This is the backported patch for the following fix to v4.4.x and v4.14.x: 19bce474c45b ("ALSA: usb-audio: Fix a stack buffer overflow bug in check_input_term")

On Fri, Aug 30, 2019 at 5:47 PM Hui Peng benquike@gmail.com wrote:

`check_input_term` recursively calls itself with input from device side (e.g., uac_input_terminal_descriptor.bCSourceID) as argument (id). In `check_input_term`, if `check_input_term` is called with the same `id` argument as the caller, it triggers endless recursive call, resulting kernel space stack overflow.

This patch fixes the bug by adding a bitmap to `struct mixer_build` to keep track of the checked ids and stop the execution if some id has been checked (similar to how parse_audio_unit handles unitid argument).

CVE: CVE-2018-15118

Reported-by: Hui Peng benquike@gmail.com Reported-by: Mathias Payer mathias.payer@nebelwelt.net Signed-off-by: Hui Peng benquike@gmail.com

---

sound/usb/mixer.c | 29 ++++++++++++++++++++++++----- 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c index 10ddec76f906..e24572fd6e30 100644 --- a/sound/usb/mixer.c +++ b/sound/usb/mixer.c @@ -81,6 +81,7 @@ struct mixer_build { unsigned char *buffer; unsigned int buflen; DECLARE_BITMAP(unitbitmap, MAX_ID_ELEMS);

•   DECLARE_BITMAP(termbitmap, MAX_ID_ELEMS);
    struct usb_audio_term oterm;
    const struct usbmix_name_map *map;
    const struct usbmix_selector_map *selector_map;
  

@@ -709,15 +710,24 @@ static int get_term_name(struct mixer_build *state, struct usb_audio_term *iterm

• parse the source unit recursively until it reaches to a terminal
• or a branched unit.

*/ -static int check_input_term(struct mixer_build *state, int id, +static int __check_input_term(struct mixer_build *state, int id, struct usb_audio_term *term) { int err; void *p1;

•   unsigned char *hdr;
  
    memset(term, 0, sizeof(*term));
  

•   while ((p1 = find_audio_control_unit(state, id)) != NULL) {
  

•           unsigned char *hdr = p1;
  

•   for (;;) {
  

•           /* a loop in the terminal chain? */
  

•           if (test_and_set_bit(id, state->termbitmap))
  

•                   return -EINVAL;
  

•           p1 = find_audio_control_unit(state, id);
  

•           if (!p1)
  

•                   break;
  

•           hdr = p1;
            term->id = id;
            switch (hdr[2]) {
            case UAC_INPUT_TERMINAL:
  

@@ -732,7 +742,7 @@ static int check_input_term(struct mixer_build *state, int id,

                            /* call recursively to verify that the
                             * referenced clock entity is valid */

•                           err = check_input_term(state,
  

d->bCSourceID, term);

•                           err = __check_input_term(state,
  

d->bCSourceID, term); if (err < 0) return err;

@@ -764,7 +774,7 @@ static int check_input_term(struct mixer_build *state, int id, case UAC2_CLOCK_SELECTOR: { struct uac_selector_unit_descriptor *d = p1; /* call recursively to retrieve the channel info */

•                   err = check_input_term(state, d->baSourceID[0],
  

term);

•                   err = __check_input_term(state, d->baSourceID[0],
  

term); if (err < 0) return err; term->type = d->bDescriptorSubtype << 16; /* virtual type */ @@ -811,6 +821,15 @@ static int check_input_term(struct mixer_build *state, int id, return -ENODEV; }

+static int check_input_term(struct mixer_build *state, int id,

•                       struct usb_audio_term *term)
  

+{

•   memset(term, 0, sizeof(*term));
  

•   memset(state->termbitmap, 0, sizeof(state->termbitmap));
  

•   return __check_input_term(state, id, term);
  

+}

/*

• Feature Unit

*/

2.17.1