media: firedtv: possible out of bounds write from a CAM response length

Hi all,

I think avc_ca_app_info() in drivers/media/firewire/firedtv-avc.c can write
past the 256 byte CA message buffer when the CI-CAM returns a large length.
I would appreciate it if you could take a look.

The copy takes its length straight from the CAM response.

	pos = get_ca_object_pos(r);
	...
	memcpy(&app_info[5], &r->operand[pos], 5 + r->operand[pos + 4]);

r->operand[] holds the raw AV/C response bytes the CAM sent, and both pos
and the length operand[pos + 4] come from those bytes. The destination
app_info is reply->msg, the 256 byte msg[] field of struct ca_msg. With a
length byte of 0xff the copy writes 5 + 255 bytes starting at app_info[5],
which runs about 9 bytes past the 256 byte buffer.

There is a FIXME right above the copy that asks for exactly this validation.
The same unbounded pattern is in avc_ca_get_mmi() a few lines down.

This looks like the unfixed sibling of CVE-2021-42739. That fix added bounds
only to avc_ca_pmt(), the host to CAM PMT path. It left the CAM to host
app_info and mmi response paths untouched, and those are the more natural
attacker direction.

The path is reachable from userspace. A CA_SEND_MSG with an APP_INFO_ENQUIRY
tag followed by a CA_GET_MSG lands in avc_ca_app_info(), which then copies
the CAM response. The attacker is a malicious or faulty FireDTV CI-CAM, or a
spoofed FireWire node that answers the request.

I reproduced it under KASAN on 7.1-rc7. A length of 0xff makes KASAN report
a slab out of bounds write past the CA message buffer. A small length stays
clean.

The fix I tried bounds pos and the device length in both avc_ca_app_info()
and avc_ca_get_mmi() so the copy stays in range.

Does this look like a real bug to you? The code predates the git history, so
I am not sure what to put in a Fixes tag. I could reference 35d2969ea3c7, the
commit that bounded avc_ca_pmt() for CVE-2021-42739, since this is the same
class left unfixed. I am happy to send a proper patch.

Thanks,
Maoyi
https://maoyixie.com/