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.
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)
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).)
