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author | Linus Torvalds <torvalds@linux-foundation.org> | 2016-10-20 00:49:40 +0200 |
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committer | Simon Shields <keepcalm444@gmail.com> | 2016-10-22 13:35:25 +1100 |
commit | 6c0837fc69be45a1a1bf91c433ac3a4955597c4e (patch) | |
tree | 323245f250fb5d9955e750eb64d708edff292eec /mm | |
parent | a20c4e114c47ab6c34025110cd44775df36079b0 (diff) | |
download | kernel_samsung_smdk4412-6c0837fc69be45a1a1bf91c433ac3a4955597c4e.tar.gz kernel_samsung_smdk4412-6c0837fc69be45a1a1bf91c433ac3a4955597c4e.tar.bz2 kernel_samsung_smdk4412-6c0837fc69be45a1a1bf91c433ac3a4955597c4e.zip |
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 4ceb5db9757a ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f404e5 ("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 abf09bed3cce ("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.
Change-Id: Id9bec3722797dff7d0ff0d9f6097c4229e31fd62
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>
[wt: s/gup.c/memory.c; s/follow_page_pte/follow_page_mask;
s/faultin_page/__get_user_page]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Diffstat (limited to 'mm')
-rw-r--r-- | mm/memory.c | 14 |
1 files changed, 12 insertions, 2 deletions
diff --git a/mm/memory.c b/mm/memory.c index 067a4f369c4..29c8112a88f 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1436,6 +1436,16 @@ int zap_vma_ptes(struct vm_area_struct *vma, unsigned long address, } EXPORT_SYMBOL_GPL(zap_vma_ptes); +/* + * 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)); +} + /** * follow_page - look up a page descriptor from a user-virtual address * @vma: vm_area_struct mapping @address @@ -1518,7 +1528,7 @@ split_fallthrough: pte = *ptep; if (!pte_present(pte)) goto no_page; - if ((flags & FOLL_WRITE) && !pte_write(pte)) + if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) goto unlock; page = vm_normal_page(vma, address, pte); @@ -1832,7 +1842,7 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, */ if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE)) - foll_flags &= ~FOLL_WRITE; + foll_flags |= FOLL_COW; cond_resched(); } |