Commit 1294d355 authored Oct 13, 2016 by Linus Torvalds Committed by Greg Kroah-Hartman Oct 20, 2016

mm: remove gup_flags FOLL_WRITE games from __get_user_pages()



commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 upstream.

This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db9 ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f4 ("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 abf09bed ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 74a2862d

include/linux/mm.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -2112,6 +2112,7 @@ static inline struct page follow_page(struct vm_area_struct vma,
		#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
		#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
		#define FOLL_MLOCK 0x1000 /* lock present pages */
		#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
		@@ -58,6 +58,16 @@ 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)
		{
		@@ -92,7 +102,7 @@ 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;
		}
		@@ -352,7 +362,7 @@ 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;
		}