From: Mike Rapoport (Microsoft) <rppt@kernel.org>
Date: Wed, 13 May 2026 08:14:16 +0000 (+0300)
Subject: userfaultfd: ensure mremap_userfaultfd_fail() releases mmap_changing
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0496a59745b0723ea74274db16fd5c8b1379b9a9;p=thirdparty%2Flinux.git

userfaultfd: ensure mremap_userfaultfd_fail() releases mmap_changing

Sashiko says:

  mremap_userfaultfd_prep() increments ctx->mmap_changing to stall
  concurrent operations, but mremap_userfaultfd_fail() does not
  decrement it before dropping the context reference.

If an mremap operation fails, ctx->mmap_changing remains elevated. This
will causes subsequent userfaultfd operations like a UFFDIO_COPY to fail
with -EAGAIN.

Decrement ctx->mmap_changing in mremap_userfaultfd_fail().

Link: https://sashiko.dev/#/patchset/20260430113512.115938-1-rppt@kernel.org
Link: https://lore.kernel.org/20260513081416.495963-1-rppt@kernel.org
Fixes: df2cc96e7701 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4b53dc4a3266..390e4b7d9cb9 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -786,6 +786,8 @@ void mremap_userfaultfd_fail(struct vm_userfaultfd_ctx *vm_ctx)
 	if (!ctx)
 		return;
 
+	atomic_dec(&ctx->mmap_changing);
+	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);
 	userfaultfd_ctx_put(ctx);
 }