syzbot reported: BUG: KMSAN: uninit-value in capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700 CPU: 0 PID: 10025 Comm: syz-executor379 Not tainted 4.20.0-rc7+ #2 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:77 [inline] dump_stack+0x173/0x1d0 lib/dump_stack.c:113 kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613 __msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:313 capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700 do_loop_readv_writev fs/read_write.c:703 [inline] do_iter_write+0x83e/0xd80 fs/read_write.c:961 vfs_writev fs/read_write.c:1004 [inline] do_writev+0x397/0x840 fs/read_write.c:1039 __do_sys_writev fs/read_write.c:1112 [inline] __se_sys_writev+0x9b/0xb0 fs/read_write.c:1109 __x64_sys_writev+0x4a/0x70 fs/read_write.c:1109 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291 entry_SYSCALL_64_after_hwframe+0x63/0xe7 [...] The problem is that capi_write() is reading past the end of the message. Fix it by checking the message's length in the needed places.
Unified · Split
| @@ -688,6 +688,9 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos · 687 unmodified lines | ||
| 688 | 688 | if (!cdev->ap.applid) |
| 689 | 689 | return -ENODEV; |
| 690 | 690 | |
| 691 | + if (count < CAPIMSG_BASELEN) | |
| 692 | + return -EINVAL; | |
| 693 | + | |
| 691 | 694 | skb = alloc_skb(count, GFP_USER); |
| 692 | 695 | if (!skb) |
| 693 | 696 | return -ENOMEM; |
| @@ -698,7 +701,8 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos · 4 unmodified lines | ||
| 698 | 701 | } |
| 699 | 702 | mlen = CAPIMSG_LEN(skb->data); |
| 700 | 703 | if (CAPIMSG_CMD(skb->data) == CAPI_DATA_B3_REQ) { |
| 701 | − if ((size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) { | |
| 704 | + if (count < CAPI_DATA_B3_REQ_LEN || | |
| 705 | + (size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) { | |
| 702 | 706 | kfree_skb(skb); |
| 703 | 707 | return -EINVAL; |
| 704 | 708 | } |
| @@ -711,6 +715,10 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos · 6 unmodified lines | ||
| 711 | 715 | CAPIMSG_SETAPPID(skb->data, cdev->ap.applid); |
| 712 | 716 | |
| 713 | 717 | if (CAPIMSG_CMD(skb->data) == CAPI_DISCONNECT_B3_RESP) { |
| 718 | + if (count < CAPI_DISCONNECT_B3_RESP_LEN) { | |
| 719 | + kfree_skb(skb); | |
| 720 | + return -EINVAL; | |
| 721 | + } | |
| 714 | 722 | mutex_lock(&cdev->lock); |
| 715 | 723 | capincci_free(cdev, CAPIMSG_NCCI(skb->data)); |
| 716 | 724 | mutex_unlock(&cdev->lock); |
| @@ -16,6 +16,7 @@ · 15 unmodified lines | ||
| 16 | 16 | #define CAPI_MSG_BASELEN 8 |
| 17 | 17 | #define CAPI_DATA_B3_REQ_LEN (CAPI_MSG_BASELEN+4+4+2+2+2) |
| 18 | 18 | #define CAPI_DATA_B3_RESP_LEN (CAPI_MSG_BASELEN+4+2) |
| 19 | +#define CAPI_DISCONNECT_B3_RESP_LEN (CAPI_MSG_BASELEN+4) | |
| 19 | 20 | |
| 20 | 21 | /*----- CAPI commands -----*/ |
| 21 | 22 | #define CAPI_ALERT 0x01 |