[PATCH] iommu/iommufd: Require write access for writable MAP_FILE mappings

[Yiming Qian]

IOMMU_IOAS_MAP_FILE pins folios from a shmem/tmpfs or hugetlb file and
uses them as the backing storage for an IOAS mapping.  When userspace sets
IOMMU_IOAS_MAP_WRITEABLE, the resulting IOMMU PTEs allow DMA writes to the
file-backed folios.

The file path currently records the IOMMU mapping as writable, but it does
not require the source file descriptor to have write permission.  It also
bypasses the address_space writable-mapping accounting used by memfd
sealing.  As a result, an O_RDONLY fd for a root-owned mode 0444 shmem file
can be mapped as DMA-writeable and a device, or the IOMMUFD selftest access
path, can write into the file page cache.  The same missing accounting also
means writable IOMMU mappings are not excluded by F_SEAL_WRITE or
F_SEAL_FUTURE_WRITE.

Treat writable MAP_FILE mappings like shared writable mappings: require an
FMODE_WRITE fd, call mapping_map_writable() when creating the backing
file-pages object, and hold that accounting until the iopt_pages object is
released.  This rejects already sealed files and prevents new write seals
from being installed while the IOMMU write mapping exists.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Yiming Qian <yimingqian591@xxxxxxxxx>
Reported-by: Keenan Dong <keenanat2000@xxxxxxxxx>
Signed-off-by: Yiming Qian <yimingqian591@xxxxxxxxx>
Signed-off-by: Keenan Dong <keenanat2000@xxxxxxxxx>
---
 drivers/iommu/iommufd/io_pagetable.h |  1 +
 drivers/iommu/iommufd/pages.c        | 18 +++++++++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iommufd/io_pagetable.h b/drivers/iommu/iommufd/io_pagetable.h
index 27e3e311d395b..63e3fd738faf2 100644
--- a/drivers/iommu/iommufd/io_pagetable.h
+++ b/drivers/iommu/iommufd/io_pagetable.h
@@ -234,6 +234,7 @@ struct iopt_pages {
 		struct {			/* IOPT_ADDRESS_FILE */
 			struct file *file;
 			unsigned long start;
+			bool mapping_writable;
 		};
 		/* IOPT_ADDRESS_DMABUF */
 		struct iopt_pages_dmabuf dmabuf;
diff --git a/drivers/iommu/iommufd/pages.c b/drivers/iommu/iommufd/pages.c
index 9bdb2945afe1e..f97d94d9eddd1 100644
--- a/drivers/iommu/iommufd/pages.c
+++ b/drivers/iommu/iommufd/pages.c
@@ -1421,13 +1421,27 @@ struct iopt_pages *iopt_alloc_file_pages(struct file *file,
 
 {
 	struct iopt_pages *pages;
+	int rc;
+
+	if (writable) {
+		if (!(file->f_mode & FMODE_WRITE))
+			return ERR_PTR(-EPERM);
+
+		rc = mapping_map_writable(file->f_mapping);
+		if (rc)
+			return ERR_PTR(rc);
+	}
 
 	pages = iopt_alloc_pages(start_byte, length, writable);
-	if (IS_ERR(pages))
+	if (IS_ERR(pages)) {
+		if (writable)
+			mapping_unmap_writable(file->f_mapping);
 		return pages;
+	}
 	pages->file = get_file(file);
 	pages->start = start - start_byte;
 	pages->type = IOPT_ADDRESS_FILE;
+	pages->mapping_writable = writable;
 	return pages;
 }
 
@@ -1668,6 +1682,8 @@ void iopt_release_pages(struct kref *kref)
 		dma_buf_put(dmabuf);
 		WARN_ON(!list_empty(&pages->dmabuf.tracker));
 	} else if (pages->type == IOPT_ADDRESS_FILE) {
+		if (pages->mapping_writable)
+			mapping_unmap_writable(pages->file->f_mapping);
 		fput(pages->file);
 	}
 	kfree(pages);