On 10/02/2013 12:50 AM, James Bottomley wrote: > On Wed, 2013-10-02 at 00:07 +0200, Helge Deller wrote: >> On 10/01/2013 11:40 PM, James Bottomley wrote: >>> On Tue, 2013-10-01 at 16:43 -0400, Tejun Heo wrote: >>>> Hello, >>>> >>>> On Tue, Oct 01, 2013 at 10:35:20PM +0200, Helge Deller wrote: >>>>> print_worker_info() includes no validity check on the pwq and wq >>>>> pointers before handing them over to the probe_kernel_read() functions. >>>>> >>>>> It seems that most architectures don't care about that, but at least on >>>>> the parisc architecture this leads to a kernel crash since accesses to >>>>> page zero are protected by the kernel for security reasons. >>>>> >>>>> Fix this problem by verifying the contents of pwq and wq before usage. >>>>> Even if probe_kernel_read() usually prevents such crashes by disabling >>>>> page faults, clean code should always include such checks. >>>>> >>>>> Without this fix issuing "echo t > /proc/sysrq-trigger" will immediately >>>>> crash the Linux kernel on the parisc architecture. >>>> >>>> Hmm... um had similar problem but the root cause here is that the arch >>>> isn't implementing probe_kernel_read() properly. We really have no >>>> idea what the pointer value may be at the dump point and that's why we >>>> use probe_kernel_read(). If something like the above is necessary for >>>> the time being, the correct place would be the arch >>>> probe_kernel_read() implementation. James, would it be difficult >>>> implement proper probe_kernel_read() on parisc? >>> >>> The problem seems to be that some traps bypass our exception table >>> handling. >> >> Yes, that's correct. >> It's trap #26 and we directly call parisc_terminate() for fault_space==0 >> without checking the exception table. >> See my patch I posted a few hours ago which fixes this: >> https://patchwork.kernel.org/patch/2971701/ > > That doesn't quite look right ... I guessed it was probably access > rights, so we should do an exception table fixup, so isn't this the fix? > because we shouldn't call do_page_fault if there's no exception table. > > diff --git a/arch/parisc/kernel/traps.c b/arch/parisc/kernel/traps.c > index 04e47c6..25a088a 100644 > --- a/arch/parisc/kernel/traps.c > +++ b/arch/parisc/kernel/traps.c > @@ -684,6 +684,8 @@ void notrace handle_interruption(int code, struct pt_regs *regs) > /* Fall Through */ > case 26: > /* PCXL: Data memory access rights trap */ > + if (!user_mode(regs) && fixup_exception(regs)) > + return; You need to check for preempt_count()!=0 too, which has been increased by pagefault_disable() inside of probe_kernel_read(). Otherwise every simple memcpy(dest,NULL,count) (*) will sucessfully be handled here and we won't trap on generic invalid memory accesses inside the kernel. But basically your patch does exactly the same as mine. Helge (*) memcpy() uses internally pa_memcpy() which defines the fixup tables. -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
