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 <[email protected]>
Acked-by: Suren Baghdasaryan <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 13 insertions, 2 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 165c58b12ccc..003619fab20e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -660,6 +660,9 @@ struct vma_numab_state {
  * per VM-area/task. A VM area is any part of the process virtual memory
  * space that has a special rule for the page-fault handlers (ie a shared
  * library, the executable area etc).
+ *
+ * Only explicitly marked struct members may be accessed by RCU readers before
+ * getting a stable reference.
  */
 struct vm_area_struct {
 	/* The first cache line has the info for VMA tree walking. */
@@ -675,7 +678,11 @@ struct vm_area_struct {
 #endif
 	};
 
-	struct mm_struct *vm_mm;	/* The address space we belong to. */
+	/*
+	 * The address space we belong to.
+	 * Unstable RCU readers are allowed to read this.
+	 */
+	struct mm_struct *vm_mm;
 	pgprot_t vm_page_prot;          /* Access permissions of this VMA. */
 
 	/*
@@ -688,7 +695,10 @@ struct vm_area_struct {
 	};
 
 #ifdef CONFIG_PER_VMA_LOCK
-	/* Flag to indicate areas detached from the mm->mm_mt tree */
+	/*
+	 * Flag to indicate areas detached from the mm->mm_mt tree.
+	 * Unstable RCU readers are allowed to read this.
+	 */
 	bool detached;
 
 	/*
@@ -706,6 +716,7 @@ struct vm_area_struct {
 	 * slowpath.
 	 */
 	int vm_lock_seq;
+	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif