[PATCH v2] comedi: Flush partial mappings in error case

[Jann Horn]

If some remap_pfn_range() calls succeeded before one failed, we still have
buffer pages mapped into the userspace page tables when we drop the buffer
reference with comedi_buf_map_put(bm). The userspace mappings are only
cleaned up later in the mmap error path.

Fix it by explicitly flushing all mappings in our VMA on the error path.

See commit 79a61cc3fc04 ("mm: avoid leaving partial pfn mappings around in
error case").

Cc: stable@xxxxxxxxxxxxxxx
Fixes: ed9eccbe8970 ("Staging: add comedi core")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Note: compile-tested only; I don't actually have comedi hardware, and I
don't know anything about comedi.
---
Changes in v2:
- only do the zapping in the pfnmap path (Ian Abbott)
- use zap_vma_ptes() instead of zap_page_range_single() (Ian Abbott)
- Link to v1: https://lore.kernel.org/r/20241014-comedi-tlb-v1-1-4b699144b438@xxxxxxxxxx
---
 drivers/comedi/comedi_fops.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index 1b481731df96..68e5301e6281 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -2407,6 +2407,16 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 
 			start += PAGE_SIZE;
 		}
+
+		/*
+		 * Leaving behind a partial mapping of a buffer we're about to
+		 * drop is unsafe, see remap_pfn_range_notrack().
+		 * We need to zap the range here ourselves instead of relying
+		 * on the automatic zapping in remap_pfn_range() because we call
+		 * remap_pfn_range() in a loop.
+		 */
+		if (retval)
+			zap_vma_ptes(vma, vma->vm_start, size);
 	}
 
 	if (retval == 0) {

---
base-commit: 6485cf5ea253d40d507cd71253c9568c5470cd27
change-id: 20241014-comedi-tlb-400246505961
-- 
Jann Horn <jannh@xxxxxxxxxx>