Re: [PATCH] nfc: fdp: reject an oversized device-reported packet length

[Simon Horman]

On Mon, Jun 15, 2026 at 03:04:02AM -0500, Bryam Vargas via B4 Relay wrote:
> From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
> 
> fdp_nci_i2c_read() reads the length of the next packet from the device
> into phy->next_read_size and uses it as the i2c_master_recv() byte count
> into a fixed on-stack buffer:
> 
> 	u8 tmp[FDP_NCI_I2C_MAX_PAYLOAD];		/* 261 bytes */
> 	...
> 	len = phy->next_read_size;
> 	r = i2c_master_recv(client, tmp, len);
> 
> When a "length packet" arrives (tmp[0] == 0 && tmp[1] == 0), the next
> length is taken verbatim from two device-supplied bytes:
> 
> 	phy->next_read_size = (tmp[2] << 8) + tmp[3] + 3;
> 
> next_read_size is a u16, so this can be driven as high as 65535 - far
> larger than the 261-byte tmp[] buffer - and it is never bounded before
> the next iteration's i2c_master_recv(). A malfunctioning, malicious or
> counterfeit FDP NFC controller (or an attacker tampering with the I2C
> bus) that sends such a length packet makes i2c_master_recv() write up to
> about 64 KB into the 261-byte on-stack buffer: a stack out-of-bounds
> write that clobbers the stack canary, saved registers and the return
> address.
> 
> Reject a next_read_size larger than the receive buffer the same way a
> corrupted packet is already handled - drop it and force resynchronization
> - so a device can never drive an over-length read.
> 
> Fixes: a06347c04c13 ("NFC: Add Intel Fields Peak NFC solution driver")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
> ---
> I reproduced the out-of-bounds write with an in-kernel test that drives
> the fdp_nci_i2c_read() buffer geometry verbatim under KASAN
> (CONFIG_KASAN_STACK=y), modelling i2c_master_recv() delivering
> next_read_size device bytes into the 261-byte tmp[] buffer:
> 
>   next_read_size = 281, no bound:
>     BUG: KASAN: stack-out-of-bounds in i2c_master_recv...
>     Write of size 281 ... [48, 309) 'tmp'   (the 261-byte buffer)
>   with the device length bounded to <= FDP_NCI_I2C_MAX_PAYLOAD (what this
>     patch enforces): no KASAN report.
>   a well-formed packet (length <= 261) is unaffected, no KASAN report.
> 
> The full device range - next_read_size = 65535 (tmp[2] = 0xff,
> tmp[3] = 0xfc; the u16 field truncates the + 3), a 65535-byte write =
> 65274 bytes past the buffer, smashing the stack canary and the return
> address - reproduces the same way under userspace AddressSanitizer on
> both -m32 and -m64.
> ---
>  drivers/nfc/fdp/i2c.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/nfc/fdp/i2c.c b/drivers/nfc/fdp/i2c.c
> index c1896a1d978c..0392bb49bb4b 100644
> --- a/drivers/nfc/fdp/i2c.c
> +++ b/drivers/nfc/fdp/i2c.c
> @@ -166,6 +166,20 @@ static int fdp_nci_i2c_read(struct fdp_i2c_phy *phy, struct sk_buff **skb)
>  		/* Packet that contains a length */
>  		if (tmp[0] == 0 && tmp[1] == 0) {
>  			phy->next_read_size = (tmp[2] << 8) + tmp[3] + 3;


Thanks Bryam,

I agree with your analysis regarding overrunning tmp and that the
fix for that is correct.

But I am concerned that there is also an expectation in the code that
next_read_size is always at least FDP_NCI_I2C_MIN_PAYLOAD (5).
But that smaller values can be achieved if either:

* tmp[2] is 0 and tmp[3] is < 2.
* the addition above overflows 16bits. e.g. both tmp[2] and tmp[3] are 255.

So I wonder if the check you are adding below should also guard
against phy->next_read_size < FDP_NCI_I2C_MIN_PAYLOAD.

> +
> +			/*
> +			 * next_read_size is taken from the device and is used
> +			 * as the i2c_master_recv() count on the next iteration.
> +			 * A value larger than the receive buffer would overflow
> +			 * tmp[]; treat it like a corrupted packet and force
> +			 * resynchronization.
> +			 */
> +			if (phy->next_read_size > FDP_NCI_I2C_MAX_PAYLOAD) {
> +				dev_dbg(&client->dev, "%s: corrupted packet\n",
> +					__func__);
> +				phy->next_read_size = FDP_NCI_I2C_MIN_PAYLOAD;
> +				goto flush;
> +			}
>  		} else {
>  			phy->next_read_size = FDP_NCI_I2C_MIN_PAYLOAD;
>  
> 
> ---
> base-commit: 8e65320d91cdc3b241d4b94855c88459b91abf66
> change-id: 20260615-b4-disp-f42dce2d-055035ea37ba
> 
> Best regards,
> -- 
> Bryam Vargas <hexlabsecurity@xxxxxxxxx>
> 
>