Re: GPF in shm_lock ipc

[Kirill A. Shutemov]

Dmitry Vyukov wrote:
> On Tue, Oct 13, 2015 at 8:30 PM, Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> > On Mon, Oct 12, 2015 at 08:18:21PM -0700, Davidlohr Bueso wrote:
> >> On Mon, 12 Oct 2015, Bueso wrote:
> >>
> >> >On Mon, 12 Oct 2015, Kirill A. Shutemov wrote:
> >> >
> >> >>On Mon, Oct 12, 2015 at 10:49:45AM -0700, Davidlohr Bueso wrote:
> >> >>>diff --git a/ipc/shm.c b/ipc/shm.c
> >> >>>index 4178727..9615f19 100644
> >> >>>--- a/ipc/shm.c
> >> >>>+++ b/ipc/shm.c
> >> >>>@@ -385,9 +385,25 @@ static struct mempolicy *shm_get_policy(struct vm_area_struct *vma,
> >> >>>static int shm_mmap(struct file *file, struct vm_area_struct *vma)
> >> >>>{
> >> >>>-  struct shm_file_data *sfd = shm_file_data(file);
> >> >>>+  struct file *vma_file = vma->vm_file;
> >> >>>+  struct shm_file_data *sfd = shm_file_data(vma_file);
> >> >>>+  struct ipc_ids *ids = &shm_ids(sfd->ns);
> >> >>>+  struct kern_ipc_perm *shp;
> >> >>>   int ret;
> >> >>>+  rcu_read_lock();
> >> >>>+  shp = ipc_obtain_object_check(ids, sfd->id);
> >> >>>+  if (IS_ERR(shp)) {
> >> >>>+          ret = -EINVAL;
> >> >>>+          goto err;
> >> >>>+  }
> >> >>>+
> >> >>>+  if (!ipc_valid_object(shp)) {
> >> >>>+          ret = -EIDRM;
> >> >>>+          goto err;
> >> >>>+  }
> >> >>>+  rcu_read_unlock();
> >> >>>+
> >> >>
> >> >>Hm. Isn't it racy? What prevents IPC_RMID from happening after this point?
> >> >
> >> >Nothing, but that is later caught by shm_open() doing similar checks. We
> >> >basically end up doing a check between ->mmap() calls, which is fair imho.
> >> >Note that this can occur anywhere in ipc as IPC_RMID is a user request/cmd,
> >> >and we try to respect it -- thus you can argue this race anywhere, which is
> >> >why we have EIDRM/EINVL. Ultimately the user should not be doing such hacks
> >> >_anyway_. So I'm not really concerned about it.
> >> >
> >> >Another similar alternative would be perhaps to make shm_lock() return an
> >> >error, and thus propagate that error to mmap return. That way we would have
> >> >a silent way out of the warning scenario (afterward we cannot race as we
> >> >hold the ipc object lock). However, the users would now have to take this
> >> >into account...
> >> >
> >> >    [validity check lockless]
> >> >    ->mmap()
> >> >    [validity check lock]
> >>
> >> Something like this, maybe. Although I could easily be missing things...
> >> I've tested it enough to see Dimitry's testcase handled ok, and put it
> >> through ltp. Also adding Manfred to the Cc, who always catches my idiotic
> >> mistakes.
> >>
> >> 8<---------------------------------------------------------------------
> >> From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> >> Date: Mon, 12 Oct 2015 19:38:34 -0700
> >> Subject: [PATCH] ipc/shm: fix handling of (re)attaching to a deleted segment
> >>
> >> There are currently two issues when dealing with segments that are
> >> marked for deletion:
> >>
> >> (i) With d0edd8528362 (ipc: convert invalid scenarios to use WARN_ON)
> >> we relaxed the system-wide impact of using a deleted segment. However,
> >> we can now perfectly well trigger the warning and then deference a nil
> >> pointer -- where shp does not exist.
> >>
> >> (ii) As of a399b29dfbaa (ipc,shm: fix shm_file deletion races) we
> >> forbid attaching/mapping a previously deleted segment; a feature once
> >> unique to Linux, but removed[1] as a side effect of lockless ipc object
> >> lookups and security checks. Similarly, Dmitry Vyukov reported[2] a
> >> simple test case that creates a new vma for a previously deleted
> >> segment, triggering the WARN_ON mentioned in (i).
> >>
> >> This patch tries to address (i) by moving the shp error check out
> >> of shm_lock() and handled by the caller instead. The benefit of this
> >> is that it allows better handling out of situations where we end up
> >> returning ERMID or EINVAL. Specifically, there are three callers
> >> of shm_lock which we must look into:
> >>
> >>  - open/close -- which we ensure to never do any operations on
> >>                  the pairs, thus becoming no-ops if found a prev
> >>                IPC_RMID.
> >>
> >>  - loosing the reference of nattch upon shmat(2) -- not feasible.
> >>
> >> In addition, the common WARN_ON call is technically removed, but
> >> we add a new one for the bogus shmat(2) case, which is definitely
> >> unacceptable to race with RMID if nattch is bumped up.
> >>
> >> To address (ii), a new shm_check_vma_validity() helper is added
> >> (for lack of a better name), which attempts to detect early on
> >> any races with RMID, before doing the full ->mmap. There is still
> >> a window between the callback and the shm_open call where we can
> >> race with IPC_RMID. If this is the case, it is handled by the next
> >> shm_lock().
> >>
> >> shm_mmap:
> >>     [shm validity checks lockless]
> >>     ->mmap()
> >>     [shm validity checks lock] <-- at this point there after there
> >>                                    is no race as we hold the ipc
> >>                                    object lock.
> >>
> >> [1] https://lkml.org/lkml/2015/10/12/483
> >> [2] https://lkml.org/lkml/2015/10/12/284
> >>
> >> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
> >> ---
> >>  ipc/shm.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
> >>  1 file changed, 73 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/ipc/shm.c b/ipc/shm.c
> >> index 4178727..47a7a67 100644
> >> --- a/ipc/shm.c
> >> +++ b/ipc/shm.c
> >> @@ -156,11 +156,10 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
> >>       struct kern_ipc_perm *ipcp = ipc_lock(&shm_ids(ns), id);
> >>       /*
> >> -      * We raced in the idr lookup or with shm_destroy().  Either way, the
> >> -      * ID is busted.
> >> +      * Callers of shm_lock() must validate the status of the returned
> >> +      * ipc object pointer (as returned by ipc_lock()), and error out as
> >> +      * appropriate.
> >>        */
> >> -     WARN_ON(IS_ERR(ipcp));
> >> -
> >>       return container_of(ipcp, struct shmid_kernel, shm_perm);
> >>  }
> >> @@ -194,6 +193,15 @@ static void shm_open(struct vm_area_struct *vma)
> >>       struct shmid_kernel *shp;
> >>       shp = shm_lock(sfd->ns, sfd->id);
> >> +     /*
> >> +      * We raced in the idr lookup or with shm_destroy().
> >> +      * Either way, the ID is busted. In the same scenario,
> >> +      * but for the close counter-part, the nattch counter
> >> +      * is never decreased, thus we can safely return.
> >> +      */
> >> +     if (IS_ERR(shp))
> >> +             return; /* no-op */
> >> +
> >>       shp->shm_atim = get_seconds();
> >>       shp->shm_lprid = task_tgid_vnr(current);
> >>       shp->shm_nattch++;
> >
> > ...
> >
> >>  static int shm_mmap(struct file *file, struct vm_area_struct *vma)
> >>  {
> >>       struct shm_file_data *sfd = shm_file_data(file);
> >>       int ret;
> >> +     /*
> >> +      * Ensure that we have not raced with IPC_RMID, such that
> >> +      * we avoid doing the ->mmap altogether. This is a preventive
> >> +      * lockless check, and thus exposed to races during the mmap.
> >> +      * However, this is later caught in shm_open(), and handled
> >> +      * accordingly.
> >> +      */
> >> +     ret = shm_check_vma_validity(vma);
> >> +     if (ret)
> >> +             return ret;
> >> +
> >>       ret = sfd->file->f_op->mmap(sfd->file, vma);
> >>       if (ret != 0)
> >>               return ret;
> >> +
> >>       sfd->vm_ops = vma->vm_ops;
> >>  #ifdef CONFIG_MMU
> >>       WARN_ON(!sfd->vm_ops->fault);
> >
> > If I read it correctly, with the patch we would ignore locking failure
> > inside shm_open() and mmap will succeed in this case. So the idea is to
> > have shm_close() no-op and therefore symmetrical. That's look fragile to
> > me. We would silently miss some other broken open/close pattern.
> >
> > I would rather propagate error to shm_mmap() caller and therefore to
> > userspace. I guess it's better to opencode shm_open() in shm_mmap() and
> > return error this way. shm_open() itself can have WARN_ON_ONCE() for
> > failure or something.
> 
> 
> Davidlohr, any updates on this? Is it committed? I don't see it in Linus tree.
> What do you think about Kirill's comments?

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