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Commit 19be0eaf authored by Linus Torvalds's avatar Linus Torvalds
Browse files

mm: remove gup_flags FOLL_WRITE games from __get_user_pages() 



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: default avatarPhil "not Paul" Oester <kernel@linuxace.com>
Acked-by: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarMichal 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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6b25e21f

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 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, 
	}

	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;

}