On Tue, Mar 10, 2009 at 4:13 PM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > Sorry, my mistake, PAE is required yes, and then 32bit Linux Kernel > will have NX enabled: > > PAE can be enabled with CONFIG_X86_PAE (and CONFIG_HIGHMEM64G - > possibly needed, which is what the kernel config file for Fedora Core > 11 has): > > In arch/x86/mm/init_32.c: > > #ifdef CONFIG_X86_PAE > set_nx(); > if (nx_enabled) > printk(KERN_INFO "NX (Execute Disable) protection: active\n"); > #endif That is indeed what happens in the kernel code. However, now I really have some doubts now after reading the Intel manual 3A. According to 3.8.5, PAE mode in x86 reserves all the bits from 36-63 to 0. Knowing that bit 63 is for NX, this means NX bit is never on, so no page can be set with NX bit. As a result, all the pages in x86 cannot prohibit execution. Meanwhile, 3.10.3 clearly mentions NX bit can be turned on in x86-64 (IA32e in Intel term). So this means NX is really only possible in 64bit OS??? But then why Linux 32 turns on NX? Could anybody confirm this confusion? Thanks, H > On Tue, Mar 10, 2009 at 12:23 PM, NAHieu <nahieu@xxxxxxxxx> wrote: >> On Mon, Mar 9, 2009 at 11:50 PM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: >>> as far as I can remember, in x86 architecture, hardware-wise, it is >>> NOT possible to enable NX. U may do anything via software, but it >>> will not be enabled. NX feature is only for 64bit OS. >>> >> >> No, NX is available for 32bit Linux, as long as PAE is enable. >> >> I am still stuck here (on 32bit Linux). It seems nobody can shed some >> lights in this problem? >> >> Thanks, >> H >> >> >>> On Mon, Mar 9, 2009 at 4:27 AM, NAHieu <nahieu@xxxxxxxxx> wrote: >>>> Hi, >>>> >>>> I inspect my Linux memory, and it seems that there is no area that >>>> prohibite execution like I expected (using NX bit in modern CPU). That >>>> really surprises me. >>>> >>>> I looked at some potential data areas exported in System.map file, like: >>>> >>>> - mark_rodata_ro >>>> - sysctl_data >>>> - new_cpu_data >>>> - boot_cpu_data >>>> >>>> And all of these areas allow to execute code (because NX=0 there). Is >>>> that really desirable? >>>> >>>> Anybody know for sure which area (easier to check if exported in >>>> System.map) doesnt allow execute? >>>> >>>> I can confirm that NX is active in my machine (reported in dmesg) >>>> >> > > > > -- > Regards, > Peter Teoh > -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
