On Thu, Jul 27, 2023 at 07:59:33PM +0200, Jann Horn wrote: > On Thu, Jul 27, 2023 at 7:22 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > Hmm. lock_vma_under_rcu() specifically checks for vma->anon_vma==NULL > > condition (see [1]) to avoid going into find_mergeable_anon_vma() (a > > check inside anon_vma_prepare() should prevent that). So, it should > > fall back to mmap_lock'ing. > > This syzkaller report applies to a tree with Willy's in-progress patch > series, where lock_vma_under_rcu() only checks for vma->anon_vma if > vma_is_anonymous() is true - it permits private non-anonymous VMAs > (which require an anon_vma for handling write faults) even if they > don't have an anon_vma. > > The commit bisected by syzkaller > (https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=a52f58b34afe095ebc5823684eb264404dad6f7b) > removes the vma_is_anonymous() check in handle_pte_fault(), so it lets > us reach do_wp_page() with a non-anonymous private VMA without > anon_vma, even though that requires allocation of an anon_vma. > > So I think this is pretty clearly an issue with Willy's in-progress > patch series that syzkaller blamed correctly. Agreed. What do we think the right solution is? Option 1: +++ b/mm/memory.c @@ -3197,6 +3197,12 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) struct mmu_notifier_range range; int ret; + if (!vma->anon_vma) { + // check if there are other things to undo here + vma_end_read(vmf->vma); + return VM_FAULT_RETRY; + } + delayacct_wpcopy_start(); Option 2: @@ -5581,7 +5587,8 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, goto inval; /* find_mergeable_anon_vma uses adjacent vmas which are not locked */ - if (vma_is_anonymous(vma) && !vma->anon_vma) + if ((vma_is_anonymous(vma) || + vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) && !vma->anon_vma) goto inval; The problem with option 2 is that we don't know whether this is a write fault or not, so we'll handle read faults on private file mappings under the mmap_lock UNTIL somebody writes to the mapping, which might be never. That seems like a bad idea. We could pass FAULT_FLAG_WRITE into lock_vma_under_rcu(), but that also seems like a bad idea. I dunno. Three bad ideas. Anyone think of a good one?
