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4335e38778
fcp_ioctl_init() allocates its response buffer with kmalloc() and copies
the whole buffer back to userspace:
buf_size = init.step0_resp_size + init.step2_resp_size;
void *resp __free(kfree) =
kmalloc(buf_size, GFP_KERNEL);
...
if (copy_to_user(arg->resp, resp, buf_size))
return -EFAULT;
Nothing clears the buffer, and the only writer of its leading
step0_resp_size bytes is the step-0 control transfer:
err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0),
FCP_USB_REQ_STEP0,
USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
0, private->bInterfaceNumber,
step0_resp, private->step0_resp_size);
if (err < 0)
return err;
usb_fill_control_urb() does not set URB_SHORT_NOT_OK, so a short or
zero-length data stage completes with status 0 and snd_usb_ctl_msg()
returns a small actual_length. The only check is err < 0, so a short
transfer is accepted as success.
snd_usb_ctl_msg() copies the full size back unconditionally:
buf = kmemdup(data, size, GFP_KERNEL);
...
memcpy(data, buf, size);
Bytes the device never wrote are therefore restored into resp unchanged
and copied to userspace. step0_resp_size and step2_resp_size are each
validated only to 1..255, so the caller also picks the slab cache, from
kmalloc-8 up to kmalloc-512.
On 7.2.0-rc5 (arm64), device answering step 0 with a zero-length data
stage, s0 = s2 = 255:
# init_on_alloc off, no spray
step0 window [0,255): nonzero=94/255
000: 00 80 60 06 00 00 ff ff 18 00 00 00 57 01 ea 01
010: 08 78 22 13 00 00 ff ff a8 c4 5f 80 00 80 ff ff
# same kernel, kmalloc-512 pre-seeded with an 8-byte tag
step0 window [0,255): nonzero=219/255 tagbytes=232
# identical run, init_on_alloc=1
step0 window [0,255): nonzero=0/255 tagbytes=0
# all three runs
step2 window [255,510): device words matched=62/62
a8 c4 5f 80 00 80 ff ff is the little-endian kernel text address
ffff8000805fc4a8. The step-2 window is unaffected, so the disclosure is
exactly the step-0 region.
Zero the buffer, and require the step-0 transfer to deliver the full
step0_resp_size bytes so a short data stage is reported as an error.
Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>
Fixes: 46757a3e7d ("ALSA: FCP: Add Focusrite Control Protocol driver")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Link: https://patch.msgid.link/20260805013804.38839-1-baul.lee@xbow.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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