[PATCH 1/3] udmabuf: fix racy memfd sealing check

[Jann Horn]

The current check_memfd_seals() is racy: Since we first do
check_memfd_seals() and then udmabuf_pin_folios() without holding any
relevant lock across both, F_SEAL_WRITE can be set in between.
This is problematic because we can end up holding pins to pages in a
write-sealed memfd.

Fix it using the inode lock, that's probably the easiest way.
In the future, we might want to consider moving this logic into memfd,
especially if anyone else wants to use memfd_pin_folios().

Reported-by: Julian Orth <ju.orth@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106
Closes: https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTgpD+StkV9w@xxxxxxxxxxxxxx
Fixes: fbb0de795078 ("Add udmabuf misc device")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 drivers/dma-buf/udmabuf.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index 8ce1f074c2d32a0a9f59ff7184359e37d56548c6..662b9a26e06668bf59ab36d07c0648c7b02ee5ae 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -436,14 +436,15 @@ static long udmabuf_create(struct miscdevice *device,
 			goto err;
 		}
 
+		inode_lock_shared(memfd->f_inode);
 		ret = check_memfd_seals(memfd);
-		if (ret < 0) {
-			fput(memfd);
-			goto err;
-		}
+		if (ret)
+			goto out_unlock;
 
 		ret = udmabuf_pin_folios(ubuf, memfd, list[i].offset,
 					 list[i].size, folios);
+out_unlock:
+		inode_unlock_shared(memfd->f_inode);
 		fput(memfd);
 		if (ret)
 			goto err;

-- 
2.47.0.338.g60cca15819-goog