On Tue, Feb 6, 2024 at 10:28 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Tue, Feb 6, 2024 at 2:09 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> > All userfaultfd operations, except write-protect, opportunistically use
> > per-vma locks to lock vmas. On failure, attempt again inside mmap_lock
> > critical section.
> >
> > Write-protect operation requires mmap_lock as it iterates over multiple
> > vmas.
> >
> > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> [...]
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b05fd28dbce1..393ab3b0d6f3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> [...]
> > +/*
> > + * lock_vma() - Lookup and lock VMA corresponding to @address.
> > + * @prepare_anon: If true, then prepare the VMA (if anonymous) with anon_vma.
> > + *
> > + * Should be called without holding mmap_lock. VMA should be unlocked after use
> > + * with unlock_vma().
> > + *
> > + * Return: A locked VMA containing @address, NULL of no VMA is found, or
> > + * -ENOMEM if anon_vma couldn't be allocated.
> > + */
> > +struct vm_area_struct *lock_vma(struct mm_struct *mm,
> > + unsigned long address,
> > + bool prepare_anon)
> > +{
> > + struct vm_area_struct *vma;
> > +
> > + vma = lock_vma_under_rcu(mm, address);
> > +
> > + if (vma)
> > + return vma;
> > +
> > + mmap_read_lock(mm);
> > + vma = vma_lookup(mm, address);
> > + if (vma) {
> > + if (prepare_anon && vma_is_anonymous(vma) &&
> > + anon_vma_prepare(vma))
> > + vma = ERR_PTR(-ENOMEM);
> > + else
> > + vma_acquire_read_lock(vma);
>
> This new code only calls anon_vma_prepare() for VMAs where
> vma_is_anonymous() is true (meaning they are private anonymous).
>
> [...]
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 74aad0831e40..64e22e467e4f 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -19,20 +19,25 @@
> > #include <asm/tlb.h>
> > #include "internal.h"
> >
> > -static __always_inline
> > -struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
> > - unsigned long dst_start,
> > - unsigned long len)
> > +/* Search for VMA and make sure it is valid. */
> > +static struct vm_area_struct *find_and_lock_dst_vma(struct mm_struct *dst_mm,
> > + unsigned long dst_start,
> > + unsigned long len)
> > {
> > - /*
> > - * Make sure that the dst range is both valid and fully within a
> > - * single existing vma.
> > - */
> > struct vm_area_struct *dst_vma;
> >
> > - dst_vma = find_vma(dst_mm, dst_start);
> > - if (!range_in_vma(dst_vma, dst_start, dst_start + len))
> > - return NULL;
> > + /* Ensure anon_vma is assigned for anonymous vma */
> > + dst_vma = lock_vma(dst_mm, dst_start, true);
>
> lock_vma() is now used by find_and_lock_dst_vma(), which is used by
> mfill_atomic().
>
> > + if (!dst_vma)
> > + return ERR_PTR(-ENOENT);
> > +
> > + if (PTR_ERR(dst_vma) == -ENOMEM)
> > + return dst_vma;
> > +
> > + /* Make sure that the dst range is fully within dst_vma. */
> > + if (dst_start + len > dst_vma->vm_end)
> > + goto out_unlock;
> >
> > /*
> > * Check the vma is registered in uffd, this is required to
> [...]
> > @@ -597,7 +599,15 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> > copied = 0;
> > folio = NULL;
> > retry:
> > - mmap_read_lock(dst_mm);
> > + /*
> > + * Make sure the vma is not shared, that the dst range is
> > + * both valid and fully within a single existing vma.
> > + */
> > + dst_vma = find_and_lock_dst_vma(dst_mm, dst_start, len);
> > + if (IS_ERR(dst_vma)) {
> > + err = PTR_ERR(dst_vma);
> > + goto out;
> > + }
> >
> > /*
> > * If memory mappings are changing because of non-cooperative
> > @@ -609,15 +619,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> > if (atomic_read(&ctx->mmap_changing))
> > goto out_unlock;
> >
> > - /*
> > - * Make sure the vma is not shared, that the dst range is
> > - * both valid and fully within a single existing vma.
> > - */
> > - err = -ENOENT;
> > - dst_vma = find_dst_vma(dst_mm, dst_start, len);
> > - if (!dst_vma)
> > - goto out_unlock;
> > -
> > err = -EINVAL;
> > /*
> > * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
> > @@ -647,16 +648,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> > uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> > goto out_unlock;
> >
> > - /*
> > - * Ensure the dst_vma has a anon_vma or this page
> > - * would get a NULL anon_vma when moved in the
> > - * dst_vma.
> > - */
> > - err = -ENOMEM;
> > - if (!(dst_vma->vm_flags & VM_SHARED) &&
> > - unlikely(anon_vma_prepare(dst_vma)))
> > - goto out_unlock;
>
> But the check mfill_atomic() used to do was different, it checked for VM_SHARED.
Thanks so much for catching this.
>
> Each VMA has one of these three types:
>
> 1. shared (marked by VM_SHARED; does not have an anon_vma)
> 2. private file-backed (needs to have anon_vma when storing PTEs)
> 3. private anonymous (what vma_is_anonymous() detects; needs to have
> anon_vma when storing PTEs)
As in the case of mfill_atomic(), it seems to me that checking for
VM_SHARED flag will cover both (2) and (3) right?
>
> This old code would call anon_vma_prepare() for both private VMA types
> (which is correct). The new code only calls anon_vma_prepare() for
> private anonymous VMAs, not for private file-backed ones. I think this
> code will probably crash with a BUG_ON() in __folio_set_anon() if you
> try to use userfaultfd to insert a PTE into a private file-backed VMA
> of a shmem file. (Which you should be able to get by creating a file
> in /dev/shm/ and then mapping that file with mmap(NULL, <size>,
> PROT_READ|PROT_WRITE, MAP_PRIVATE, <fd>, 0).)
