udmabuf: fix racy memfd sealing check

JIRA: https://issues.redhat.com/browse/RHEL-89519
Conflicts: A merge conflict due to missing upstream commit c87a126
	   ("udmabuf: reuse folio array when pin folios")

commit 9cb189a
Author: Jann Horn <jannh@google.com>
Date:   Wed, 4 Dec 2024 17:26:19 +0100

    udmabuf: fix racy memfd sealing check

    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@gmail.com>
    Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106
    Closes: https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTgpD+StkV9w@mail.gmail.com
    Fixes: fbb0de7 ("Add udmabuf misc device")
    Cc: stable@vger.kernel.org
    Signed-off-by: Jann Horn <jannh@google.com>
    Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
    Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20241204-udmabuf-fixes-v2-1-23887289de1c@google.com

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

drivers/dma-buf/udmabuf.c

@@ -423,14 +423,19 @@ static long udmabuf_create(struct miscdevice *device,
 			goto err;
 		}
 
+		/*
+		 * Take the inode lock to protect against concurrent
+		 * memfd_add_seals(), which takes this lock in write mode.
+		 */
+		inode_lock_shared(file_inode(memfd));
 		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);
+out_unlock:
+		inode_unlock_shared(file_inode(memfd));
 		fput(memfd);
 		if (ret)
 			goto err;