mm: remove gup_flags FOLL_WRITE games from __get_user_pages() (19be0eaf) · Commits · 戴 / test

This is an ancient bug that was actually attempted to be fixed once (badly) by me eleven years ago in commit ("Fix get_user_pages() race for write access") but that was then undone due to problems on s390 by commit ("fix get_user_pages bug"). In the meantime, the s390 situation has long been fixed, and we can now fix it by checking the pte_dirty() bit properly (and do it better). The s390 dirty bit was implemented in ("s390/mm: implement software dirty bits") which made it into v3.9. Earlier kernels will have to look at the page state itself. Also, the VM has become more scalable, and what used a purely theoretical race back then has become easier to trigger. To fix it, we introduce a new internal FOLL_COW flag to mark the "yes, we already did a COW" rather than play racy games with FOLL_WRITE that is very fundamental, and then use the pte dirty flag to validate that the FOLL_COW flag is still valid. Reported-and-tested-by:

Phil "not Paul" Oester <kernel@linuxace.com> Acked-by:

Hugh Dickins <hughd@google.com> Reviewed-by:

Michal Hocko <mhocko@suse.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Kees Cook <keescook@chromium.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Willy Tarreau <w@1wt.eu> Cc: Nick Piggin <npiggin@gmail.com> Cc: Greg Thelen <gthelen@google.com> Cc: stable@vger.kernel.org Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -2232,6 +2232,7 @@ static inline struct page follow_page(struct vm_area_struct vma,
		#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
		#define FOLL_MLOCK 0x1000 /* lock present pages */
		#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
		#define FOLL_COW 0x4000 /* internal GUP flag */

		typedef int (pte_fn_t)(pte_t pte, pgtable_t token, unsigned long addr,
		void *data);

mm/gup.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -60,6 +60,16 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
		return -EEXIST;
		}

		/*
		* FOLL_FORCE can write to even 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));
		}

		static struct page follow_page_pte(struct vm_area_struct vma,
		unsigned long address, pmd_t *pmd, unsigned int flags)
		{
		@@ -95,7 +105,7 @@ retry:
		}
		if ((flags & FOLL_NUMA) && pte_protnone(pte))
		goto no_page;
		if ((flags & FOLL_WRITE) && !pte_write(pte)) {
		if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
		pte_unmap_unlock(ptep, ptl);
		return NULL;
		}
		@@ -412,7 +422,7 @@ static int faultin_page(struct task_struct tsk, struct vm_area_struct vma,
		* reCOWed by userspace write).
		*/
		if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
		*flags &= ~FOLL_WRITE;
		*flags \|= FOLL_COW;
		return 0;
		}

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