[PATCH] x86/kprobe: Fix crash when probe cs call

[Jinke Han]

When I used eBPF to probe the call instructions within a function,
we encountered a kernel crash.

The ebpf tool probes the 257 offset 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

The scene of kernel crash is as follows:

[73665.737181] BUG: unable to handle page fault for address: 00000000000f41c9
[73665.744253] #PF: supervisor write access in kernel mode
[73665.749643] #PF: error_code(0x0002) - not-present page
[73665.754843] PGD 0 P4D 0
[73665.757390] Oops: 0002 [#1] SMP NOPTI
[73665.761073] CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
[73665.782671] 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, 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.

Cc: stable@xxxxxxxxxx
Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Signed-off-by: Jinke Han <jinkehan@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 4e5f8c1736ec..b50e4a60bdfe 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -511,9 +511,11 @@ NOKPROBE_SYMBOL(kprobe_emulate_ret);
 static void kprobe_emulate_call(struct kprobe *p, struct pt_regs *regs)
 {
 	unsigned long func = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
+	unsigned long ip = func;
 
 	func += p->ainsn.rel32;
-	int3_emulate_call(regs, func);
+	int3_emulate_push(regs, ip);
+	int3_emulate_jmp(regs, func);
 }
 NOKPROBE_SYMBOL(kprobe_emulate_call);
 
-- 
2.34.1