userfaultfd: retry copying with locks dropped in mfill_atomic_pte_copy()

Implementation of UFFDIO_COPY for anonymous memory might fail to copy data
from userspace buffer when the destination VMA is locked (either with
mm_lock or with per-VMA lock).

In that case, mfill_atomic() releases the locks, retries copying the data
with locks dropped and then re-locks the destination VMA and
re-establishes PMD.

Since this retry-reget dance is only relevant for UFFDIO_COPY and it never
happens for other UFFDIO_ operations, make it a part of
mfill_atomic_pte_copy() that actually implements UFFDIO_COPY for anonymous
memory.

As a temporal safety measure to avoid breaking biscection
mfill_atomic_pte_copy() makes sure to never return -ENOENT so that the
loop in mfill_atomic() won't retry copiyng outside of mmap_lock.  This is
removed later when shmem implementation will be updated later and the loop
in mfill_atomic() will be adjusted.

[akpm@linux-foundation.org: update mfill_copy_folio_retry()]
Link: https://lore.kernel.org/20260316173829.1126728-1-avagin@google.com
Link: https://lore.kernel.org/20260306171815.3160826-6-rppt@kernel.org
Link: https://lore.kernel.org/20260402041156.1377214-6-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nikita Kalyazin <kalyazin@amazon.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Carlier <devnexen@gmail.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index bcba57dc1aeee3adb73e13cb13a22c4cd76d018d..4857be5a7fa209354646cc65d4d68a749d61fbae 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -405,35 +405,63 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
        return ret;
 }
 
+static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio)
+{
+       unsigned long src_addr = state->src_addr;
+       void *kaddr;
+       int err;
+
+       /* retry copying with mm_lock dropped */
+       mfill_put_vma(state);
+
+       kaddr = kmap_local_folio(folio, 0);
+       err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE);
+       kunmap_local(kaddr);
+       if (unlikely(err))
+               return -EFAULT;
+
+       flush_dcache_folio(folio);
+
+       /* reget VMA and PMD, they could change underneath us */
+       err = mfill_get_vma(state);
+       if (err)
+               return err;
+
+       err = mfill_establish_pmd(state);
+       if (err)
+               return err;
+
+       return 0;
+}
+
 static int mfill_atomic_pte_copy(struct mfill_state *state)
 {
-       struct vm_area_struct *dst_vma = state->vma;
        unsigned long dst_addr = state->dst_addr;
        unsigned long src_addr = state->src_addr;
        uffd_flags_t flags = state->flags;
-       pmd_t *dst_pmd = state->pmd;
        struct folio *folio;
        int ret;
 
-       if (!state->folio) {
-               ret = -ENOMEM;
-               folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma,
-                                       dst_addr);
-               if (!folio)
-                       goto out;
+       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, state->vma, dst_addr);
+       if (!folio)
+               return -ENOMEM;
 
-               ret = mfill_copy_folio_locked(folio, src_addr);
+       ret = -ENOMEM;
+       if (mem_cgroup_charge(folio, state->vma->vm_mm, GFP_KERNEL))
+               goto out_release;
 
-               /* fallback to copy_from_user outside mmap_lock */
-               if (unlikely(ret)) {
-                       ret = -ENOENT;
-                       state->folio = folio;
-                       /* don't free the page */
-                       goto out;
-               }
-       } else {
-               folio = state->folio;
-               state->folio = NULL;
+       ret = mfill_copy_folio_locked(folio, src_addr);
+       if (unlikely(ret)) {
+               /*
+                * Fallback to copy_from_user outside mmap_lock.
+                * If retry is successful, mfill_copy_folio_locked() returns
+                * with locks retaken by mfill_get_vma().
+                * If there was an error, we must mfill_put_vma() anyway and it
+                * will take care of unlocking if needed.
+                */
+               ret = mfill_copy_folio_retry(state, folio);
+               if (ret)
+                       goto out_release;
        }
 
        /*
@@ -443,17 +471,16 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
         */
        __folio_mark_uptodate(folio);
 
-       ret = -ENOMEM;
-       if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
-               goto out_release;
-
-       ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
+       ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
                                       &folio->page, true, flags);
        if (ret)
                goto out_release;
 out:
        return ret;
 out_release:
+       /* Don't return -ENOENT so that our caller won't retry */
+       if (ret == -ENOENT)
+               ret = -EFAULT;
        folio_put(folio);
        goto out;
 }