The following reproducer program (must be compiled with -O0 as gcc optimizes out the infinite recursion) crashes the kernel with this panic:

95.388 [#0 sigtest(35:35)]: copy_to_user(0x01300ff0, 0xc7c29cb0, 4) failed at V0x01300ff0
[sigtest(35:35)]: ASSERTION FAILED: result.is_success()
[sigtest(35:35)]: ../../Kernel/Thread.cpp:808 in void Kernel::push_value_on_user_stack(FlatPtr&, FlatPtr)
[sigtest(35:35)]: KERNEL PANIC! :^(
[sigtest(35:35)]: Aborted
[sigtest(35:35)]: at ../../Kernel/Arch/x86/common/CPU.cpp:35 in void abort()
[sigtest(35:35)]: Kernel + 0x007c9726  Kernel::__panic(char const*, unsigned int, char const*) +0x115
[sigtest(35:35)]: Kernel + 0x0099082d  abort.localalias +0x244
[sigtest(35:35)]: Kernel + 0x009905e9  abort.localalias +0x0
[sigtest(35:35)]: Kernel + 0x00937ec6  Kernel::push_value_on_user_stack(unsigned int&, unsigned int) +0x99
[sigtest(35:35)]: Kernel + 0x0093f6cd  .L8448 +0x3f1
[sigtest(35:35)]: Kernel + 0x0093fc99  Kernel::Thread::send_urgent_signal_to_self(unsigned char) +0x95
[sigtest(35:35)]: Kernel + 0x00993157  page_fault_handler +0x690
[sigtest(35:35)]: Kernel + 0x0099090e  page_fault_asm_entry +0x26

reproducer:

#include <signal.h>
#include <unistd.h>

void handler(int);
void handler(int)
{
    write(2, "stack overflow\n", 15);
    _exit(1);
}

unsigned infinite_recursion(unsigned x);
unsigned infinite_recursion(unsigned x) {
    return infinite_recursion(x)+1;
}

int main()
{
    struct sigaction sa = {
        .sa_handler = handler,
        .sa_mask = 0,
        .sa_flags = 0,
    };
    sigfillset(&sa.sa_mask);
    sigaction(SIGSEGV, &sa, 0);
    infinite_recursion(0);
}