + * NOTE! We allow read-only gup_fast() here, but you'd better be + * careful about possible COW pages. You'll get _a_ COW page, but + * not necessarily the one you intended to get depending on what + * COW event happens after this. COW may break the page copy in a + * random direction. */ if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && @@ -2766,10 +2791,17 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages, if (unlikely(!access_ok((void __user *)start, len))) return -EFAULT; + /* + * The FAST_GUP case requires FOLL_WRITE even for pure reads, + * because get_user_pages() may need to cause an early COW in + * order to avoid confusing the normal COW routines. So only + * targets that are already writable are safe to do by just + * looking at the page tables. + */ if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) { local_irq_disable(); - gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned); + gup_pgd_range(addr, end, gup_flags | FOLL_WRITE, pages, &nr_pinned); local_irq_enable(); ret = nr_pinned; } |