mm, memfd: fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

F_SEAL_FUTURE_WRITE has unexpected behavior when used with MAP_PRIVATE:
A private mapping created after the memfd file that gets sealed with
F_SEAL_FUTURE_WRITE loses the copy-on-write at fork behavior, meaning
children and parent share the same memory, even though the mapping is
private.

The reason for this is due to the code below:

static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
{
struct shmem_inode_info *info = SHMEM_I(file_inode(file));

if (info->seals & F_SEAL_FUTURE_WRITE) {
/*
* New PROT_WRITE and MAP_SHARED mmaps are not allowed when
* "future write" seal active.
*/
if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
return -EPERM;

/*
* Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED
* read-only mapping, take care to not allow mprotect to revert
* protections.
*/
vma->vm_flags &= ~(VM_MAYWRITE);
}
...
}

And for the mm to know if a mapping is copy-on-write:

static inline bool is_cow_mapping(vm_flags_t flags)
{
return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}

The patch fixes the issue by making the mprotect revert protection
happen only for shared mappings. For private mappings, using mprotect
will have no effect on the seal behavior.

The F_SEAL_FUTURE_WRITE feature was introduced in v5.1 so v5.3.x stable
kernels would need a backport.

[[e-mail]: reflow comment, per Christoph]
Link: http://lkml.kernel.org/r/[e-mail]
Fixes: ab3948f58ff84 ("mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd")
Signed-off-by: Nicolas Geoffray <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/shmem.c

···
 			return -EPERM;
 
 		/*
-		 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED
-		 * read-only mapping, take care to not allow mprotect to revert
-		 * protections.
+		 * Since an F_SEAL_FUTURE_WRITE sealed memfd can be mapped as
+		 * MAP_SHARED and read-only, take care to not allow mprotect to
+		 * revert protections on such mappings. Do this only for shared
+		 * mappings. For private mappings, don't need to mask
+		 * VM_MAYWRITE as we still want them to be COW-writable.
 		 */
-		vma->vm_flags &= ~(VM_MAYWRITE);
+		if (vma->vm_flags & VM_SHARED)
+			vma->vm_flags &= ~(VM_MAYWRITE);
 	}
 
 	file_accessed(file);