+ mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings
has been added to the -mm mm-unstable branch.  Its filename is
     mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings
Date: Mon, 2 Jan 2023 17:08:55 +0100

Let's stop using VM_MAYSHARE for MAP_PRIVATE mappings and use
VM_MAYOVERLAY instead.  Rewrite determine_vm_flags() to make the whole
logic easier to digest, and to cleanly separate MAP_PRIVATE vs. 
MAP_SHARED.

No functional change intended.

Link: https://lkml.kernel.org/r/20230102160856.500584-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |    7 +++++
 mm/nommu.c         |   51 +++++++++++++++++++++++++------------------
 2 files changed, 36 insertions(+), 22 deletions(-)

--- a/include/linux/mm.h~mm-nommu-dont-use-vm_mayshare-for-map_private-mappings
+++ a/include/linux/mm.h
@@ -276,7 +276,12 @@ extern unsigned int kobjsize(const void
 #define VM_MAYSHARE	0x00000080
 
 #define VM_GROWSDOWN	0x00000100	/* general info on the segment */
+#ifdef CONFIG_MMU
 #define VM_UFFD_MISSING	0x00000200	/* missing pages tracking */
+#else /* CONFIG_MMU */
+#define VM_MAYOVERLAY	0x00000200	/* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */
+#define VM_UFFD_MISSING	0
+#endif /* CONFIG_MMU */
 #define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */
 #define VM_UFFD_WP	0x00001000	/* wrprotect pages tracking */
 
@@ -1396,7 +1401,7 @@ static inline bool is_nommu_shared_mappi
 	 * ptrace does not apply. Note that there is no mprotect() to upgrade
 	 * write permissions later.
 	 */
-	return flags & VM_MAYSHARE;
+	return flags & (VM_MAYSHARE | VM_MAYOVERLAY);
 }
 #endif
 
--- a/mm/nommu.c~mm-nommu-dont-use-vm_mayshare-for-map_private-mappings
+++ a/mm/nommu.c
@@ -892,29 +892,36 @@ static unsigned long determine_vm_flags(
 	unsigned long vm_flags;
 
 	vm_flags = calc_vm_prot_bits(prot, 0) | calc_vm_flag_bits(flags);
-	/* vm_flags |= mm->def_flags; */
 
-	if (!(capabilities & NOMMU_MAP_DIRECT)) {
-		/* attempt to share read-only copies of mapped file chunks */
+	if (!file) {
+		/*
+		 * MAP_ANONYMOUS. MAP_SHARED is mapped to MAP_PRIVATE, because
+		 * there is no fork().
+		 */
 		vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
-		if (file && !(prot & PROT_WRITE))
-			vm_flags |= VM_MAYSHARE;
+	} else if (flags & MAP_PRIVATE) {
+		/* MAP_PRIVATE file mapping */
+		if (capabilities & NOMMU_MAP_DIRECT)
+			vm_flags |= (capabilities & NOMMU_VMFLAGS);
+		else
+			vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
+
+		if (!(prot & PROT_WRITE) && !current->ptrace)
+			/*
+			 * R/O private file mapping which cannot be used to
+			 * modify memory, especially also not via active ptrace
+			 * (e.g., set breakpoints) or later by upgrading
+			 * permissions (no mprotect()). We can try overlaying
+			 * the file mapping, which will work e.g., on chardevs,
+			 * ramfs/tmpfs/shmfs and romfs/cramf.
+			 */
+			vm_flags |= VM_MAYOVERLAY;
 	} else {
-		/* overlay a shareable mapping on the backing device or inode
-		 * if possible - used for chardevs, ramfs/tmpfs/shmfs and
-		 * romfs/cramfs */
-		vm_flags |= VM_MAYSHARE | (capabilities & NOMMU_VMFLAGS);
-		if (flags & MAP_SHARED)
-			vm_flags |= VM_SHARED;
+		/* MAP_SHARED file mapping: NOMMU_MAP_DIRECT is set. */
+		vm_flags |= VM_SHARED | VM_MAYSHARE |
+			    (capabilities & NOMMU_VMFLAGS);
 	}
 
-	/* refuse to let anyone share private mappings with this process if
-	 * it's being traced - otherwise breakpoints set in it may interfere
-	 * with another untraced process
-	 */
-	if ((flags & MAP_PRIVATE) && current->ptrace)
-		vm_flags &= ~VM_MAYSHARE;
-
 	return vm_flags;
 }
 
@@ -952,9 +959,11 @@ static int do_mmap_private(struct vm_are
 	void *base;
 	int ret, order;
 
-	/* invoke the file's mapping function so that it can keep track of
-	 * shared mappings on devices or memory
-	 * - VM_MAYSHARE will be set if it may attempt to share
+	/*
+	 * Invoke the file's mapping function so that it can keep track of
+	 * shared mappings on devices or memory. VM_MAYOVERLAY will be set if
+	 * it may attempt to share, which will make is_nommu_shared_mapping()
+	 * happy.
 	 */
 	if (capabilities & NOMMU_MAP_DIRECT) {
 		ret = call_mmap(vma->vm_file, vma);
_

Patches currently in -mm which might be from david@redhat.com are

mm-hugetlb-fix-pte-marker-handling-in-hugetlb_change_protection.patch
mm-hugetlb-fix-uffd-wp-handling-for-migration-entries-in-hugetlb_change_protection.patch
mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range-fix.patch
mm-mprotect-drop-pgprot_t-parameter-from-change_protection.patch
mm-mprotect-drop-pgprot_t-parameter-from-change_protection-fix.patch
selftests-vm-cow-add-cow-tests-for-collapsing-of-pte-mapped-anon-thp.patch
mm-nommu-factor-out-check-for-nommu-shared-mappings-into-is_nommu_shared_mapping.patch
mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch
drivers-misc-open-dice-dont-touch-vm_mayshare.patch