linux-kernel - Re: [PATCH v3 1/2] media: venus: fix TOCTOU vulnerability when reading packets from shared memory

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Message-ID: <b0c48989-4ce7-4338-b4bb-565ea8b6cd82@linaro.org>
Date: Thu, 15 May 2025 11:28:10 +0100
From: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To: Vikash Garodia <quic_vgarodia@...cinc.com>,
 Dikshita Agarwal <quic_dikshita@...cinc.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Stanimir Varbanov <stanimir.varbanov@...aro.org>,
 Hans Verkuil <hans.verkuil@...co.com>
Cc: linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Vedang Nagar <quic_vnagar@...cinc.com>
Subject: Re: [PATCH v3 1/2] media: venus: fix TOCTOU vulnerability when
 reading packets from shared memory

On 15/05/2025 10:56, Vikash Garodia wrote:
> memcpy(hfi_dev->pkt_buf, rd_ptr from shared queue, dwords..)
> 
> pkt_hdr = (struct hfi_pkt_hdr *) (hfi_dev->pkt_buf);
> 
> if ((pkt_hdr->size >> 2) != dwords)
>      return -EINVAL;

Yeah it would be better wrt the commit log.

But does it really give additional data-confidence - I don't believe it 
does.

=> The firmware can update the pkt header after our 
subsequent-to-memcpy() check.

Again this is a data-lifetime expectation question.

You validate the received data against a maximum size reading to a 
buffer you know the size of - and do it once.

The firmware might corrupt that data in-between but that is not 
catastrophic for the APSS which has a buffer of a known size containing 
potential bad data.

Fine - and additional check after the mempcy() only imparts 
verisimilitude - only validates our data at the time of the check.

my-linear-uninterrupted-context:

memcpy();

if(*rd_ptr >> 2 > len) <- doesn't branch
     return -EBAD

if(*rd_ptr >> 2 > len) <- does branch firmware went nuts
     return -EBAD

Superficially you might say this addresses the problem

if (*rd_ptr > MAX)
     return -EBAD;

memcpy();

if (*rd_ptr > MAX)
     return -EBAD;

But what if the "malicious" firmware only updated the data in the 
packet, not the length - or another field we are not checking ?

As I say if this can happen

if (*rd_ptr > MAX)
     return -EBAD;

memcpy();

if (*rd_ptr > MAX)  // good
     return -EBAD;

if (*rd_ptr > MAX) //bad
     return -EBAD;

then this can happen

if (*rd_ptr > MAX)
     return -EBAD;

memcpy();

if (*rd_ptr > MAX) // good
     return -EBAD;

if (*rd_ptr > MAX) //good
     return -EBAD;

if (*rd_ptr > MAX) //bad
     return -EBAD;

We need to have a crisp and clear definition of the data-lifetime. Since 
we don't have a checksum element in the header the only check that makes 
sense is to validate the buffer size

data_len = *ptr_val >> 2;
if (data_len > max)
     return BAD;

Using the data_len in memcpy if the *ptr_val can change is _NOT_ TOCTOU

This is TOCTOU

if (*ptr_val > max)
     return EBAD;

memcpy(dst, src, *ptr_val);

Because I validated the content of the pointer and then I relied on the 
data that pointer pointed to, which might have changed.

TBH I think the entire premise of this patch is bogus.

---
bod