Re: [RFC PATCH RESEND 06/28] mm: mark VMA as locked whenever vma->vm_flags are modified

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Sep 6, 2022 at 7:27 AM Laurent Dufour <ldufour@linux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > VMA flag modifications should be done under VMA lock to prevent concurrent
> > page fault handling in that area.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  fs/proc/task_mmu.c | 1 +
> >  fs/userfaultfd.c   | 6 ++++++
> >  mm/madvise.c       | 1 +
> >  mm/mlock.c         | 2 ++
> >  mm/mmap.c          | 1 +
> >  mm/mprotect.c      | 1 +
> >  6 files changed, 12 insertions(+)
>
> There are few changes also done in the driver's space, for instance:
>
> *** arch/x86/kernel/cpu/sgx/driver.c:
> sgx_mmap[98]                   vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND |
> VM_DONTDUMP | VM_IO;
> *** arch/x86/kernel/cpu/sgx/virt.c:
> sgx_vepc_mmap[108]             vma->vm_flags |= VM_PFNMAP | VM_IO |
> VM_DONTDUMP | VM_DONTCOPY;
> *** drivers/dax/device.c:
> dax_mmap[311]                  vma->vm_flags |= VM_HUGEPAGE;
>
> I guess these changes to vm_flags should be protected as well, or to be
> checked one by one.

Thanks for noting these! I'll add necessary locking here and will look
for other places I might have missed.

>
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 4e0023643f8b..ceffa5c2c650 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1285,6 +1285,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
> >                       for (vma = mm->mmap; vma; vma = vma->vm_next) {
> >                               if (!(vma->vm_flags & VM_SOFTDIRTY))
> >                                       continue;
> > +                             vma_mark_locked(vma);
> >                               vma->vm_flags &= ~VM_SOFTDIRTY;
> >                               vma_set_page_prot(vma);
> >                       }
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 175de70e3adf..fe557b3d1c07 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -620,6 +620,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
> >               mmap_write_lock(mm);
> >               for (vma = mm->mmap; vma; vma = vma->vm_next)
> >                       if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> > +                             vma_mark_locked(vma);
> >                               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> >                               vma->vm_flags &= ~__VM_UFFD_FLAGS;
> >                       }
> > @@ -653,6 +654,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> >
> >       octx = vma->vm_userfaultfd_ctx.ctx;
> >       if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> > +             vma_mark_locked(vma);
> >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> >               vma->vm_flags &= ~__VM_UFFD_FLAGS;
> >               return 0;
> > @@ -734,6 +736,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> >               atomic_inc(&ctx->mmap_changing);
> >       } else {
> >               /* Drop uffd context if remap feature not enabled */
> > +             vma_mark_locked(vma);
> >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> >               vma->vm_flags &= ~__VM_UFFD_FLAGS;
> >       }
> > @@ -891,6 +894,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
> >                       vma = prev;
> >               else
> >                       prev = vma;
> > +             vma_mark_locked(vma);
> >               vma->vm_flags = new_flags;
> >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> >       }
> > @@ -1449,6 +1453,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> >                * the next vma was merged into the current one and
> >                * the current one has not been updated yet.
> >                */
> > +             vma_mark_locked(vma);
> >               vma->vm_flags = new_flags;
> >               vma->vm_userfaultfd_ctx.ctx = ctx;
> >
> > @@ -1630,6 +1635,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> >                * the next vma was merged into the current one and
> >                * the current one has not been updated yet.
> >                */
> > +             vma_mark_locked(vma);
> >               vma->vm_flags = new_flags;
> >               vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 5f0f0948a50e..a173f0025abd 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -181,6 +181,7 @@ static int madvise_update_vma(struct vm_area_struct *vma,
> >       /*
> >        * vm_flags is protected by the mmap_lock held in write mode.
> >        */
> > +     vma_mark_locked(vma);
> >       vma->vm_flags = new_flags;
> >       if (!vma->vm_file) {
> >               error = replace_anon_vma_name(vma, anon_name);
> > diff --git a/mm/mlock.c b/mm/mlock.c
> > index b14e929084cc..f62e1a4d05f2 100644
> > --- a/mm/mlock.c
> > +++ b/mm/mlock.c
> > @@ -380,6 +380,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
> >        */
> >       if (newflags & VM_LOCKED)
> >               newflags |= VM_IO;
> > +     vma_mark_locked(vma);
> >       WRITE_ONCE(vma->vm_flags, newflags);
> >
> >       lru_add_drain();
> > @@ -456,6 +457,7 @@ static int mlock_fixup(struct vm_area_struct *vma, struct vm_area_struct **prev,
> >
> >       if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
> >               /* No work to do, and mlocking twice would be wrong */
> > +             vma_mark_locked(vma);
> >               vma->vm_flags = newflags;
> >       } else {
> >               mlock_vma_pages_range(vma, start, end, newflags);
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 693e6776be39..f89c9b058105 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1818,6 +1818,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  out:
> >       perf_event_mmap(vma);
> >
> > +     vma_mark_locked(vma);
> >       vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
> >       if (vm_flags & VM_LOCKED) {
> >               if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
>
> I guess, this doesn't really impact, but the call to vma_mark_locked(vma)
> may be done only in the case the vm_flags field is touched.
> Something like this:
>
>         vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
>         if (vm_flags & VM_LOCKED) {
>                 if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
>                                         is_vm_hugetlb_page(vma) ||
> -                                       vma == get_gate_vma(current->mm))
> +                                       vma == get_gate_vma(current->mm)) {
> +                       vma_mark_locked(vma);
>                         vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
> -               else
> +               } else
>                         mm->locked_vm += (len >> PAGE_SHIFT);
>         }
>
>
> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index bc6bddd156ca..df47fc21b0e4 100644
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -621,6 +621,7 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
> >        * vm_flags and vm_page_prot are protected by the mmap_lock
> >        * held in write mode.
> >        */
> > +     vma_mark_locked(vma);
> >       vma->vm_flags = newflags;
> >       /*
> >        * We want to check manually if we can change individual PTEs writable
>

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