The following commit has been merged into the core/urgent branch of tip: Commit-ID: 2667ed10d9f01e250ba806276740782c89d77fda Gitweb: https://git.kernel.org/tip/2667ed10d9f01e250ba806276740782c89d77fda Author: Fenghua Yu <fenghua.yu@xxxxxxxxx> AuthorDate: Thu, 28 Apr 2022 11:00:41 -07:00 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitterDate: Sun, 01 May 2022 10:17:17 +02:00 mm: Fix PASID use-after-free issue The PASID is being freed too early. It needs to stay around until after device drivers that might be using it have had a chance to clear it out of the hardware. The relevant refcounts are: mmget() /mmput() refcount the mm's address space mmgrab()/mmdrop() refcount the mm itself The PASID is currently tied to the life of the mm's address space and freed in __mmput(). This makes logical sense because the PASID can't be used once the address space is gone. But, this misses an important point: even after the address space is gone, the PASID will still be programmed into a device. Device drivers might, for instance, still need to flush operations that are outstanding and need to use that PASID. They do this at file->release() time. Device drivers call the IOMMU driver to hold a reference on the mm itself and drop it at file->release() time. But, the IOMMU driver holds a reference on the mm itself, not the address space. The address space (and the PASID) is long gone by the time the driver tries to clean up. This is effectively a use-after-free bug on the PASID. To fix this, move the PASID free operation from __mmput() to __mmdrop(). This ensures that the IOMMU driver's existing mmgrab() keeps the PASID allocated until it drops its mm reference. Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit") Reported-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx> Suggested-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> Suggested-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx> Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> Link: https://lore.kernel.org/r/20220428180041.806809-1-fenghua.yu@xxxxxxxxx --- kernel/fork.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/fork.c b/kernel/fork.c index 9796897..35a3bef 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -792,6 +792,7 @@ void __mmdrop(struct mm_struct *mm) mmu_notifier_subscriptions_destroy(mm); check_mm(mm); put_user_ns(mm->user_ns); + mm_pasid_drop(mm); free_mm(mm); } EXPORT_SYMBOL_GPL(__mmdrop); @@ -1190,7 +1191,6 @@ static inline void __mmput(struct mm_struct *mm) } if (mm->binfmt) module_put(mm->binfmt->module); - mm_pasid_drop(mm); mmdrop(mm); }
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