From: Jann Horn jannh@google.com
commit b1f02b95758d05b799731d939e76a0bd6da312db upstream.
mm->mm_lock_seq effectively functions as a read/write lock; therefore it must be used with acquire/release semantics.
A specific example is the interaction between userfaultfd_register() and lock_vma_under_rcu().
userfaultfd_register() does the following from the point where it changes a VMA's flags to the point where concurrent readers are permitted again (in a simple scenario where only a single private VMA is accessed and no merging/splitting is involved):
userfaultfd_register userfaultfd_set_vm_flags vm_flags_reset vma_start_write down_write(&vma->vm_lock->lock) vma->vm_lock_seq = mm_lock_seq [marks VMA as busy] up_write(&vma->vm_lock->lock) vm_flags_init [sets VM_UFFD_* in __vm_flags] vma->vm_userfaultfd_ctx.ctx = ctx mmap_write_unlock vma_end_write_all WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1) [unlocks VMA]
There are no memory barriers in between the __vm_flags update and the mm->mm_lock_seq update that unlocks the VMA, so the unlock can be reordered to above the `vm_flags_init()` call, which means from the perspective of a concurrent reader, a VMA can be marked as a userfaultfd VMA while it is not VMA-locked. That's bad, we definitely need a store-release for the unlock operation.
The non-atomic write to vma->vm_lock_seq in vma_start_write() is mostly fine because all accesses to vma->vm_lock_seq that matter are always protected by the VMA lock. There is a racy read in vma_start_read() though that can tolerate false-positives, so we should be using WRITE_ONCE() to keep things tidy and data-race-free (including for KCSAN).
On the other side, lock_vma_under_rcu() works as follows in the relevant region for locking and userfaultfd check:
lock_vma_under_rcu vma_start_read vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq) [early bailout] down_read_trylock(&vma->vm_lock->lock) vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq) [main check] userfaultfd_armed checks vma->vm_flags & __VM_UFFD_FLAGS
Here, the interesting aspect is how far down the mm->mm_lock_seq read can be reordered - if this read is reordered down below the vma->vm_flags access, this could cause lock_vma_under_rcu() to partly operate on information that was read while the VMA was supposed to be locked. To prevent this kind of downwards bleeding of the mm->mm_lock_seq read, we need to read it with a load-acquire.
Some of the comment wording is based on suggestions by Suren.
BACKPORT WARNING: One of the functions changed by this patch (which I've written against Linus' tree) is vma_try_start_write(), but this function no longer exists in mm/mm-everything. I don't know whether the merged version of this patch will be ordered before or after the patch that removes vma_try_start_write(). If you're backporting this patch to a tree with vma_try_start_write(), make sure this patch changes that function.
Link: https://lkml.kernel.org/r/20230721225107.942336-1-jannh@google.com Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it") Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/mm.h | 29 +++++++++++++++++++++++------ include/linux/mm_types.h | 28 ++++++++++++++++++++++++++++ include/linux/mmap_lock.h | 10 ++++++++-- 3 files changed, 59 insertions(+), 8 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 2dd73e4f3d8e..406ab9ea818f 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -641,8 +641,14 @@ static inline void vma_numab_state_free(struct vm_area_struct *vma) {} */ static inline bool vma_start_read(struct vm_area_struct *vma) { - /* Check before locking. A race might cause false locked result. */ - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) + /* + * Check before locking. A race might cause false locked result. + * We can use READ_ONCE() for the mm_lock_seq here, and don't need + * ACQUIRE semantics, because this is just a lockless check whose result + * we don't rely on for anything - the mm_lock_seq read against which we + * need ordering is below. + */ + if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq)) return false;
if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) @@ -653,8 +659,13 @@ static inline bool vma_start_read(struct vm_area_struct *vma) * False unlocked result is impossible because we modify and check * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq * modification invalidates all existing locks. + * + * We must use ACQUIRE semantics for the mm_lock_seq so that if we are + * racing with vma_end_write_all(), we only start reading from the VMA + * after it has been unlocked. + * This pairs with RELEASE semantics in vma_end_write_all(). */ - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { + if (unlikely(vma->vm_lock_seq == smp_load_acquire(&vma->vm_mm->mm_lock_seq))) { up_read(&vma->vm_lock->lock); return false; } @@ -676,7 +687,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq) * current task is holding mmap_write_lock, both vma->vm_lock_seq and * mm->mm_lock_seq can't be concurrently modified. */ - *mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq); + *mm_lock_seq = vma->vm_mm->mm_lock_seq; return (vma->vm_lock_seq == *mm_lock_seq); }
@@ -688,7 +699,13 @@ static inline void vma_start_write(struct vm_area_struct *vma) return;
down_write(&vma->vm_lock->lock); - vma->vm_lock_seq = mm_lock_seq; + /* + * We should use WRITE_ONCE() here because we can have concurrent reads + * from the early lockless pessimistic check in vma_start_read(). + * We don't really care about the correctness of that early check, but + * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy. + */ + WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq); up_write(&vma->vm_lock->lock); }
@@ -702,7 +719,7 @@ static inline bool vma_try_start_write(struct vm_area_struct *vma) if (!down_write_trylock(&vma->vm_lock->lock)) return false;
- vma->vm_lock_seq = mm_lock_seq; + WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq); up_write(&vma->vm_lock->lock); return true; } diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index de10fc797c8e..5e74ce4a28cd 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -514,6 +514,20 @@ struct vm_area_struct { };
#ifdef CONFIG_PER_VMA_LOCK + /* + * 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; struct vma_lock *vm_lock;
@@ -679,6 +693,20 @@ struct mm_struct { * by mmlist_lock */ #ifdef CONFIG_PER_VMA_LOCK + /* + * This field has lock-like semantics, meaning it is sometimes + * accessed with ACQUIRE/RELEASE semantics. + * Roughly speaking, incrementing the sequence number is + * equivalent to releasing locks on VMAs; reading the sequence + * number can be part of taking a read lock on a VMA. + * + * Can be modified under write mmap_lock using RELEASE + * semantics. + * Can be read with no other protection when holding write + * mmap_lock. + * Can be read with ACQUIRE semantics if not holding write + * mmap_lock. + */ int mm_lock_seq; #endif
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index aab8f1b28d26..e05e167dbd16 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -76,8 +76,14 @@ static inline void mmap_assert_write_locked(struct mm_struct *mm) static inline void vma_end_write_all(struct mm_struct *mm) { mmap_assert_write_locked(mm); - /* No races during update due to exclusive mmap_lock being held */ - WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1); + /* + * Nobody can concurrently modify mm->mm_lock_seq due to exclusive + * mmap_lock being held. + * We need RELEASE semantics here to ensure that preceding stores into + * the VMA take effect before we unlock it with this store. + * Pairs with ACQUIRE semantics in vma_start_read(). + */ + smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1); } #else static inline void vma_end_write_all(struct mm_struct *mm) {}