Re: [PATCH] Fix a stack buffer overflow bug check_input_term

[Takashi Iwai <tiwai@suse.de>]

On Thu, 15 Aug 2019 08:13:57 +0200,
Takashi Iwai wrote:
> 
> On Thu, 15 Aug 2019 06:35:49 +0200,
> Hui Peng 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 by `check_input_term` and stop
> > the execution if some id has been checked (similar to how
> > parse_audio_unit handles unitid argument).
> > 
> > Reported-by: Hui Peng <benquike@gmail.com>
> > Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
> > Signed-off-by: Hui Peng <benquike@gmail.com>
> 
> The fix looks almost good, but we need to be careful about the
> bitmap check.  In theory, it's possible that multiple nodes point to
> the same input terminal, and your patch would break that scenario.
> For fixing that, we need to zero-clear the termbitmap at each first
> invocation of check_input_term(), something like below.
> 
> Could you check whether this works?

Thinking of this further, there is another possible infinite loop.
Namely, when the feature unit in the input terminal chain points to
itself as the source, it'll loop endlessly without the stack
overflow.

So the check of the termbitmap should be inside the loop.
The revised patch is below.


thanks,

Takashi

-- 8< --
From: Hui Peng <benquike@gmail.com>
Subject: [PATCH] ALSA: usb-audio: Fix a stack buffer overflow bug
 check_input_term

`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 by `check_input_term` and stop
the execution if some id has been checked (similar to how
parse_audio_unit handles unitid argument).

[ The termbitmap needs to be cleared at each first check of the input
  terminal, so the function got split now.  Also, for catching another
  endless loop in the input terminal chain -- where the feature unit
  points to itself as its source -- the termbitmap check is moved
  inside the parser loop. -- tiwai ]

Reported-by: Hui Peng <benquike@gmail.com>
Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
Signed-off-by: Hui Peng <benquike@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/usb/mixer.c | 36 ++++++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index ea487378be17..aa8b046aa91f 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -68,6 +68,7 @@ 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;
@@ -775,16 +776,23 @@ static int uac_mixer_unit_get_channels(struct mixer_build *state,
  * 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,
-			    struct usb_audio_term *term)
+static int __check_input_term(struct mixer_build *state, int id,
+			      struct usb_audio_term *term)
 {
 	int protocol = state->mixer->protocol;
 	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))
+			break;
+
+		p1 = find_audio_control_unit(state, id);
+		if (!p1)
+			break;
+		hdr = p1;
 		term->id = id;
 
 		if (protocol == UAC_VERSION_1 || protocol == UAC_VERSION_2) {
@@ -802,7 +810,7 @@ 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;
 
@@ -836,7 +844,7 @@ 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 = UAC3_SELECTOR_UNIT << 16; /* virtual type */
@@ -899,7 +907,7 @@ 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;
 
@@ -950,7 +958,7 @@ static int check_input_term(struct mixer_build *state, int id,
 			case UAC3_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 = UAC3_SELECTOR_UNIT << 16; /* virtual type */
@@ -966,7 +974,7 @@ static int check_input_term(struct mixer_build *state, int id,
 					return -EINVAL;
 
 				/* 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;
 
@@ -984,6 +992,14 @@ 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
  */
-- 
2.16.4