linux-kernel - [PATCH] cdrom: Avoid barrier_nospec() in cdrom_ioctl_media

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Message-ID: <1d86f4d9d8fba68e5ca64cdeac2451b95a8bf872.1729202937.git.jpoimboe@kernel.org>
Date: Thu, 17 Oct 2024 15:09:02 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Phillip Potter <phil@...lpotter.co.uk>
Cc: linux-kernel@...r.kernel.org,
	Jordy Zomer <jordyzomer@...gle.com>,
	Jens Axboe <axboe@...nel.dk>
Subject: [PATCH] cdrom: Avoid barrier_nospec() in cdrom_ioctl_media_changed()

The barrier_nospec() after the array bounds check is overkill and
painfully slow for arches which implement it.

Furthermore, most arches don't implement it, so they remain exposed to
Spectre v1 (which can affect pretty much any CPU with branch
prediction).

Instead, clamp the user pointer to a valid range so it's guaranteed to
be a valid array index even when the bounds check mispredicts.

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 drivers/cdrom/cdrom.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index 9b0f37d4b9d4..6a99a459b80b 100644
--- a/drivers/cdrom/cdrom.c
+++ b/drivers/cdrom/cdrom.c
@@ -2313,7 +2313,7 @@ static int cdrom_ioctl_media_changed(struct cdrom_device_info *cdi,
 		return -EINVAL;
 
 	/* Prevent arg from speculatively bypassing the length check */
-	barrier_nospec();
+	arg = array_index_nospec(arg, cdi->capacity);
 
 	info = kmalloc(sizeof(*info), GFP_KERNEL);
 	if (!info)
-- 
2.47.0