[PATCH 1/2] memfd: Fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

From: Nicolas Geoffray <ngeoffray@google.com>

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.

Cc: kernel-team@android.com
Signed-off-by: Nicolas Geoffray <ngeoffray@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
Google bug: 143833776

 mm/shmem.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 447fd575587c..6ac5e867ef13 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2214,11 +2214,14 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 			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);
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog