On Mon, Aug 11, 2014 at 5:24 AM, Denys Vlasenko <[email protected]> wrote: > On 08/11/2014 12:42 AM, Andy Lutomirski wrote: >> On Mon, Aug 11, 2014 at 12:00 AM, Denys Vlasenko <[email protected]> wrote: >>> On 08/09/2014 12:59 AM, Andy Lutomirski wrote: >>>>> + * When returning through fast path, userspace sees rcx = return address, >>>>> + * r11 = rflags. When returning through iret (e.g. if audit is active), >>>>> + * these registers may contain garbage. >>>>> + * For ptrace we manage to avoid that: when we hit slow path on entry, >>>>> + * we do save rcx and r11 in pt_regs, so ptrace on exit also sees them. >>>>> + * If slow path is entered only on exit, there will be garbage. >>>> >>>> I don't like this. At least the current code puts something >>>> deterministic in there (-1) for the slow path, even though it's wrong >>>> and makes the slow path behave visibly differently from the fast path. >>>> >>>> Leaking uninitialized data here is extra bad, though. Keep in mind >>>> that, when a syscall entry is interrupted before fully setting up >>>> pt_regs, the interrupt frame overlaps task_pt_regs, so it's possible, >>>> depending on the stack slot ordering, for a kernel secret >>>> (kernel_stack?) to end up somewhere in task_pt_regs. >>> >>> It's easy to fix. We jump off fast path to slow path here: >>> >>> movl TI_flags+THREAD_INFO(%rsp,SIZEOF_PTREGS),%edx >>> andl %edi,%edx >>> jnz sysret_careful >>> >>> This is the only use of "sysret_careful" label. >>> Therefore, there we don't need to think about any other scenarios >>> besides "we are returning from syscall here". >> >> I may be missing something here (on vacation, not really testing >> things, no big monitor, etc), but how is this compatible with things >> like rt_sigreturn? rt_sigreturn is call from the fastpath, via a >> stub, and it returns through int_ret_from_syscall. The C part needs >> to modify all the regs, and those regs need to stick, so fixing up rcx >> and r11 after rt_sigreturn can't work. > > Code at "sysret_careful" label is only reachable > on fast path return. > We don't go down this code path after rt_sigreturn. > stub_rt_sigreturn manually steers into iret code path instead: > > ENTRY(stub_rt_sigreturn) > CFI_STARTPROC > addq $8, %rsp > DEFAULT_FRAME 0 > SAVE_EXTRA_REGS > STORE_IRET_FRAME_CS_SS > call sys_rt_sigreturn > movq %rax,RAX(%rsp) > RESTORE_EXTRA_REGS > jmp int_ret_from_sys_call <==== NOTE THIS > > So, we don't do any rcx/r11 fixups after sys_rt_sigreturn.
Oh, right. rt_sigreturn overwrites all regs, so it doesn't need a fixup in advance. That still leaves fork and everything that calls ptrace_event, though. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
