mm: fix (harmless) type confusion in lock_vma_under_rcu()

There is a (harmless) type confusion in lock_vma_under_rcu(): After
vma_start_read(), we have taken the VMA lock but don't know yet whether
the VMA has already been detached and scheduled for RCU freeing.  At this
point, ->vm_start and ->vm_end are accessed.

vm_area_struct contains a union such that ->vm_rcu uses the same memory as
->vm_start and ->vm_end; so accessing ->vm_start and ->vm_end of a
detached VMA is illegal and leads to type confusion between union members.

Fix it by reordering the vma->detached check above the address checks, and
document the rules for RCU readers accessing VMAs.

This will probably change the number of observed VMA_LOCK_MISS events
(since previously, trying to access a detached VMA whose ->vm_rcu has been
scheduled would bail out when checking the fault address against the
rcu_head members reinterpreted as VMA bounds).

Link: https://lkml.kernel.org/r/20240805-fix-vma-lock-type-confusion-v1-1-9f25443a9a71@google.com
Fixes: 50ee32537206 ("mm: introduce lock_vma_under_rcu to be used from arch-specific code")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 661168a46b43..46a44dc702fa 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5997,10 +5997,6 @@ retry:
 	if (!vma_start_read(vma))
 		goto inval;
 
-	/* Check since vm_start/vm_end might change before we lock the VMA */
-	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
-		goto inval_end_read;
-
 	/* Check if the VMA got isolated after we found it */
 	if (vma->detached) {
 		vma_end_read(vma);
@@ -6008,6 +6004,16 @@ retry:
 		/* The area was replaced with another one */
 		goto retry;
 	}
+	/*
+	 * At this point, we have a stable reference to a VMA: The VMA is
+	 * locked and we know it hasn't already been isolated.
+	 * From here on, we can access the VMA without worrying about which
+	 * fields are accessible for RCU readers.
+	 */
+
+	/* Check since vm_start/vm_end might change before we lock the VMA */
+	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
+		goto inval_end_read;
 
 	rcu_read_unlock();
 	return vma;