FAILED: patch "[PATCH] misc: fastrpc: don't publish fd before copy_to_user()" failed to apply to 6.18-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x a4a1a2bfcb29785292d634d7787edc6fb550714d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090820-renovator-enjoyable-ee8a@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a4a1a2bfcb29785292d634d7787edc6fb550714d Mon Sep 17 00:00:00 2001
From: Baineng Shou <shoubaineng@gmail.com>
Date: Mon, 17 Aug 2026 13:04:55 +0800
Subject: [PATCH] misc: fastrpc: don't publish fd before copy_to_user()
 succeeds
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

fastrpc_ioctl_alloc_dmabuf() calls dma_buf_fd() which installs the fd
into the caller's fd table before copy_to_user() copies the fd number
back to userspace.  If copy_to_user() fails, the fd is already visible
to other threads in the same process but the ioctl returns -EFAULT.
The existing comment in the code even acknowledges the problem:

  "The usercopy failed, but we can't do much about it, as dma_buf_fd()
   already called fd_install()..."

Now that dma_buf_fd_install() is available (introduced to fix the same
issue in dma-heap), apply the same pattern here: reserve the fd with
get_unused_fd_flags(), attempt copy_to_user(), and only on success call
dma_buf_fd_install() to publish it atomically with the tracepoint.  On
copy_to_user() failure, put_unused_fd() and dma_buf_put() cleanly
unwind without any user-visible side effects.

Fixes: 6cffd79504ce ("misc: fastrpc: Add support for dmabuf exporter")
Cc: stable@vger.kernel.org
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Baineng Shou <shoubaineng@gmail.com>
Link: https://lore.kernel.org/r/20260817050457.1005285-3-shoubaineng@gmail.com
Signed-off-by: Christian König <christian.koenig@amd.com>

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index eb6c2a78d3c7..d7a9e12f5575 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -1712,24 +1712,20 @@ static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
 		return err;
 	}
 
-	bp.fd = dma_buf_fd(buf->dmabuf, O_ACCMODE);
+	bp.fd = get_unused_fd_flags(O_ACCMODE);
 	if (bp.fd < 0) {
 		dma_buf_put(buf->dmabuf);
-		return -EINVAL;
+		return bp.fd;
 	}
 
 	if (copy_to_user(argp, &bp, sizeof(bp))) {
-		/*
-		 * The usercopy failed, but we can't do much about it, as
-		 * dma_buf_fd() already called fd_install() and made the
-		 * file descriptor accessible for the current process. It
-		 * might already be closed and dmabuf no longer valid when
-		 * we reach this point. Therefore "leak" the fd and rely on
-		 * the process exit path to do any required cleanup.
-		 */
+		put_unused_fd(bp.fd);
+		dma_buf_put(buf->dmabuf);
 		return -EFAULT;
 	}
 
+	dma_buf_fd_install(buf->dmabuf, bp.fd);
+
 	return 0;
 }