This patch adds the ability to atomically set VMA flags with only the mmap
read/VMA read lock held.
As this could be hugely problematic for VMA flags in general given that
all other accesses are non-atomic and serialised by the mmap/VMA locks, we
implement this with a strict allow-list - that is, only designated flags
are allowed to do this.
We make VM_MAYBE_GUARD one of these flags.
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
include/linux/mm.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 44 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 36b9418c00fc..03776aab3837 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -518,6 +518,9 @@ extern unsigned int kobjsize(const void *objp);
/* This mask represents all the VMA flag bits used by mlock */
#define VM_LOCKED_MASK (VM_LOCKED | VM_LOCKONFAULT)
+/* These flags can be updated atomically via VMA/mmap read lock. */
+#define VM_ATOMIC_SET_ALLOWED VM_MAYBE_GUARD
+
/* Arch-specific flags to clear when updating VM flags on protection change */
#ifndef VM_ARCH_CLEAR
# define VM_ARCH_CLEAR VM_NONE
@@ -860,6 +863,47 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
__vm_flags_mod(vma, set, clear);
}
+static inline bool __vma_flag_atomic_valid(struct vm_area_struct *vma,
+ int bit)
+{
+ const vm_flags_t mask = BIT(bit);
+
+ /* Only specific flags are permitted */
+ if (WARN_ON_ONCE(!(mask & VM_ATOMIC_SET_ALLOWED)))
+ return false;
+
+ return true;
+}
+
+/*
+ * Set VMA flag atomically. Requires only VMA/mmap read lock. Only specific
+ * valid flags are allowed to do this.
+ */
+static inline void vma_flag_set_atomic(struct vm_area_struct *vma, int bit)
+{
+ /* mmap read lock/VMA read lock must be held. */
+ if (!rwsem_is_locked(&vma->vm_mm->mmap_lock))
+ vma_assert_locked(vma);
+
+ if (__vma_flag_atomic_valid(vma, bit))
+ set_bit(bit, &ACCESS_PRIVATE(vma, __vm_flags));
+}
+
+/*
+ * Test for VMA flag atomically. Requires no locks. Only specific valid flags
+ * are allowed to do this.
+ *
+ * This is necessarily racey, so callers must ensure that serialisation is
+ * achieved through some other means, or that races are permissible.
+ */
+static inline bool vma_flag_test_atomic(struct vm_area_struct *vma, int bit)
+{
+ if (__vma_flag_atomic_valid(vma, bit))
+ return test_bit(bit, &vma->vm_flags);
+
+ return false;
+}
+
static inline void vma_set_anonymous(struct vm_area_struct *vma)
{
vma->vm_ops = NULL;
--
2.51.2