On Thu, Oct 12, 2023 at 06:04:30PM +0100, Lorenzo Stoakes wrote:

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@gmail.com

---

mm/mmap.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-)

[snip]

Andrew, could you apply the following -fix patch to this? As a bug was detected in the implementation [0] - I was being over-zealous in setting the writable_file_mapping flag and had falsely assumed vma->vm_file == file in all instances of the cleanup. The fix is to only set it in one place.

[0]: https://lore.kernel.org/all/CA+G9fYtL7wK-dE-Tnz4t-GWmQb50EPYa=TWGjpgYU2Z=oeA...

----8<---- From 7feea6faada5b10a872c24755cc630220cba619a Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes lstoakes@gmail.com Date: Mon, 16 Oct 2023 17:17:13 +0100 Subject: [PATCH] mm: perform the mapping_map_writable() check after call_mmap()

Do not set writable_file_mapping in an instance where it is not appropriate to do so.

Signed-off-by: Lorenzo Stoakes lstoakes@gmail.com --- mm/mmap.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c index 7f45a08e7973..8b57e42fd980 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2923,10 +2923,8 @@ 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);