[PATCH] i3c: dw: avoid shift-out-of-bounds when DAA assigns no devices

From: Jakub Kicinski <kuba@kernel.org>
To: linux-i3c@lists.infradead.org
Cc: Jakub Kicinski <kuba@kernel.org>,
	alexandre.belloni@bootlin.com, Frank.Li@nxp.com,
	adrianhoyin.ng@altera.com, ustc.gu@gmail.com,
	bbrezillon@kernel.org, vitor.soares@synopsys.com
Subject: [PATCH] i3c: dw: avoid shift-out-of-bounds when DAA assigns no devices
Date: Mon, 29 Jun 2026 16:36:50 -0700	[thread overview]
Message-ID: <20260629233650.2148827-1-kuba@kernel.org> (raw)

On an empty bus ENTDAA assigns nothing, so cmd->rx_len (the count
of addresses left unassigned) equals master->maxdevs.
The GENMASK() index master->maxdevs - cmd->rx_len - 1 then becomes -1,
which trips up UBSAN. We hit this on a Gigabyte/AMD server on
every boot:

    UBSAN: shift-out-of-bounds in drivers/i3c/master/dw-i3c-master.c:905:12
    shift exponent 64 is too large for 64-bit type 'long unsigned int'
    CPU: 7 UID: 0 PID: 963 Comm: (udev-worker) Not tainted 7.0.11-200.fc44.x86_64 #1 PREEMPT(lazy)
    Hardware name: Giga Computing E163-Z34-AAH1-000/MZ33-DC1-000, BIOS R32_F45 04/01/2026
    Call Trace:
     <TASK>
     dump_stack_lvl+0x5d/0x80
     ubsan_epilogue+0x5/0x2b
     __ubsan_handle_shift_out_of_bounds.cold+0xd7/0x1ab
     dw_i3c_master_daa.cold+0x1b/0x96 [dw_i3c_master]
     i3c_master_do_daa_ext.part.0+0x3e/0xf0 [i3c]

Skip the mask when no new device was assigned.

Fixes: 1dd728f5d4d4 ("i3c: master: Add driver for Synopsys DesignWare IP")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: alexandre.belloni@bootlin.com
CC: Frank.Li@nxp.com
CC: adrianhoyin.ng@altera.com
CC: ustc.gu@gmail.com
CC: bbrezillon@kernel.org
CC: vitor.soares@synopsys.com
---
 drivers/i3c/master/dw-i3c-master.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/i3c/master/dw-i3c-master.c b/drivers/i3c/master/dw-i3c-master.c
index 2f8c0c4683e0..03321edcd0d3 100644
--- a/drivers/i3c/master/dw-i3c-master.c
+++ b/drivers/i3c/master/dw-i3c-master.c
@@ -888,7 +888,14 @@ static int dw_i3c_master_daa(struct i3c_master_controller *m)
 	if (!wait_for_completion_timeout(&xfer->comp, XFER_TIMEOUT))
 		dw_i3c_master_dequeue_xfer(master, xfer);
 
-	newdevs = GENMASK(master->maxdevs - cmd->rx_len - 1, 0);
+	/*
+	 * cmd->rx_len holds the number of addresses ENTDAA left unassigned.
+	 * On an empty bus rx_len == maxdevs, so avoid GENMASK(-1, 0).
+	 */
+	if (cmd->rx_len >= master->maxdevs)
+		newdevs = 0;
+	else
+		newdevs = GENMASK(master->maxdevs - cmd->rx_len - 1, 0);
 	newdevs &= ~olddevs;
 
 	for (pos = 0; pos < master->maxdevs; pos++) {
-- 
2.54.0


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