linux-kernel - [PATCH 5.10 061/116] misc: fastrpc: avoid double fput() on failed usercopy

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Date:   Mon, 14 Feb 2022 10:26:00 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christian König <christian.koenig@....com>,
        Mathias Krause <minipli@...ecurity.net>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.10 061/116] misc: fastrpc: avoid double fput() on failed usercopy

From: Mathias Krause <minipli@...ecurity.net>

[ Upstream commit 46963e2e0629cb31c96b1d47ddd89dc3d8990b34 ]

If the copy back to userland fails for the FASTRPC_IOCTL_ALLOC_DMA_BUFF
ioctl(), we shouldn't assume that 'buf->dmabuf' is still valid. In fact,
dma_buf_fd() called fd_install() before, i.e. "consumed" one reference,
leaving us with none.

Calling dma_buf_put() will therefore put a reference we no longer own,
leading to a valid file descritor table entry for an already released
'file' object which is a straight use-after-free.

Simply avoid calling dma_buf_put() and rely on the process exit code to
do the necessary cleanup, if needed, i.e. if the file descriptor is
still valid.

Fixes: 6cffd79504ce ("misc: fastrpc: Add support for dmabuf exporter")
Acked-by: Christian König <christian.koenig@....com>
Signed-off-by: Mathias Krause <minipli@...ecurity.net>
Link: https://lore.kernel.org/r/20220127130218.809261-1-minipli@grsecurity.net
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/misc/fastrpc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index ef49ac8d91019..d0471fec37fbb 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -1284,7 +1284,14 @@ static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
 	}
 
 	if (copy_to_user(argp, &bp, sizeof(bp))) {
-		dma_buf_put(buf->dmabuf);
+		/*
+		 * 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+		 * might already be closed and dmabuf no longer valid when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.
+		 */
 		return -EFAULT;
 	}
 
-- 
2.34.1