linux-hardening - [PATCH] usb: host: xhci: mvebu: Iterate over array indexes instead of using pointer math

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Date:   Sat,  4 Feb 2023 10:36:52 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Mathias Nyman <mathias.nyman@...el.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH] usb: host: xhci: mvebu: Iterate over array indexes instead of using pointer math

Walking the dram->cs array was seen as accesses beyond the first array
item by the compiler. Instead, use the array index directly. This allows
for run-time bounds checking under CONFIG_UBSAN_BOUNDS as well. Seen
with GCC 13 with -fstrict-flex-arrays:

In function 'xhci_mvebu_mbus_config',
    inlined from 'xhci_mvebu_mbus_init_quirk' at ../drivers/usb/host/xhci-mvebu.c:66:2:
../drivers/usb/host/xhci-mvebu.c:37:28: warning: array subscript 0 is outside array bounds of 'const struct mbus_dram_window[0]' [-Warray-bounds=]
   37 |                 writel(((cs->size - 1) & 0xffff0000) | (cs->mbus_attr << 8) |
      |                          ~~^~~~~~

Cc: Mathias Nyman <mathias.nyman@...el.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-usb@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/usb/host/xhci-mvebu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-mvebu.c b/drivers/usb/host/xhci-mvebu.c
index 60651a50770f..87f1597a0e5a 100644
--- a/drivers/usb/host/xhci-mvebu.c
+++ b/drivers/usb/host/xhci-mvebu.c
@@ -32,7 +32,7 @@ static void xhci_mvebu_mbus_config(void __iomem *base,
 
 	/* Program each DRAM CS in a seperate window */
 	for (win = 0; win < dram->num_cs; win++) {
-		const struct mbus_dram_window *cs = dram->cs + win;
+		const struct mbus_dram_window *cs = &dram->cs[win];
 
 		writel(((cs->size - 1) & 0xffff0000) | (cs->mbus_attr << 8) |
 		       (dram->mbus_dram_target_id << 4) | 1,
-- 
2.34.1