On Tue, Jul 19, 2016 at 1:14 PM, Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: > On 07/19/2016 09:31 PM, Kees Cook wrote: >> On Tue, Jul 19, 2016 at 2:21 AM, Christian Borntraeger >> <borntraeger@xxxxxxxxxx> wrote: >>> On 07/15/2016 11:44 PM, Kees Cook wrote: >>>> +config HAVE_ARCH_LINEAR_KERNEL_MAPPING >>>> + bool >>>> + help >>>> + An architecture should select this if it has a secondary linear >>>> + mapping of the kernel text. This is used to verify that kernel >>>> + text exposures are not visible under CONFIG_HARDENED_USERCOPY. >>> >>> I have trouble parsing this. (What does secondary linear mapping mean?) >> >> I likely need help clarifying this language... >> >>> So let me give an example below >>> >>>> + >>> [...] >>>> +/* Is this address range in the kernel text area? */ >>>> +static inline const char *check_kernel_text_object(const void *ptr, >>>> + unsigned long n) >>>> +{ >>>> + unsigned long textlow = (unsigned long)_stext; >>>> + unsigned long texthigh = (unsigned long)_etext; >>>> + >>>> + if (overlaps(ptr, n, textlow, texthigh)) >>>> + return "<kernel text>"; >>>> + >>>> +#ifdef HAVE_ARCH_LINEAR_KERNEL_MAPPING >>>> + /* Check against linear mapping as well. */ >>>> + if (overlaps(ptr, n, (unsigned long)__va(__pa(textlow)), >>>> + (unsigned long)__va(__pa(texthigh)))) >>>> + return "<linear kernel text>"; >>>> +#endif >>>> + >>>> + return NULL; >>>> +} >>> >>> s390 has an address space for user (primary address space from 0..4TB/8PB) and a separate >>> address space (home space from 0..4TB/8PB) for the kernel. In this home space the kernel >>> mapping is virtual containing the physical memory as well as vmalloc memory (creating aliases >>> into the physical one). The kernel text is mapped from _stext to _etext in this mapping. >>> So I assume this would qualify for HAVE_ARCH_LINEAR_KERNEL_MAPPING ? >> >> If I understand your example, yes. In the home space you have two >> addresses that reference the kernel image? > > No, there is only one address that points to the kernel. > As we have no kernel ASLR yet, and the kernel mapping is > a 1:1 mapping from 0 to memory end and the kernel is only > from _stext to _etext. The vmalloc area contains modules > and vmalloc but not a 2nd kernel mapping. > > But thanks for your example, now I understood. If we have only > one address >>>> + if (overlaps(ptr, n, textlow, texthigh)) >>>> + return "<kernel text>"; > > This is just enough. > > So what about for the CONFIG text: > > An architecture should select this if the kernel mapping has a secondary > linear mapping of the kernel text - in other words more than one virtual > kernel address that points to the kernel image. This is used to verify > that kernel text exposures are not visible under CONFIG_HARDENED_USERCOPY. Sounds good, I've adjusted it for now. >> I wonder if I can avoid the CONFIG entirely if I just did a >> __va(__pa(_stext)) != _stext test... would that break anyone? > > Can this be resolved on all platforms at compile time? Well, I think it still needs a runtime check (compile-time may not be able to tell about kaslr, or who knows what else). I would really like to avoid the CONFIG if possible, though. Would this do the right thing on s390? This appears to work where I'm able to test it (32/64 x86, 32/64 arm): unsigned long textlow = (unsigned long)_stext; unsigned long texthigh = (unsigned long)_etext; unsigned long textlow_linear = (unsigned long)__va(__pa(textlow); unsigned long texthigh_linear = (unsigned long)__va(__pa(texthigh); if (overlaps(ptr, n, textlow, texthigh)) return "<kernel text>"; /* Check against possible secondary linear mapping as well. */ if (textlow != textlow_linear && overlaps(ptr, n, textlow_linear, texthigh_linear)) return "<linear kernel text>"; return NULL; -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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