Message-Id: <20240805-fix-vma-lock-type-confusion-v1-1-9f25443a9a71@google.com>
Date: Mon, 05 Aug 2024 14:52:03 +0200
From: Jann Horn <jannh@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Suren Baghdasaryan <surenb@...gle.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org, 
 linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: [PATCH] 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).

Fixes: 50ee32537206 ("mm: introduce lock_vma_under_rcu to be used from arch-specific code")
Signed-off-by: Jann Horn <jannh@...gle.com>
---
sidenote: I'm not entirely sure why we handle detached VMAs and moved
VMAs differently here (detached VMAs retry, moved VMAs bail out), but
that's kinda out of scope of this patch.
---
 include/linux/mm_types.h | 15 +++++++++++++--
 mm/memory.c              | 14 ++++++++++----
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 485424979254..498cdf3121d0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -657,58 +657,69 @@ struct vma_numab_state {
 
 /*
  * This struct describes a virtual memory area. There is one of these
  * 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. */
 
 	union {
 		struct {
 			/* VMA covers [vm_start; vm_end) addresses within mm */
 			unsigned long vm_start;
 			unsigned long vm_end;
 		};
 #ifdef CONFIG_PER_VMA_LOCK
 		struct rcu_head vm_rcu;	/* Used for deferred freeing. */
 #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. */
 
 	/*
 	 * Flags, see mm.h.
 	 * To modify use vm_flags_{init|reset|set|clear|mod} functions.
 	 */
 	union {
 		const vm_flags_t vm_flags;
 		vm_flags_t __private __vm_flags;
 	};
 
 #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;
 
 	/*
 	 * Can only be written (using WRITE_ONCE()) while holding both:
 	 *  - mmap_lock (in write mode)
 	 *  - vm_lock->lock (in write mode)
 	 * Can be read reliably while holding one of:
 	 *  - mmap_lock (in read or write mode)
 	 *  - vm_lock->lock (in read or write mode)
 	 * Can be read unreliably (using READ_ONCE()) for pessimistic bailout
 	 * while holding nothing (except RCU to keep the VMA struct allocated).
 	 *
 	 * This sequence counter is explicitly allowed to overflow; sequence
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
 	int vm_lock_seq;
+	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif
 
 	/*
 	 * For areas with an address space and backing store,
 	 * linkage into the address_space->i_mmap interval tree.
diff --git a/mm/memory.c b/mm/memory.c
index 34f8402d2046..3f4232b985a1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5996,23 +5996,29 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
 	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);
 		count_vm_vma_lock_event(VMA_LOCK_MISS);
 		/* 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;
 
 inval_end_read:
 	vma_end_read(vma);

---
base-commit: de9c2c66ad8e787abec7c9d7eff4f8c3cdd28aed
change-id: 20240805-fix-vma-lock-type-confusion-0a956d9d31ae
-- 
Jann Horn <jannh@...gle.com>

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