Hi Alex, After more thinking, looks like adding a r-b tree is still not enough to solve the potential problem for free a range of PASID in one ioctl. If caller gives [0, MAX_UNIT] in the free request, kernel anyhow should loop all the PASIDs and search in the r-b tree. Even VFIO can track the smallest/largest allocated PASID, and limit the free range to an accurate range, it is still no efficient. For example, user has allocated two PASIDs ( 1 and 999), and user gives the [0, MAX_UNIT] range in free request. VFIO will limit the free range to be [1, 999], but still needs to loop PASID 1 - 999, and search in r-b tree. So I'm wondering can we fall back to prior proposal which only free one PASID for a free request. how about your opinion? https://lore.kernel.org/linux-iommu/20200416084031.7266ad40@xxxxxxxxx/ Regards, Yi Liu > From: Liu, Yi L <yi.l.liu@xxxxxxxxx> > Sent: Thursday, July 9, 2020 10:26 AM > > Hi Kevin, > > > From: Tian, Kevin <kevin.tian@xxxxxxxxx> > > Sent: Thursday, July 9, 2020 10:18 AM > > > > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx> > > > Sent: Thursday, July 9, 2020 10:08 AM > > > > > > Hi Kevin, > > > > > > > From: Tian, Kevin <kevin.tian@xxxxxxxxx> > > > > Sent: Thursday, July 9, 2020 9:57 AM > > > > > > > > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx> > > > > > Sent: Thursday, July 9, 2020 8:32 AM > > > > > > > > > > Hi Alex, > > > > > > > > > > > Alex Williamson <alex.williamson@xxxxxxxxxx> > > > > > > Sent: Thursday, July 9, 2020 3:55 AM > > > > > > > > > > > > On Wed, 8 Jul 2020 08:16:16 +0000 "Liu, Yi L" > > > > > > <yi.l.liu@xxxxxxxxx> wrote: > > > > > > > > > > > > > Hi Alex, > > > > > > > > > > > > > > > From: Liu, Yi L < yi.l.liu@xxxxxxxxx> > > > > > > > > Sent: Friday, July 3, 2020 2:28 PM > > > > > > > > > > > > > > > > Hi Alex, > > > > > > > > > > > > > > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > > > > > > > Sent: Friday, July 3, 2020 5:19 AM > > > > > > > > > > > > > > > > > > On Wed, 24 Jun 2020 01:55:19 -0700 Liu Yi L > > > > > > > > > <yi.l.liu@xxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > This patch allows user space to request PASID > > > > > > > > > > allocation/free, > > > e.g. > > > > > > > > > > when serving the request from the guest. > > > > > > > > > > > > > > > > > > > > PASIDs that are not freed by userspace are > > > > > > > > > > automatically freed > > > > > when > > > > > > > > > > the IOASID set is destroyed when process exits. > > > > > > > [...] > > > > > > > > > > +static int vfio_iommu_type1_pasid_request(struct > > > > > > > > > > +vfio_iommu > > > > > *iommu, > > > > > > > > > > + unsigned long arg) { > > > > > > > > > > + struct vfio_iommu_type1_pasid_request req; > > > > > > > > > > + unsigned long minsz; > > > > > > > > > > + > > > > > > > > > > + minsz = offsetofend(struct > > vfio_iommu_type1_pasid_request, > > > > > > range); > > > > > > > > > > + > > > > > > > > > > + if (copy_from_user(&req, (void __user *)arg, minsz)) > > > > > > > > > > + return -EFAULT; > > > > > > > > > > + > > > > > > > > > > + if (req.argsz < minsz || (req.flags & > > > > > > ~VFIO_PASID_REQUEST_MASK)) > > > > > > > > > > + return -EINVAL; > > > > > > > > > > + > > > > > > > > > > + if (req.range.min > req.range.max) > > > > > > > > > > > > > > > > > > Is it exploitable that a user can spin the kernel for a > > > > > > > > > long time in the case of a free by calling this with [0, > > > > > > > > > MAX_UINT] regardless of their > > > > > > actual > > > > > > > > allocations? > > > > > > > > > > > > > > > > IOASID can ensure that user can only free the PASIDs > > > > > > > > allocated to the > > > > > user. > > > > > > but > > > > > > > > it's true, kernel needs to loop all the PASIDs within the > > > > > > > > range provided by user. > > > > > > it > > > > > > > > may take a long time. is there anything we can do? one > > > > > > > > thing may limit > > > > > the > > > > > > range > > > > > > > > provided by user? > > > > > > > > > > > > > > thought about it more, we have per-VM pasid quota (say > > > > > > > 1000), so even if user passed down [0, MAX_UNIT], kernel > > > > > > > will only loop the > > > > > > > 1000 pasids at most. do you think we still need to do something on it? > > > > > > > > > > > > How do you figure that? vfio_iommu_type1_pasid_request() > > > > > > accepts the user's min/max so long as (max > min) and passes > > > > > > that to vfio_iommu_type1_pasid_free(), then to > > > > > > vfio_pasid_free_range() which loops as: > > > > > > > > > > > > ioasid_t pasid = min; > > > > > > for (; pasid <= max; pasid++) > > > > > > ioasid_free(pasid); > > > > > > > > > > > > A user might only be able to allocate 1000 pasids, but > > > > > > apparently they can ask to free all they want. > > > > > > > > > > > > It's also not obvious to me that calling ioasid_free() is only > > > > > > allowing the user to free their own passid. Does it? It > > > > > > would be a pretty > > > > > > > > Agree. I thought ioasid_free should at least carry a token since > > > > the user > > > space is > > > > only allowed to manage PASIDs in its own set... > > > > > > > > > > gaping hole if a user could free arbitrary pasids. A r-b tree > > > > > > of passids might help both for security and to bound spinning in a loop. > > > > > > > > > > oh, yes. BTW. instead of r-b tree in VFIO, maybe we can add an > > > > > ioasid_set parameter for ioasid_free(), thus to prevent the user > > > > > from freeing PASIDs that doesn't belong to it. I remember Jacob > > > > > mentioned it > > > before. > > > > > > > > > > > > > check current ioasid_free: > > > > > > > > spin_lock(&ioasid_allocator_lock); > > > > ioasid_data = xa_load(&active_allocator->xa, ioasid); > > > > if (!ioasid_data) { > > > > pr_err("Trying to free unknown IOASID %u\n", ioasid); > > > > goto exit_unlock; > > > > } > > > > > > > > Allow an user to trigger above lock paths with MAX_UINT times > > > > might still > > > be bad. > > > > > > yeah, how about the below two options: > > > > > > - comparing the max - min with the quota before calling ioasid_free(). > > > If max - min > current quota of the user, then should fail it. If > > > max - min < quota, then call ioasid_free() one by one. still trigger > > > the above lock path with quota times. > > > > This is definitely wrong. [min, max] is about the range of the PASID > > value, while quota is about the number of allocated PASIDs. It's a bit > > weird to mix two together. > > got it. > > > btw what is the main purpose of allowing batch PASID free requests? > > Can we just simplify to allow one PASID in each free just like how is > > it done in allocation path? > > it's an intention to reuse the [min, max] range as allocation path. currently, we > don't have such request as far as I can see. > > > > > > > - pass the max and min to ioasid_free(), let ioasid_free() decide. should > > > be able to avoid trigger the lock multiple times, and ioasid has have a > > > track on how may PASIDs have been allocated, if max - min is larger than > > > the allocated number, should fail anyway. > > > > What about Alex's r-b tree suggestion? Is there any downside in you mind? > > no downside, I was just wanting to reuse the tracks in ioasid_set. I can add a r-b > for allocated PASIDs and find the PASIDs in the r-b tree only do free for the > PASIDs found in r-b tree, others in the range would be ignored. > does it look good? > > Regards, > Yi Liu > > > Thanks, > > Kevin
