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. > 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? -- 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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