linux-kernel - Re: [Linux-kernel-mentees] [PATCH] Fix uninit-value in hci_chan_lookup

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Message-Id: <2AEF86AF-8F80-4B01-B506-D2ABC654DBC7@holtmann.org>
Date:   Sat, 26 Sep 2020 09:46:49 +0200
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Anant Thazhemadam <anant.thazhemadam@...il.com>
Cc:     linux-kernel-mentees@...ts.linuxfoundation.org,
        Johan Hedberg <johan.hedberg@...il.com>,
        linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [Linux-kernel-mentees] [PATCH] Fix uninit-value in
 hci_chan_lookup_handle

Hi Anant,

> When the amount of data stored in the location corresponding to
> iov_iter *from is less then 4, some data seems to go uninitialized.
> 
> Updating this condition accordingly, makes sense both intuitively and 
> logically as well, since the other check for extreme condition done is if
> len > HCI_MAX_FRAME_SIZE, which is HCI_MAX_ACL_SIZE (which is 1024) + 4;
> which itself gives some idea about what must be the ideal mininum size.
> 
> Reported-and-tested by: syzbot+4c14a8f574461e1c3659@...kaller.appspotmail.com
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
> ---
> If there is some explicit reason why len < 4 doesn't work, and only len < 2 works, 
> please do let me know.
> The commit message that introduced the initial change 
> (512b2268156a4e15ebf897f9a883bdee153a54b7) wasn't exactly very helpful in this 
> respect, and I couldn't find a whole lot of discussion regarding this either.
> 
> drivers/bluetooth/hci_vhci.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/bluetooth/hci_vhci.c b/drivers/bluetooth/hci_vhci.c
> index 8ab26dec5f6e..0c49821d7b98 100644
> --- a/drivers/bluetooth/hci_vhci.c
> +++ b/drivers/bluetooth/hci_vhci.c
> @@ -159,7 +159,7 @@ static inline ssize_t vhci_get_user(struct vhci_data *data,
> 	__u8 pkt_type, opcode;
> 	int ret;
> 
> -	if (len < 2 || len > HCI_MAX_FRAME_SIZE)
> +	if (len < 4 || len > HCI_MAX_FRAME_SIZE)
> 		return -EINVAL;

so the minimum requirement is to have an 1 octet packet indicator. If some other functions are missing length checks, then it should be fixed there. I see that in case of HCI_VENDOR_PKT we need to check for the 2nd octet to be present, but I have no idea why you require 3 octets minimum. As I said, if so, then the other functions that get called require proper range checks.

Regards

Marcel