[PATCH mm-stable RFC 0/2] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

[PATCH mm-stable RFC 0/2] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

[David Hildenbrand]

Trying to reduce the confusion around VM_SHARED and VM_MAYSHARE first
requires !CONFIG_MMU to stop using VM_MAYSHARE for MAP_PRIVATE mappings.
CONFIG_MMU only sets VM_MAYSHARE for MAP_SHARED mappings.

This paves the way for further VM_MAYSHARE and VM_SHARED cleanups: for
example, renaming VM_MAYSHARED to VM_MAP_SHARED to make it cleaner what
is actually means.

Let's first get the weird case out of the way and not use VM_MAYSHARE in
MAP_PRIVATE mappings, using a new VM_MAYOVERLAY flag instead.

I am not a NOMMU expert, but my basic testing with risc64-nommu with
buildroot under QEMU revealed no surprises.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>

David Hildenbrand (2):
  mm/nommu: factor out check for NOMMU shared mappings into
    is_nommu_shared_mapping()
  mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

 drivers/char/mem.c    |  2 +-
 fs/cramfs/inode.c     |  2 +-
 fs/proc/task_nommu.c  |  2 +-
 fs/ramfs/file-nommu.c |  2 +-
 fs/romfs/mmap-nommu.c |  2 +-
 include/linux/mm.h    | 20 ++++++++++++++
 io_uring/io_uring.c   |  2 +-
 mm/nommu.c            | 62 +++++++++++++++++++++++++------------------
 8 files changed, 62 insertions(+), 32 deletions(-)

-- 
2.38.1

[PATCH mm-stable RFC 1/2] mm/nommu: factor out check for NOMMU shared mappings into is_nommu_shared_mapping()

[David Hildenbrand]

We want to stop using VM_MAYSHARE in private mappings to pave the way
for clarifying the semantics of VM_MAYSHARE vs. VM_SHARED and reduce
the confusion. While CONFIG_MMU uses VM_MAYSHARE to represent MAP_SHARED,
!CONFIG_MMU also sets VM_MAYSHARE for selected R/O private file mappings
that are an effective overlay of a file mapping.

Let's factor out all relevant VM_MAYSHARE checks in !CONFIG_MMU code into
is_nommu_shared_mapping() first.

Note that whenever VM_SHARED is set, VM_MAYSHARE must be set as well
(unless there is a serious BUG). So there is not need to test for VM_SHARED
manually.

No functional change intended.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/char/mem.c    |  2 +-
 fs/cramfs/inode.c     |  2 +-
 fs/proc/task_nommu.c  |  2 +-
 fs/ramfs/file-nommu.c |  2 +-
 fs/romfs/mmap-nommu.c |  2 +-
 include/linux/mm.h    | 15 +++++++++++++++
 io_uring/io_uring.c   |  2 +-
 mm/nommu.c            | 11 ++++++-----
 8 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 83bf2a4dcb57..ffb101d349f0 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -343,7 +343,7 @@ static unsigned zero_mmap_capabilities(struct file *file)
 /* can't do an in-place private mapping if there's no MMU */
 static inline int private_mapping_ok(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & VM_MAYSHARE;
+	return is_nommu_shared_mapping(vma->vm_flags);
 }
 #else
 
diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index 61ccf7722fc3..50e4e060db68 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -437,7 +437,7 @@ static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
 
 static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned long cramfs_physmem_get_unmapped_area(struct file *file,
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 2fd06f52b6a4..0ec35072a8e5 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -38,7 +38,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 		}
 
 		if (atomic_read(&mm->mm_count) > 1 ||
-		    vma->vm_flags & VM_MAYSHARE) {
+		    is_nommu_shared_mapping(vma->vm_flags)) {
 			sbytes += size;
 		} else {
 			bytes += size;
diff --git a/fs/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index cb240eac5036..cd4537692751 100644
--- a/fs/ramfs/file-nommu.c
+++ b/fs/ramfs/file-nommu.c
@@ -264,7 +264,7 @@ static unsigned long ramfs_nommu_get_unmapped_area(struct file *file,
  */
 static int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if (!(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
+	if (!is_nommu_shared_mapping(vma->vm_flags))
 		return -ENOSYS;
 
 	file_accessed(file);
diff --git a/fs/romfs/mmap-nommu.c b/fs/romfs/mmap-nommu.c
index 2c4a23113fb5..4578dc45e50a 100644
--- a/fs/romfs/mmap-nommu.c
+++ b/fs/romfs/mmap-nommu.c
@@ -63,7 +63,7 @@ static unsigned long romfs_get_unmapped_area(struct file *file,
  */
 static int romfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned romfs_mmap_capabilities(struct file *file)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index f3f196e4d66d..734d0bc7c7c6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1363,6 +1363,21 @@ static inline bool is_cow_mapping(vm_flags_t flags)
 	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
 }
 
+#ifndef CONFIG_MMU
+static inline bool is_nommu_shared_mapping(vm_flags_t flags)
+{
+	/*
+	 * NOMMU shared mappings are ordinary MAP_SHARED mappings and selected
+	 * R/O MAP_PRIVATE file mappings that are an effective R/O overlay of
+	 * a file mapping. R/O MAP_PRIVATE mappings might still modify
+	 * underlying memory if ptrace is active, so this is only possible if
+	 * ptrace does not apply. Note that there is no mprotect() to upgrade
+	 * write permissions later.
+	 */
+	return flags & VM_MAYSHARE;
+}
+#endif
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index b521186efa5c..84fa23b1f782 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3174,7 +3174,7 @@ static __cold int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 
 static int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -EINVAL;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -EINVAL;
 }
 
 static unsigned int io_uring_nommu_mmap_capabilities(struct file *file)
diff --git a/mm/nommu.c b/mm/nommu.c
index 214c70e1d059..6c4bdc07a776 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -958,9 +958,10 @@ static int do_mmap_private(struct vm_area_struct *vma,
 	 */
 	if (capabilities & NOMMU_MAP_DIRECT) {
 		ret = call_mmap(vma->vm_file, vma);
+		/* shouldn't return success if we're not sharing */
+		if (WARN_ON_ONCE(!is_nommu_shared_mapping(vma->vm_flags)))
+			ret = -ENOSYS;
 		if (ret == 0) {
-			/* shouldn't return success if we're not sharing */
-			BUG_ON(!(vma->vm_flags & VM_MAYSHARE));
 			vma->vm_region->vm_top = vma->vm_region->vm_end;
 			return 0;
 		}
@@ -1106,7 +1107,7 @@ unsigned long do_mmap(struct file *file,
 	 *   these cases, sharing is handled in the driver or filesystem rather
 	 *   than here
 	 */
-	if (vm_flags & VM_MAYSHARE) {
+	if (is_nommu_shared_mapping(vm_flags)) {
 		struct vm_region *pregion;
 		unsigned long pglen, rpglen, pgend, rpgend, start;
 
@@ -1116,7 +1117,7 @@ unsigned long do_mmap(struct file *file,
 		for (rb = rb_first(&nommu_region_tree); rb; rb = rb_next(rb)) {
 			pregion = rb_entry(rb, struct vm_region, vm_rb);
 
-			if (!(pregion->vm_flags & VM_MAYSHARE))
+			if (!is_nommu_shared_mapping(pregion->vm_flags))
 				continue;
 
 			/* search for overlapping mappings on the same file */
@@ -1597,7 +1598,7 @@ static unsigned long do_mremap(unsigned long addr,
 	if (vma->vm_end != vma->vm_start + old_len)
 		return (unsigned long) -EFAULT;
 
-	if (vma->vm_flags & VM_MAYSHARE)
+	if (is_nommu_shared_mapping(vma->vm_flags))
 		return (unsigned long) -EPERM;
 
 	if (new_len > vma->vm_region->vm_end - vma->vm_region->vm_start)
-- 
2.38.1

[PATCH mm-stable RFC 2/2] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

[David Hildenbrand]

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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/mm.h |  7 ++++++-
 mm/nommu.c         | 51 +++++++++++++++++++++++++++-------------------
 2 files changed, 36 insertions(+), 22 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 734d0bc7c7c6..c229eee6211c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -276,7 +276,12 @@ extern unsigned int kobjsize(const void *objp);
 #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 */
 
@@ -1374,7 +1379,7 @@ static inline bool is_nommu_shared_mapping(vm_flags_t flags)
 	 * 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
 
diff --git a/mm/nommu.c b/mm/nommu.c
index 6c4bdc07a776..5c628c868648 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -892,29 +892,36 @@ static unsigned long determine_vm_flags(struct file *file,
 	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_area_struct *vma,
 	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);
-- 
2.38.1