[PATCH] HID: mcp2221: Fix heap buffer overflow in mcp2221_raw_event()

[PATCH] HID: mcp2221: Fix heap buffer overflow in mcp2221_raw_event()

[Benoit Sevens]

From: Benoît Sevens <bsevens@google.com>

A heap buffer overflow can occur in the mcp2221_raw_event() function
when handling I2C read responses. The driver failed to check if the
total incoming data length fits within the originally allocated buffer
`mcp->rxbuf`.

Fix this by introducing `rxbuf_len` to `struct mcp2221` to keep track
of the allocated buffer size. Initialize it in `mcp_i2c_smbus_read()`
and `mcp_smbus_xfer()`, and ensure the copied data length combined with
the current index does not exceed this length in `mcp2221_raw_event()`.

Signed-off-by: Benoît Sevens <bsevens@google.com>
---
 drivers/hid/hid-mcp2221.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/hid-mcp2221.c b/drivers/hid/hid-mcp2221.c
index ef3b5c77c38e..744561e65079 100644
--- a/drivers/hid/hid-mcp2221.c
+++ b/drivers/hid/hid-mcp2221.c
@@ -119,6 +119,7 @@ struct mcp2221 {
 	struct completion wait_in_report;
 	struct delayed_work init_work;
 	u8 *rxbuf;
+	int rxbuf_len;
 	u8 txbuf[64];
 	int rxbuf_idx;
 	int status;
@@ -323,12 +324,14 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 		mcp->txbuf[3] = (u8)(msg->addr << 1);
 		total_len = msg->len;
 		mcp->rxbuf = msg->buf;
+		mcp->rxbuf_len = msg->len;
 	} else {
 		mcp->txbuf[1] = smbus_len;
 		mcp->txbuf[2] = 0;
 		mcp->txbuf[3] = (u8)(smbus_addr << 1);
 		total_len = smbus_len;
 		mcp->rxbuf = smbus_buf;
+		mcp->rxbuf_len = smbus_len;
 	}
 
 	ret = mcp_send_data_req_status(mcp, mcp->txbuf, 4);
@@ -538,6 +541,7 @@ static int mcp_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 
 			mcp->rxbuf_idx = 0;
 			mcp->rxbuf = data->block;
+			mcp->rxbuf_len = sizeof(data->block);
 			mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
 			ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
 			if (ret)
@@ -561,6 +565,7 @@ static int mcp_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 
 			mcp->rxbuf_idx = 0;
 			mcp->rxbuf = data->block;
+			mcp->rxbuf_len = sizeof(data->block);
 			mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
 			ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
 			if (ret)
@@ -908,7 +913,9 @@ static int mcp2221_raw_event(struct hid_device *hdev,
 			}
 			if (data[2] == MCP2221_I2C_READ_COMPL ||
 			    data[2] == MCP2221_I2C_READ_PARTIAL) {
-				if (!mcp->rxbuf || mcp->rxbuf_idx < 0 || data[3] > 60) {
+				if (!mcp->rxbuf || mcp->rxbuf_idx < 0 ||
+				    data[3] > 60 ||
+				    mcp->rxbuf_idx + data[3] > mcp->rxbuf_len) {
 					mcp->status = -EINVAL;
 					break;
 				}
-- 
2.54.0.rc0.605.g598a273b03-goog

Re: [PATCH] HID: mcp2221: Fix heap buffer overflow in mcp2221_raw_event()

[Benoît Sevens]

Hi,

Just checking in to see if anyone had a chance to look at this patch,
or if there is any feedback I can address.

Thanks!

Re: [PATCH] HID: mcp2221: Fix heap buffer overflow in mcp2221_raw_event()

[Jiri Kosina]

On Wed, 15 Apr 2026, Benoit Sevens wrote:

> From: Benoît Sevens <bsevens@google.com>
> 
> A heap buffer overflow can occur in the mcp2221_raw_event() function
> when handling I2C read responses. The driver failed to check if the
> total incoming data length fits within the originally allocated buffer
> `mcp->rxbuf`.
> 
> Fix this by introducing `rxbuf_len` to `struct mcp2221` to keep track
> of the allocated buffer size. Initialize it in `mcp_i2c_smbus_read()`
> and `mcp_smbus_xfer()`, and ensure the copied data length combined with
> the current index does not exceed this length in `mcp2221_raw_event()`.
> 
> Signed-off-by: Benoît Sevens <bsevens@google.com>

Unfortunately the patch seems to have been somehow mangled by your mail 
client:

	patching file drivers/hid/hid-mcp2221.c
	Hunk #1 succeeded at 126 (offset 7 lines).
	Hunk #2 FAILED at 324.
	patch: **** malformed patch at line 173: u16 addr,

Fix for the same issue has later been submitted by Florian Pradines [1], 
so I will be taking that one and adding you as Reported-by: or so, ok?

Thanks.

[1] https://lore.kernel.org/all/20260509094517.2691246-1-florian.pradines@gmail.com/

-- 
Jiri Kosina
SUSE Labs

Re: [PATCH] HID: mcp2221: Fix heap buffer overflow in mcp2221_raw_event()

[Benoît Sevens]

On Tue, 12 May 2026 at 17:47, Jiri Kosina <jikos@kernel.org> wrote:
>
> On Wed, 15 Apr 2026, Benoit Sevens wrote:
>
> > From: Benoît Sevens <bsevens@google.com>
> >
> > A heap buffer overflow can occur in the mcp2221_raw_event() function
> > when handling I2C read responses. The driver failed to check if the
> > total incoming data length fits within the originally allocated buffer
> > `mcp->rxbuf`.
> >
> > Fix this by introducing `rxbuf_len` to `struct mcp2221` to keep track
> > of the allocated buffer size. Initialize it in `mcp_i2c_smbus_read()`
> > and `mcp_smbus_xfer()`, and ensure the copied data length combined with
> > the current index does not exceed this length in `mcp2221_raw_event()`.
> >
> > Signed-off-by: Benoît Sevens <bsevens@google.com>
>
> Unfortunately the patch seems to have been somehow mangled by your mail
> client:
>
>         patching file drivers/hid/hid-mcp2221.c
>         Hunk #1 succeeded at 126 (offset 7 lines).
>         Hunk #2 FAILED at 324.
>         patch: **** malformed patch at line 173: u16 addr,
>
> Fix for the same issue has later been submitted by Florian Pradines [1],
> so I will be taking that one and adding you as Reported-by: or so, ok?
>
> Thanks.
>
> [1] https://lore.kernel.org/all/20260509094517.2691246-1-florian.pradines@gmail.com/
>
> --
> Jiri Kosina
> SUSE Labs
>

Hi Jiri,

That sounds good to me. Thanks for adding me in the Reported-by

One thing though: Florian's patch doesn't set `mcp->rxbuf_size` in all
places. I believe it should be set in every place where `mcp->rxbuf`
is set. There are 2 places in `mcp_smbus_xfer` where it is missing
(see my original patch):

```c
@@ -538,6 +541,7 @@ static int mcp_smbus_xfer(struct i2c_adapter
*adapter, u16 addr,

                        mcp->rxbuf_idx = 0;
                        mcp->rxbuf = data->block;
+                       mcp->rxbuf_size = sizeof(data->block);
                        mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
                        ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
                        if (ret)
@@ -561,6 +565,7 @@ static int mcp_smbus_xfer(struct i2c_adapter
*adapter, u16 addr,

                        mcp->rxbuf_idx = 0;
                        mcp->rxbuf = data->block;
+                       mcp->rxbuf_size = sizeof(data->block);
                        mcp->txbuf[0] = MCP2221_I2C_GET_DATA;
                        ret = mcp_send_data_req_status(mcp, mcp->txbuf, 1);
                        if (ret)
```

Thanks,

Benoit