Re: [PATCH] ALSA: usb-audio: fix OOB write in snd_usbmidi_novation_output()

On Sun, 23 Aug 2026 15:55:48 +0200,
Marouane El Moufid wrote:
> 
> snd_usbmidi_novation_output() lays out a two-byte header at
> transfer_buffer[0..1] and passes &transfer_buffer[2] together with a
> length of ep->max_transfer - 2 to snd_rawmidi_transmit():
> 
> 	count = snd_rawmidi_transmit(ep->ports[0].substream,
> 				     &transfer_buffer[2],
> 				     ep->max_transfer - 2);
> 
> ep->max_transfer comes from the output endpoint's wMaxPacketSize via
> usb_maxpacket(). A malformed or malicious device can advertise a bulk
> OUT endpoint with a wMaxPacketSize of 1 - the USB core only clamps this
> value downwards - so ep->max_transfer becomes 1 and the count argument
> becomes -1.
> 
> snd_rawmidi_transmit() passes the negative count on to
> __snd_rawmidi_transmit_peek(), where "if (count1 > count) count1 = count"
> leaves count1 negative; get_aligned_size() keeps it negative for a
> byte-stream substream, so the following memcpy(buffer, ..., count1) runs
> with a (size_t)-1 length and writes far past the transfer buffer, which
> was allocated with usb_alloc_coherent(ep->max_transfer).
> 
> This is the same class of bug that was fixed for snd_usbmidi_akai_output()
> in commit 0970274613fb ("ALSA: usb-audio: fix OOB write in
> snd_usbmidi_akai_output()"); the novation output routine was left
> unguarded. Bail out when the endpoint cannot hold the two-byte header
> plus at least one payload byte.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: [email protected]
> Signed-off-by: Marouane El Moufid <[email protected]>

Applied now.  Thanks.


Takashi