Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
On Tue, Jan 17, 2023 at 12:36 PM Jann Horn wrote: > > On Tue, Jan 17, 2023 at 8:51 PM Jann Horn wrote: > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > > > Due to the possibility of handle_userfault dropping mmap_lock, avoid fault > > > handling under VMA lock and retry holding mmap_lock. This can be handled > > > more gracefully in the future. > > > > > > Signed-off-by: Suren Baghdasaryan > > > Suggested-by: Peter Xu > > > --- > > > mm/memory.c | 7 +++ > > > 1 file changed, 7 insertions(+) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index 20806bc8b4eb..12508f4d845a 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct > > > mm_struct *mm, > > > if (!vma->anon_vma) > > > goto inval; > > > > > > + /* > > > +* Due to the possibility of userfault handler dropping > > > mmap_lock, avoid > > > +* it for now and fall back to page fault handling under > > > mmap_lock. > > > +*/ > > > + if (userfaultfd_armed(vma)) > > > + goto inval; > > > > This looks racy wrt concurrent userfaultfd_register(). I think you'll > > want to do the userfaultfd_armed(vma) check _after_ locking the VMA, > > I still think this change is needed... Yes, I think you are right. I'll move the check after locking the VMA. Thanks! > > > and ensure that the userfaultfd code write-locks the VMA before > > changing the __VM_UFFD_FLAGS in vma->vm_flags. > > Ah, but now I see you already took care of this half of the issue with > the reset_vm_flags() change in > https://lore.kernel.org/linux-mm/20230109205336.3665937-16-sur...@google.com/ > . > > > > > if (!vma_read_trylock(vma)) > > > goto inval; > > > > > > -- > > > 2.39.0 > > >
Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
On Tue, Jan 17, 2023 at 8:51 PM Jann Horn wrote: > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > > Due to the possibility of handle_userfault dropping mmap_lock, avoid fault > > handling under VMA lock and retry holding mmap_lock. This can be handled > > more gracefully in the future. > > > > Signed-off-by: Suren Baghdasaryan > > Suggested-by: Peter Xu > > --- > > mm/memory.c | 7 +++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 20806bc8b4eb..12508f4d845a 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct > > mm_struct *mm, > > if (!vma->anon_vma) > > goto inval; > > > > + /* > > +* Due to the possibility of userfault handler dropping mmap_lock, > > avoid > > +* it for now and fall back to page fault handling under mmap_lock. > > +*/ > > + if (userfaultfd_armed(vma)) > > + goto inval; > > This looks racy wrt concurrent userfaultfd_register(). I think you'll > want to do the userfaultfd_armed(vma) check _after_ locking the VMA, I still think this change is needed... > and ensure that the userfaultfd code write-locks the VMA before > changing the __VM_UFFD_FLAGS in vma->vm_flags. Ah, but now I see you already took care of this half of the issue with the reset_vm_flags() change in https://lore.kernel.org/linux-mm/20230109205336.3665937-16-sur...@google.com/ . > > if (!vma_read_trylock(vma)) > > goto inval; > > > > -- > > 2.39.0 > >
Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > Due to the possibility of handle_userfault dropping mmap_lock, avoid fault > handling under VMA lock and retry holding mmap_lock. This can be handled > more gracefully in the future. > > Signed-off-by: Suren Baghdasaryan > Suggested-by: Peter Xu > --- > mm/memory.c | 7 +++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 20806bc8b4eb..12508f4d845a 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct > mm_struct *mm, > if (!vma->anon_vma) > goto inval; > > + /* > +* Due to the possibility of userfault handler dropping mmap_lock, > avoid > +* it for now and fall back to page fault handling under mmap_lock. > +*/ > + if (userfaultfd_armed(vma)) > + goto inval; This looks racy wrt concurrent userfaultfd_register(). I think you'll want to do the userfaultfd_armed(vma) check _after_ locking the VMA, and ensure that the userfaultfd code write-locks the VMA before changing the __VM_UFFD_FLAGS in vma->vm_flags. > if (!vma_read_trylock(vma)) > goto inval; > > -- > 2.39.0 >
[PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
Due to the possibility of handle_userfault dropping mmap_lock, avoid fault handling under VMA lock and retry holding mmap_lock. This can be handled more gracefully in the future. Signed-off-by: Suren Baghdasaryan Suggested-by: Peter Xu --- mm/memory.c | 7 +++ 1 file changed, 7 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index 20806bc8b4eb..12508f4d845a 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, if (!vma->anon_vma) goto inval; + /* +* Due to the possibility of userfault handler dropping mmap_lock, avoid +* it for now and fall back to page fault handling under mmap_lock. +*/ + if (userfaultfd_armed(vma)) + goto inval; + if (!vma_read_trylock(vma)) goto inval; -- 2.39.0
