[PATCH 6.18.y] mm/userfaultfd: wake waiters when registration modes are removed

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit 4eee9c389b072e161549d46dcb1ae10b0222717e ]

UFFDIO_REGISTER can replace MISSING or MINOR tracking without waking
faults that are already queued. For example, switching an anonymous
range from MISSING to WP leaves a missing-fault waiter asleep until
userspace explicitly wakes it.

When a registration change removes a UFFD mode, wake waiters in the
affected interval after updating the VMA. The VMA is write-locked at
this point, so no new faults can queue under the old modes and woken
threads cannot retry until mmap_write_unlock(). Waiters for retained
modes in the same interval may also wake and refault.

The hang reproduces for MISSING-to-WP and MINOR-to-MISSING transitions,
with both read and unread fault events.

Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Cc: stable@vger.kernel.org
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-25-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: replaced the upstream wakeup wrapper with a direct cross-file wake_userfault() call using a shared range structure ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/userfaultfd.c              |  9 ++-------
 include/linux/userfaultfd_k.h |  8 ++++++++
 mm/userfaultfd.c              | 12 ++++++++++++
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 7e40eb9131bd7..22d4a7ad54575 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -71,11 +71,6 @@ struct userfaultfd_wait_queue {
 	bool waken;
 };
 
-struct userfaultfd_wake_range {
-	unsigned long start;
-	unsigned long len;
-};
-
 /* internal indication that UFFD_API ioctl was successfully executed */
 #define UFFD_FEATURE_INITIALIZED		(1u << 31)
 
@@ -1191,8 +1186,8 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx,
 	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
 }
 
-static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
-					   struct userfaultfd_wake_range *range)
+void wake_userfault(struct userfaultfd_ctx *ctx,
+		    struct userfaultfd_wake_range *range)
 {
 	unsigned seq;
 	bool need_wakeup;
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 796a380664bd8..d65effb5cbff2 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -78,6 +78,14 @@ struct userfaultfd_ctx {
 	struct mm_struct *mm;
 };
 
+struct userfaultfd_wake_range {
+	unsigned long start;
+	unsigned long len;
+};
+
+void wake_userfault(struct userfaultfd_ctx *ctx,
+		    struct userfaultfd_wake_range *range);
+
 extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
 
 /* A combined operation mode + behavior flags. */
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 579445bddf561..5846fbf8baa09 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2064,6 +2064,8 @@ int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
 		prev = vma;
 
 	for_each_vma_range(vmi, vma, end) {
+		const bool any_mode_removed = vma->vm_flags & __VM_UFFD_FLAGS & ~vm_flags;
+
 		cond_resched();
 
 		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async));
@@ -2098,6 +2100,16 @@ int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
 		 */
 		userfaultfd_set_ctx(vma, ctx, vm_flags);
 
+		/* Retry faults for removed modes while the VMA is write-locked. */
+		if (any_mode_removed) {
+			struct userfaultfd_wake_range range = {
+				.start = start,
+				.len = vma_end - start,
+			};
+
+			wake_userfault(ctx, &range);
+		}
+
 		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
 			hugetlb_unshare_all_pmds(vma);
 
-- 
2.53.0