[PATCH 04/14] mm: add VM_UFFD_RWP VMA flag

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

Preparatory patch for userfaultfd read-write protection (RWP). RWP
extends userfaultfd protection from plain write-protection (WP) to
full read-write protection: accesses to an RWP-protected range --
reads as well as writes -- trap through userfaultfd.

RWP marks ranges by combining PAGE_NONE with the uffd PTE bit, so
the flag is only meaningful when both primitives exist. A new
CONFIG_USERFAULTFD_RWP Kconfig symbol auto-selects when CONFIG_64BIT,
CONFIG_ARCH_HAS_PTE_PROTNONE, and CONFIG_HAVE_ARCH_USERFAULTFD_WP
are all set; call sites that gate on the flag depend on the symbol.
Elsewhere VM_UFFD_RWP aliases VM_NONE and every downstream check
folds to dead code.

Nothing sets the flag yet.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
---
 Documentation/filesystems/proc.rst |  1 +
 fs/proc/task_mmu.c                 |  3 +++
 include/linux/mm.h                 | 28 +++++++++++++++++----------
 include/linux/userfaultfd_k.h      | 31 +++++++++++++++++++++++++-----
 include/trace/events/mmflags.h     |  7 +++++++
 mm/Kconfig                         |  9 +++++++++
 6 files changed, 64 insertions(+), 15 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index db6167befb7b..db28207c5290 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -607,6 +607,7 @@ encoded manner. The codes are the following:
     um    userfaultfd missing tracking
     uw    userfaultfd wr-protect tracking
     ui    userfaultfd minor fault
+    ur    userfaultfd read-write-protect tracking
     ss    shadow/guarded control stack page
     sl    sealed
     lf    lock on fault pages
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5827074962e7..fbaede228201 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1206,6 +1206,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
 		[ilog2(VM_UFFD_MINOR)]	= "ui",
 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
+#ifdef CONFIG_USERFAULTFD_RWP
+		[ilog2(VM_UFFD_RWP)]	= "ur",
+#endif
 #ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK
 		[ilog2(VM_SHADOW_STACK)] = "ss",
 #endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0b776907152e..3f53d1e978c0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -353,6 +353,7 @@ enum {
 #endif
 	DECLARE_VMA_BIT(UFFD_MINOR, 41),
 	DECLARE_VMA_BIT(SEALED, 42),
+	DECLARE_VMA_BIT(UFFD_RWP, 43),
 	/* Flags that reuse flags above. */
 	DECLARE_VMA_BIT_ALIAS(PKEY_BIT0, HIGH_ARCH_0),
 	DECLARE_VMA_BIT_ALIAS(PKEY_BIT1, HIGH_ARCH_1),
@@ -496,6 +497,11 @@ enum {
 #else
 #define VM_UFFD_MINOR	VM_NONE
 #endif
+#ifdef CONFIG_USERFAULTFD_RWP
+#define VM_UFFD_RWP		INIT_VM_FLAG(UFFD_RWP)
+#else
+#define VM_UFFD_RWP		VM_NONE
+#endif
 #ifdef CONFIG_64BIT
 #define VM_ALLOW_ANY_UNCACHED	INIT_VM_FLAG(ALLOW_ANY_UNCACHED)
 #define VM_SEALED		INIT_VM_FLAG(SEALED)
@@ -633,22 +639,24 @@ enum {
  * reconsistuted upon page fault, so necessitate page table copying upon fork.
  *
  * Note that these flags should be compared with the DESTINATION VMA not the
- * source, as VM_UFFD_WP may not be propagated to destination, while all other
- * flags will be.
+ * source: VM_UFFD_WP and VM_UFFD_RWP may be cleared on the destination
+ * (dup_userfaultfd() -> userfaultfd_reset_ctx() when the parent context did
+ * not negotiate UFFD_FEATURE_EVENT_FORK), while all other flags propagate.
  *
  * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
  *                           reasonably reconstructed on page fault.
  *
  *              VM_UFFD_WP - Encodes metadata about an installed uffd
- *                           write protect handler, which cannot be
- *                           reconstructed on page fault.
+ *              VM_UFFD_RWP  write- or read-write-protect handler, which
+ *                           cannot be reconstructed on page fault.
  *
- *                           We always copy pgtables when dst_vma has uffd-wp
- *                           enabled even if it's file-backed
- *                           (e.g. shmem). Because when uffd-wp is enabled,
- *                           pgtable contains uffd-wp protection information,
- *                           that's something we can't retrieve from page cache,
- *                           and skip copying will lose those info.
+ *                           We always copy pgtables when dst_vma has the
+ *                           uffd PTE bit in use even if it's file-backed
+ *                           (e.g. shmem). Because when the uffd bit is
+ *                           in use, the pgtable contains the protection
+ *                           information, that's something we can't
+ *                           retrieve from page cache, and skip copying
+ *                           will lose those info.
  *
  *          VM_MAYBE_GUARD - Could contain page guard region markers which
  *                           by design are a property of the page tables
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 98f546e83cd2..fcf308dba311 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -21,10 +21,11 @@
 #include <linux/hugetlb_inline.h>
 
 /* The set of all possible UFFD-related VM flags. */
-#define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR)
+#define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR | \
+			 VM_UFFD_RWP)
 
 #define __VMA_UFFD_FLAGS mk_vma_flags(VMA_UFFD_MISSING_BIT, VMA_UFFD_WP_BIT, \
-				      VMA_UFFD_MINOR_BIT)
+				      VMA_UFFD_MINOR_BIT, VMA_UFFD_RWP_BIT)
 
 /*
  * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
@@ -192,7 +193,7 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
  */
 static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_UFFD_WP | VM_UFFD_MINOR);
+	return vma->vm_flags & (VM_UFFD_WP | VM_UFFD_MINOR | VM_UFFD_RWP);
 }
 
 /*
@@ -222,6 +223,16 @@ static inline bool userfaultfd_minor(struct vm_area_struct *vma)
 	return vma->vm_flags & VM_UFFD_MINOR;
 }
 
+static inline bool userfaultfd_rwp(struct vm_area_struct *vma)
+{
+	return vma->vm_flags & VM_UFFD_RWP;
+}
+
+static inline bool userfaultfd_protected(struct vm_area_struct *vma)
+{
+	return userfaultfd_wp(vma) || userfaultfd_rwp(vma);
+}
+
 static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
 				      pte_t pte)
 {
@@ -364,6 +375,16 @@ static inline bool userfaultfd_minor(struct vm_area_struct *vma)
 	return false;
 }
 
+static inline bool userfaultfd_rwp(struct vm_area_struct *vma)
+{
+	return false;
+}
+
+static inline bool userfaultfd_protected(struct vm_area_struct *vma)
+{
+	return false;
+}
+
 static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
 				      pte_t pte)
 {
@@ -457,8 +478,8 @@ static inline bool userfaultfd_wp_use_markers(struct vm_area_struct *vma)
 }
 
 /*
- * Returns true if this is a swap pte and was uffd-wp wr-protected in either
- * forms (pte marker or a normal swap pte), false otherwise.
+ * Returns true if this swap pte carries uffd-tracked state in either
+ * form (pte marker or a normal swap pte), false otherwise.
  */
 static inline bool pte_swp_uffd_any(pte_t pte)
 {
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index a6e5a44c9b42..bfface3d0203 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -194,6 +194,12 @@ IF_HAVE_PG_ARCH_3(arch_3)
 # define IF_HAVE_UFFD_MINOR(flag, name)
 #endif
 
+#ifdef CONFIG_USERFAULTFD_RWP
+# define IF_HAVE_UFFD_RWP(flag, name) {flag, name},
+#else
+# define IF_HAVE_UFFD_RWP(flag, name)
+#endif
+
 #if defined(CONFIG_64BIT) || defined(CONFIG_PPC32)
 # define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name},
 #else
@@ -215,6 +221,7 @@ IF_HAVE_UFFD_MINOR(VM_UFFD_MINOR,	"uffd_minor"	)		\
 	{VM_PFNMAP,			"pfnmap"	},		\
 	{VM_MAYBE_GUARD,		"maybe_guard"	},		\
 	{VM_UFFD_WP,			"uffd_wp"	},		\
+IF_HAVE_UFFD_RWP(VM_UFFD_RWP,		"uffd_rwp"	)		\
 	{VM_LOCKED,			"locked"	},		\
 	{VM_IO,				"io"		},		\
 	{VM_SEQ_READ,			"seqread"	},		\
diff --git a/mm/Kconfig b/mm/Kconfig
index e8bf1e9e6ad9..ccf534a8cbc9 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1347,6 +1347,15 @@ config HAVE_ARCH_USERFAULTFD_MINOR
 	help
 	  Arch has userfaultfd minor fault support
 
+config USERFAULTFD_RWP
+	def_bool y
+	depends on 64BIT && ARCH_HAS_PTE_PROTNONE && HAVE_ARCH_USERFAULTFD_WP
+	help
+	  Userfaultfd read-write protection (UFFDIO_RWPROTECT) delivers a
+	  userfaultfd notification on every access -- read or write -- to a
+	  protected range, letting userspace observe the working set of a
+	  process.
+
 menuconfig USERFAULTFD
 	bool "Enable userfaultfd() system call"
 	depends on MMU
-- 
2.51.2