On Fri, 19 Jun 2020 at 01:32, Thomas Gleixner tglx@linutronix.de wrote:
Cyril Hrubis chrubis@suse.cz writes:
What is does is to write:
(void*)1 to u_debugreg[0] (void*)1 to u_debugreg[7] do_debug addr to u_debugreg[0]Looking at the kernel code the write to register 7 enables the breakpoints and what we attempt here is to change an invalid address to a valid one after we enabled the breakpoint but that's as far I can go.
So does anyone has an idea how to trigger the bug without the do_debug function address? Would any valid kernel function address suffice?
According to https://www.openwall.com/lists/oss-security/2018/05/01/3 the trigger is to set the breakpoint to do_debug() and then execute INT1, aka. ICEBP which ends up in do_debug() ....
In principle each kernel address is ok, but do_debug() is interesting due to the recursion issue because user space can reach it by executing INT1.
So you might check for exc_debug() if do_debug() is not available and make the whole thing fail gracefully with a usefu error message.
My two cents, LTP test case ptrace08 fails on x86_64 and i386.
ptrace08.c:62: BROK: Cannot find address of kernel symbol "do_debug"
This error is coming from test case setup KERNEL_SYM = do_debug
if (strcmp(symname, KERNEL_SYM)) tst_brk(TBROK, "Cannot find address of kernel symbol "%s"", KERNEL_SYM);
Test case got pass when DEBUG_INFO config enabled
CONFIG_DEBUG_INFO=y
ptrace08.c:68: INFO: Kernel symbol "do_debug" found at 0xd8898410
Full test log, https://lkft.validation.linaro.org/scheduler/job/1483117#L1325