In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap() handler to do so. We would otherwise fail the mapping_map_writable() check before we had the opportunity to avoid it. This patch moves this check after the call_mmap() invocation. Only memfd actively denies write access causing a potential failure here (in memfd_add_seals()), so there should be no impact on non-memfd cases. This patch makes the userland-visible change that MAP_SHARED, PROT_READ mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed. There is a delicate situation with cleanup paths assuming that a writable mapping must have occurred in circumstances where it may now not have. In order to ensure we do not accidentally mark a writable file unwritable by mistake, we explicitly track whether we have a writable mapping and unmap only if we do. Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238 Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> --- mm/mmap.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 0041e3631f6c..7f45a08e7973 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2752,6 +2752,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr, unsigned long charged = 0; unsigned long end = addr + len; unsigned long merge_start = addr, merge_end = end; + bool writable_file_mapping = false; pgoff_t vm_pgoff; int error; VMA_ITERATOR(vmi, mm, addr); @@ -2846,17 +2847,19 @@ 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; - } - vma->vm_file = get_file(file); error = call_mmap(file, vma); if (error) goto unmap_and_free_vma; + if (vma_is_shared_maywrite(vma)) { + error = mapping_map_writable(file->f_mapping); + if (error) + goto close_and_free_vma; + + writable_file_mapping = true; + } + /* * Expansion is handled above, merging is handled below. * Drivers should not alter the address of the VMA. @@ -2920,8 +2923,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr, mm->map_count++; if (vma->vm_file) { i_mmap_lock_write(vma->vm_file->f_mapping); - if (vma_is_shared_maywrite(vma)) + if (vma_is_shared_maywrite(vma)) { mapping_allow_writable(vma->vm_file->f_mapping); + writable_file_mapping = true; + } flush_dcache_mmap_lock(vma->vm_file->f_mapping); vma_interval_tree_insert(vma, &vma->vm_file->f_mapping->i_mmap); @@ -2937,7 +2942,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr, /* Once vma denies write, undo our temporary denial count */ unmap_writable: - if (file && is_shared_maywrite(vm_flags)) + if (writable_file_mapping) mapping_unmap_writable(file->f_mapping); file = vma->vm_file; ksm_add_vma(vma); @@ -2985,7 +2990,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr, unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true); } - if (file && is_shared_maywrite(vm_flags)) + if (writable_file_mapping) mapping_unmap_writable(file->f_mapping); free_vma: vm_area_free(vma); -- 2.42.0
