x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

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) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 100 insertions, 0 deletions

diff --git a/Documentation/virt/ne_overview.rst b/Documentation/virt/ne_overview.rst
new file mode 100644
index 000000000..74c2f5919
--- /dev/null
+++ b/Documentation/virt/ne_overview.rst
@@ -0,0 +1,100 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+Nitro Enclaves
+==============
+
+Overview
+========
+
+Nitro Enclaves (NE) is a new Amazon Elastic Compute Cloud (EC2) capability
+that allows customers to carve out isolated compute environments within EC2
+instances [1].
+
+For example, an application that processes sensitive data and runs in a VM,
+can be separated from other applications running in the same VM. This
+application then runs in a separate VM than the primary VM, namely an enclave.
+It runs alongside the VM that spawned it. This setup matches low latency
+applications needs.
+
+The current supported architectures for the NE kernel driver, available in the
+upstream Linux kernel, are x86 and ARM64.
+
+The resources that are allocated for the enclave, such as memory and CPUs, are
+carved out of the primary VM. Each enclave is mapped to a process running in the
+primary VM, that communicates with the NE kernel driver via an ioctl interface.
+
+In this sense, there are two components:
+
+1. An enclave abstraction process - a user space process running in the primary
+VM guest that uses the provided ioctl interface of the NE driver to spawn an
+enclave VM (that's 2 below).
+
+There is a NE emulated PCI device exposed to the primary VM. The driver for this
+new PCI device is included in the NE driver.
+
+The ioctl logic is mapped to PCI device commands e.g. the NE_START_ENCLAVE ioctl
+maps to an enclave start PCI command. The PCI device commands are then
+translated into  actions taken on the hypervisor side; that's the Nitro
+hypervisor running on the host where the primary VM is running. The Nitro
+hypervisor is based on core KVM technology.
+
+2. The enclave itself - a VM running on the same host as the primary VM that
+spawned it. Memory and CPUs are carved out of the primary VM and are dedicated
+for the enclave VM. An enclave does not have persistent storage attached.
+
+The memory regions carved out of the primary VM and given to an enclave need to
+be aligned 2 MiB / 1 GiB physically contiguous memory regions (or multiple of
+this size e.g. 8 MiB). The memory can be allocated e.g. by using hugetlbfs from
+user space [2][3][7]. The memory size for an enclave needs to be at least
+64 MiB. The enclave memory and CPUs need to be from the same NUMA node.
+
+An enclave runs on dedicated cores. CPU 0 and its CPU siblings need to remain
+available for the primary VM. A CPU pool has to be set for NE purposes by an
+user with admin capability. See the cpu list section from the kernel
+documentation [4] for how a CPU pool format looks.
+
+An enclave communicates with the primary VM via a local communication channel,
+using virtio-vsock [5]. The primary VM has virtio-pci vsock emulated device,
+while the enclave VM has a virtio-mmio vsock emulated device. The vsock device
+uses eventfd for signaling. The enclave VM sees the usual interfaces - local
+APIC and IOAPIC - to get interrupts from virtio-vsock device. The virtio-mmio
+device is placed in memory below the typical 4 GiB.
+
+The application that runs in the enclave needs to be packaged in an enclave
+image together with the OS ( e.g. kernel, ramdisk, init ) that will run in the
+enclave VM. The enclave VM has its own kernel and follows the standard Linux
+boot protocol [6][8].
+
+The kernel bzImage, the kernel command line, the ramdisk(s) are part of the
+Enclave Image Format (EIF); plus an EIF header including metadata such as magic
+number, eif version, image size and CRC.
+
+Hash values are computed for the entire enclave image (EIF), the kernel and
+ramdisk(s). That's used, for example, to check that the enclave image that is
+loaded in the enclave VM is the one that was intended to be run.
+
+These crypto measurements are included in a signed attestation document
+generated by the Nitro Hypervisor and further used to prove the identity of the
+enclave; KMS is an example of service that NE is integrated with and that checks
+the attestation doc.
+
+The enclave image (EIF) is loaded in the enclave memory at offset 8 MiB. The
+init process in the enclave connects to the vsock CID of the primary VM and a
+predefined port - 9000 - to send a heartbeat value - 0xb7. This mechanism is
+used to check in the primary VM that the enclave has booted. The CID of the
+primary VM is 3.
+
+If the enclave VM crashes or gracefully exits, an interrupt event is received by
+the NE driver. This event is sent further to the user space enclave process
+running in the primary VM via a poll notification mechanism. Then the user space
+enclave process can exit.
+
+[1] https://aws.amazon.com/ec2/nitro/nitro-enclaves/
+[2] https://www.kernel.org/doc/html/latest/admin-guide/mm/hugetlbpage.html
+[3] https://lwn.net/Articles/807108/
+[4] https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
+[5] https://man7.org/linux/man-pages/man7/vsock.7.html
+[6] https://www.kernel.org/doc/html/latest/x86/boot.html
+[7] https://www.kernel.org/doc/html/latest/arm64/hugetlbpage.html
+[8] https://www.kernel.org/doc/html/latest/arm64/booting.html