On 06/19, Andy Lutomirski wrote:
>
> Something's clearly buggy there,
The usage of __X32_SYSCALL_BIT doesn't look right too. Nothing serious
but still.
Damn, initially I thought I have found the serious bug in entry_64.S
and it took me some time to understand why my exploit doesn't work ;)
So I learned that
andl $__SYSCALL_MASK, %eax
in entry_SYSCALL_64_fastpath() zero-extends %rax and thus
cmpl $__NR_syscall_max, %eax
...
call *sys_call_table(, %rax, 8)
is correct (rax <= __NR_syscall_max).
OK, so entry_64.S simply "ignores" the upper bits if CONFIG_X86_X32_ABI.
Fine, but this doesn't match the
if (likely((nr & __SYSCALL_MASK) < NR_syscalls))
check in do_syscall_64(). So this test-case
#include <stdio.h>
int main(void)
{
// __NR_exit == 0x3c
asm volatile ("movq $0xFFFFFFFF0000003c, %rax; syscall");
printf("I didn't exit because I am traced\n");
return 0;
}
silently exits if not traced, otherwise it calls printf().
Should we do something or we do not care?
Oleg.
