[hotfix,6.12,1/2] fork: do not invoke uffd on fork if error occurs

Commit Message

Lorenzo Stoakes Oct. 15, 2024, 5:56 p.m. UTC

Currently on fork we expose the virtual address space of a process to
userland unconditionally if uffd is registered in VMAs, regardless of
whether an error arose in the fork.

This is performed in dup_userfaultfd_complete() which is invoked
unconditionally, and performs two duties - invoking registered handlers for
the UFFD_EVENT_FORK event via dup_fctx(), and clearing down
userfaultfd_fork_ctx objects established in dup_userfaultfd().

This is problematic, because the virtual address space may not yet be
correctly initialised if an error arose.

The change in commit d24062914837 ("fork: use __mt_dup() to duplicate maple
tree in dup_mmap()") makes this more pertinent as we may be in a state
where entries in the maple tree are not yet consistent.

We address this by, on fork error, ensuring that we roll back state that we
would otherwise expect to clean up through the event being handled by
userland and perform the memory freeing duty otherwise performed by
dup_userfaultfd_complete().

We do this by implementing a new function, dup_userfaultfd_fail(), which
performs the same loop, only decrementing reference counts.

Note that we perform mmgrab() on the parent and child mm's, however
userfaultfd_ctx_put() will mmdrop() this once the reference count drops to
zero, so we will avoid memory leaks correctly here.

Reported-by: Jann Horn <[e-mail]>
Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()")
Reviewed-by: Jann Horn <[e-mail]>
Reviewed-by: Liam R. Howlett <[e-mail]>
Cc: [e-mail]
Signed-off-by: Lorenzo Stoakes <[e-mail]>
---
 fs/userfaultfd.c              | 28 ++++++++++++++++++++++++++++
 include/linux/userfaultfd_k.h |  5 +++++
 kernel/fork.c                 |  5 ++++-
 3 files changed, 37 insertions(+), 1 deletion(-)

--
2.46.2

Patch

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 68cdd89c97a3..7c0bd0b55f88 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -692,6 +692,34 @@  void dup_userfaultfd_complete(struct list_head *fcs)
 	}
 }

+void dup_userfaultfd_fail(struct list_head *fcs)
+{
+	struct userfaultfd_fork_ctx *fctx, *n;
+
+	/*
+	 * An error has occurred on fork, we will tear memory down, but have
+	 * allocated memory for fctx's and raised reference counts for both the
+	 * original and child contexts (and on the mm for each as a result).
+	 *
+	 * These would ordinarily be taken care of by a user handling the event,
+	 * but we are no longer doing so, so manually clean up here.
+	 *
+	 * mm tear down will take care of cleaning up VMA contexts.
+	 */
+	list_for_each_entry_safe(fctx, n, fcs, list) {
+		struct userfaultfd_ctx *octx = fctx->orig;
+		struct userfaultfd_ctx *ctx = fctx->new;
+
+		atomic_dec(&octx->mmap_changing);
+		VM_BUG_ON(atomic_read(&octx->mmap_changing) < 0);
+		userfaultfd_ctx_put(octx);
+		userfaultfd_ctx_put(ctx);
+
+		list_del(&fctx->list);
+		kfree(fctx);
+	}
+}
+
 void mremap_userfaultfd_prep(struct vm_area_struct *vma,
 			     struct vm_userfaultfd_ctx *vm_ctx)
 {
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 9fc6ce15c499..cb40f1a1d081 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -249,6 +249,7 @@  static inline bool vma_can_userfault(struct vm_area_struct *vma,

 extern int dup_userfaultfd(struct vm_area_struct *, struct list_head *);
 extern void dup_userfaultfd_complete(struct list_head *);
+void dup_userfaultfd_fail(struct list_head *);

 extern void mremap_userfaultfd_prep(struct vm_area_struct *,
 				    struct vm_userfaultfd_ctx *);
@@ -351,6 +352,10 @@  static inline void dup_userfaultfd_complete(struct list_head *l)
 {
 }

+static inline void dup_userfaultfd_fail(struct list_head *l)
+{
+}
+
 static inline void mremap_userfaultfd_prep(struct vm_area_struct *vma,
 					   struct vm_userfaultfd_ctx *ctx)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index 89ceb4a68af2..597b477dd491 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -775,7 +775,10 @@  static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	mmap_write_unlock(mm);
 	flush_tlb_mm(oldmm);
 	mmap_write_unlock(oldmm);
-	dup_userfaultfd_complete(&uf);
+	if (!retval)
+		dup_userfaultfd_complete(&uf);
+	else
+		dup_userfaultfd_fail(&uf);
 fail_uprobe_end:
 	uprobe_end_dup_mmap();
 	return retval;