From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- Documentation/input/uinput.rst | 245 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 245 insertions(+) create mode 100644 Documentation/input/uinput.rst (limited to 'Documentation/input/uinput.rst') diff --git a/Documentation/input/uinput.rst b/Documentation/input/uinput.rst new file mode 100644 index 000000000..30fe80e32 --- /dev/null +++ b/Documentation/input/uinput.rst @@ -0,0 +1,245 @@ +============= +uinput module +============= + +Introduction +============ + +uinput is a kernel module that makes it possible to emulate input devices +from userspace. By writing to /dev/uinput (or /dev/input/uinput) device, a +process can create a virtual input device with specific capabilities. Once +this virtual device is created, the process can send events through it, +that will be delivered to userspace and in-kernel consumers. + +Interface +========= + +:: + + linux/uinput.h + +The uinput header defines ioctls to create, set up, and destroy virtual +devices. + +libevdev +======== + +libevdev is a wrapper library for evdev devices that provides interfaces to +create uinput devices and send events. libevdev is less error-prone than +accessing uinput directly, and should be considered for new software. + +For examples and more information about libevdev: +https://www.freedesktop.org/software/libevdev/doc/latest/ + +Examples +======== + +Keyboard events +--------------- + +This first example shows how to create a new virtual device, and how to +send a key event. All default imports and error handlers were removed for +the sake of simplicity. + +.. code-block:: c + + #include + + void emit(int fd, int type, int code, int val) + { + struct input_event ie; + + ie.type = type; + ie.code = code; + ie.value = val; + /* timestamp values below are ignored */ + ie.time.tv_sec = 0; + ie.time.tv_usec = 0; + + write(fd, &ie, sizeof(ie)); + } + + int main(void) + { + struct uinput_setup usetup; + + int fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK); + + + /* + * The ioctls below will enable the device that is about to be + * created, to pass key events, in this case the space key. + */ + ioctl(fd, UI_SET_EVBIT, EV_KEY); + ioctl(fd, UI_SET_KEYBIT, KEY_SPACE); + + memset(&usetup, 0, sizeof(usetup)); + usetup.id.bustype = BUS_USB; + usetup.id.vendor = 0x1234; /* sample vendor */ + usetup.id.product = 0x5678; /* sample product */ + strcpy(usetup.name, "Example device"); + + ioctl(fd, UI_DEV_SETUP, &usetup); + ioctl(fd, UI_DEV_CREATE); + + /* + * On UI_DEV_CREATE the kernel will create the device node for this + * device. We are inserting a pause here so that userspace has time + * to detect, initialize the new device, and can start listening to + * the event, otherwise it will not notice the event we are about + * to send. This pause is only needed in our example code! + */ + sleep(1); + + /* Key press, report the event, send key release, and report again */ + emit(fd, EV_KEY, KEY_SPACE, 1); + emit(fd, EV_SYN, SYN_REPORT, 0); + emit(fd, EV_KEY, KEY_SPACE, 0); + emit(fd, EV_SYN, SYN_REPORT, 0); + + /* + * Give userspace some time to read the events before we destroy the + * device with UI_DEV_DESTROY. + */ + sleep(1); + + ioctl(fd, UI_DEV_DESTROY); + close(fd); + + return 0; + } + +Mouse movements +--------------- + +This example shows how to create a virtual device that behaves like a physical +mouse. + +.. code-block:: c + + #include + + /* emit function is identical to of the first example */ + + int main(void) + { + struct uinput_setup usetup; + int i = 50; + + int fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK); + + /* enable mouse button left and relative events */ + ioctl(fd, UI_SET_EVBIT, EV_KEY); + ioctl(fd, UI_SET_KEYBIT, BTN_LEFT); + + ioctl(fd, UI_SET_EVBIT, EV_REL); + ioctl(fd, UI_SET_RELBIT, REL_X); + ioctl(fd, UI_SET_RELBIT, REL_Y); + + memset(&usetup, 0, sizeof(usetup)); + usetup.id.bustype = BUS_USB; + usetup.id.vendor = 0x1234; /* sample vendor */ + usetup.id.product = 0x5678; /* sample product */ + strcpy(usetup.name, "Example device"); + + ioctl(fd, UI_DEV_SETUP, &usetup); + ioctl(fd, UI_DEV_CREATE); + + /* + * On UI_DEV_CREATE the kernel will create the device node for this + * device. We are inserting a pause here so that userspace has time + * to detect, initialize the new device, and can start listening to + * the event, otherwise it will not notice the event we are about + * to send. This pause is only needed in our example code! + */ + sleep(1); + + /* Move the mouse diagonally, 5 units per axis */ + while (i--) { + emit(fd, EV_REL, REL_X, 5); + emit(fd, EV_REL, REL_Y, 5); + emit(fd, EV_SYN, SYN_REPORT, 0); + usleep(15000); + } + + /* + * Give userspace some time to read the events before we destroy the + * device with UI_DEV_DESTROY. + */ + sleep(1); + + ioctl(fd, UI_DEV_DESTROY); + close(fd); + + return 0; + } + + +uinput old interface +-------------------- + +Before uinput version 5, there wasn't a dedicated ioctl to set up a virtual +device. Programs supporting older versions of uinput interface need to fill +a uinput_user_dev structure and write it to the uinput file descriptor to +configure the new uinput device. New code should not use the old interface +but interact with uinput via ioctl calls, or use libevdev. + +.. code-block:: c + + #include + + /* emit function is identical to of the first example */ + + int main(void) + { + struct uinput_user_dev uud; + int version, rc, fd; + + fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK); + rc = ioctl(fd, UI_GET_VERSION, &version); + + if (rc == 0 && version >= 5) { + /* use UI_DEV_SETUP */ + return 0; + } + + /* + * The ioctls below will enable the device that is about to be + * created, to pass key events, in this case the space key. + */ + ioctl(fd, UI_SET_EVBIT, EV_KEY); + ioctl(fd, UI_SET_KEYBIT, KEY_SPACE); + + memset(&uud, 0, sizeof(uud)); + snprintf(uud.name, UINPUT_MAX_NAME_SIZE, "uinput old interface"); + write(fd, &uud, sizeof(uud)); + + ioctl(fd, UI_DEV_CREATE); + + /* + * On UI_DEV_CREATE the kernel will create the device node for this + * device. We are inserting a pause here so that userspace has time + * to detect, initialize the new device, and can start listening to + * the event, otherwise it will not notice the event we are about + * to send. This pause is only needed in our example code! + */ + sleep(1); + + /* Key press, report the event, send key release, and report again */ + emit(fd, EV_KEY, KEY_SPACE, 1); + emit(fd, EV_SYN, SYN_REPORT, 0); + emit(fd, EV_KEY, KEY_SPACE, 0); + emit(fd, EV_SYN, SYN_REPORT, 0); + + /* + * Give userspace some time to read the events before we destroy the + * device with UI_DEV_DESTROY. + */ + sleep(1); + + ioctl(fd, UI_DEV_DESTROY); + + close(fd); + return 0; + } + -- cgit v1.3.1