[PATCH] net: cdc_ncm: Deal with too low values of dwNtbOutMaxSize

[Tudor Ambarus]

Currently in cdc_ncm_check_tx_max(), if dwNtbOutMaxSize is lower than
the calculated "min" value, but greater than zero, the logic sets
tx_max to dwNtbOutMaxSize. This is then used to allocate a new SKB in
cdc_ncm_fill_tx_frame() where all the data is handled.

For small values of dwNtbOutMaxSize the memory allocated during
alloc_skb(dwNtbOutMaxSize, GFP_ATOMIC) will have the same size, due to
how size is aligned at alloc time:
	size = SKB_DATA_ALIGN(size);
        size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
Thus we hit the same bug that we tried to squash with
commit 2be6d4d16a084 ("net: cdc_ncm: Allow for dwNtbOutMaxSize to be unset or zero")

Low values of dwNtbOutMaxSize do not cause an issue presently because at
alloc_skb() time more memory (512b) is allocated than required for the
SKB headers alone (320b), leaving some space (512b - 320b = 192b)
for CDC data (172b).

However, if more elements (for example 3 x u64 = [24b]) were added to
one of the SKB header structs, say 'struct skb_shared_info',
increasing its original size (320b [320b aligned]) to something larger
(344b [384b aligned]), then suddenly the CDC data (172b) no longer
fits in the spare SKB data area (512b - 384b = 128b).

Consequently the SKB bounds checking semantics fails and panics:

skbuff: skb_over_panic: text:ffffffff831f755b len:184 put:172 head:ffff88811f1c6c00 data:ffff88811f1c6c00 tail:0xb8 end:0x80 dev:<NULL>
------------[ cut here ]------------
kernel BUG at net/core/skbuff.c:113!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 57 Comm: kworker/0:2 Not tainted 5.15.106-syzkaller-00249-g19c0ed55a470 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/14/2023
Workqueue: mld mld_ifc_work
RIP: 0010:skb_panic net/core/skbuff.c:113 [inline]
RIP: 0010:skb_over_panic+0x14c/0x150 net/core/skbuff.c:118
[snip]
Call Trace:
 <TASK>
 skb_put+0x151/0x210 net/core/skbuff.c:2047
 skb_put_zero include/linux/skbuff.h:2422 [inline]
 cdc_ncm_ndp16 drivers/net/usb/cdc_ncm.c:1131 [inline]
 cdc_ncm_fill_tx_frame+0x11ab/0x3da0 drivers/net/usb/cdc_ncm.c:1308
 cdc_ncm_tx_fixup+0xa3/0x100

Deal with too low values of dwNtbOutMaxSize, clamp it in the range
[USB_CDC_NCM_NTB_MIN_OUT_SIZE, CDC_NCM_NTB_MAX_SIZE_TX]. We ensure
enough data space is allocated to handle CDC data by making sure
dwNtbOutMaxSize is not smaller than USB_CDC_NCM_NTB_MIN_OUT_SIZE.

Fixes: 289507d3364f ("net: cdc_ncm: use sysfs for rx/tx aggregation tuning")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: syzbot+9f575a1f15fc0c01ed69@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?extid=b982f1059506db48409d
Link: https://lore.kernel.org/all/20211202143437.1411410-1-lee.jones@xxxxxxxxxx/
Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
---
 drivers/net/usb/cdc_ncm.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 6ce8f4f0c70e..db05622f1f70 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -181,9 +181,12 @@ static u32 cdc_ncm_check_tx_max(struct usbnet *dev, u32 new_tx)
 	else
 		min = ctx->max_datagram_size + ctx->max_ndp_size + sizeof(struct usb_cdc_ncm_nth32);
 
-	max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_TX, le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize));
-	if (max == 0)
+	if (le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize) == 0)
 		max = CDC_NCM_NTB_MAX_SIZE_TX; /* dwNtbOutMaxSize not set */
+	else
+		max = clamp_t(u32, le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize),
+			      USB_CDC_NCM_NTB_MIN_OUT_SIZE,
+			      CDC_NCM_NTB_MAX_SIZE_TX);
 
 	/* some devices set dwNtbOutMaxSize too low for the above default */
 	min = min(min, max);
@@ -1244,6 +1247,9 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 			 * further.
 			 */
 			if (skb_out == NULL) {
+				/* If even the smallest allocation fails, abort. */
+				if (ctx->tx_curr_size == USB_CDC_NCM_NTB_MIN_OUT_SIZE)
+					goto alloc_failed;
 				ctx->tx_low_mem_max_cnt = min(ctx->tx_low_mem_max_cnt + 1,
 							      (unsigned)CDC_NCM_LOW_MEM_MAX_CNT);
 				ctx->tx_low_mem_val = ctx->tx_low_mem_max_cnt;
@@ -1262,13 +1268,8 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 			skb_out = alloc_skb(ctx->tx_curr_size, GFP_ATOMIC);
 
 			/* No allocation possible so we will abort */
-			if (skb_out == NULL) {
-				if (skb != NULL) {
-					dev_kfree_skb_any(skb);
-					dev->net->stats.tx_dropped++;
-				}
-				goto exit_no_skb;
-			}
+			if (!skb_out)
+				goto alloc_failed;
 			ctx->tx_low_mem_val--;
 		}
 		if (ctx->is_ndp16) {
@@ -1461,6 +1462,11 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 
 	return skb_out;
 
+alloc_failed:
+	if (skb) {
+		dev_kfree_skb_any(skb);
+		dev->net->stats.tx_dropped++;
+	}
 exit_no_skb:
 	/* Start timer, if there is a remaining non-empty skb */
 	if (ctx->tx_curr_skb != NULL && n > 0)
-- 
2.40.1.606.ga4b1b128d6-goog