In order for an F_SEAL_WRITE sealed memfd mapping to have an opportunity to clear VM_MAYWRITE in seal_check_write() we must be able to invoke either the shmem_mmap() or hugetlbfs_file_mmap() f_ops->mmap() handler to do so. We would otherwise fail the mapping_map_writable() check before we had the opportunity to clear VM_MAYWRITE. However, the existing logic in mmap_region() performs this check BEFORE calling call_mmap() (which invokes file->f_ops->mmap()). We must enforce this check AFTER the function call. In order to avoid any risk of breaking call_mmap() handlers which assume this will have been done first, we continue to mark the file writable first, simply deferring enforcement of it failing until afterwards. This enables mmap(..., PROT_READ, MAP_SHARED, fd, 0) mappings for memfd's sealed via F_SEAL_WRITE to succeed, whereas previously they were not permitted. Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238 Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> --- mm/mmap.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 6f6856b3267a..9fbee92aaaee 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2767,17 +2767,25 @@ unsigned long mmap_region(struct file *file, unsigned long addr, vma->vm_pgoff = pgoff; if (file) { - if (is_shared_maywrite(vm_flags)) { - error = mapping_map_writable(file->f_mapping); - if (error) - goto free_vma; - } + int writable_error = 0; + + if (vma_is_shared_maywrite(vma)) + writable_error = mapping_map_writable(file->f_mapping); vma->vm_file = get_file(file); error = call_mmap(file, vma); if (error) goto unmap_and_free_vma; + /* + * call_mmap() may have changed VMA flags, so retry this check + * if it failed before. + */ + if (writable_error && vma_is_shared_maywrite(vma)) { + error = writable_error; + goto close_and_free_vma; + } + /* * Expansion is handled above, merging is handled below. * Drivers should not alter the address of the VMA. -- 2.42.0
