[PATCH 15/39] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if kernel-owned

For ordinary files the only way the VMA_MAYWRITE_BIT flag is cleared is if
the underlying file is itself read-only.

This means that mprotect() cannot mark a shared mapping of a read-only file
as read/write, as doing so would violate the read only attribute, and
permit writes.

In general, we do not want file systems to be able to do this for
read/write files.

Doing so would violate fundamental user expectation of file attributes and
likely break userspace.

However, drivers pose a tricky problem here - the /dev/xxx file may be
read/write but provide access to a resource which is fundamentally
read-only.

Therefore we must allow drivers to be able to clear VMA_MAYWRITE_BIT.

To achieve both of these things, restrict this ability to kernel-owned
mappings as identified by vma_flags_is_kernel_owned().

This constrains this ability to drivers which own the mapping's contents,
whether memory-mapped I/O, kernel-allocated pages, or ordinary pages they
map themselves, and so define its semantics.

Every in-tree mmap hook which clears VMA_MAYWRITE_BIT, some twenty sites
across drivers, filesystems and bpf, establishes a kernel-owned mapping,
with usbmon and the ALSA PCM status page converted earlier in this series
to do so.

Note that drivers may, if they do not gate on VMA_SHARED_BIT, be able to
disable MAP_PRIVATE-file-backed mapping CoW semantics.

This is perhaps not always intended, but we retain this capacity to
maintain existing behaviour.

As all drivers which clear VMA_MAYWRITE_BIT establish kernel-owned
mappings, no functional change is intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/vma.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/vma.c b/mm/vma.c
index 9e45fc83f74c..526428753218 100644
--- a/mm/vma.c
+++ b/mm/vma.c

@@ -2780,6 +2780,11 @@ static int mmap_validate(unsigned long prev_start,
 	if (WARN_ON_ONCE(!was_maywrite && is_maywrite))
 		return -EINVAL;
 
+	/* Only kernel-owned mappings may clear VMA_MAYWRITE_BIT. */
+	if (!vma_flags_is_kernel_owned(curr_flags) &&
+	    WARN_ON_ONCE(was_maywrite && !is_maywrite))
+		return -EINVAL;
+
 	return mmap_validate_vma_flags(curr_flags);
 }
 

-- 
2.55.0