Messages in this thread | | | Date | Mon, 24 Mar 2014 12:30:08 +0000 | From | Dave Martin <> | Subject | Re: [PATCH 2/2] ARM: mm: keep rodata non-executable |
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On Sun, Mar 23, 2014 at 06:47:36PM +0000, Laura Abbott wrote: > On 2/17/2014 4:34 AM, Dave Martin wrote: > > On Fri, Feb 14, 2014 at 11:11:07AM -0800, Kees Cook wrote: > >> On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin <Dave.Martin@arm.com> wrote: > >>> On Thu, Feb 13, 2014 at 05:04:10PM -0800, Kees Cook wrote: > >>>> Introduce "CONFIG_DEBUG_RODATA" to mostly match the x86 config, though > >>>> the behavior is different: it depends on STRICT_KERNMEM_PERMS, which > >>>> sets rodata read-only (but executable), where as this option additionally > >>>> splits rodata from the kernel text (resulting in potentially more memory > >>>> lost to padding) and sets it non-executable as well. The end result is > >>>> that on builds with CONFIG_DEBUG_RODATA=y (like x86) the rodata with be > >>>> marked purely read-only. > >>> > >>> This triggers an Oops in kexec, because we have a block of code in .text > >>> which is a template for generating baremetal code to relocate the new > >>> kernel, and some literal words are written into it before copying. > >> > >> You're writing into the text area? I would imagine that > >> CONFIG_ARM_KERNMEM_PERMS would break that. However, that's not the > >> right place to be building code -- shouldn't the module area be used > >> for that? > >>
[...] > FWIW, we've hit issues not just with kexec but kprobes as well. The same > problems exist with this series:
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> We had some functions that allowed the text to be temporarily made writable but something > uniform for kexec would be useful as well (our kexec solution has been 'turn it off')
kexec doesn't rely on poking the kernel text: the fact that it does this is just a side-effect of the way it is currently implemented.
I would like to fix it -- it's currently on my todo list, but I consider it non-urgent. depending on !KEXEC seems reasonable for now.
People building a hardened system may choose do disable kexec for other reasons, but that's a separate issue entirely.
kprobes is a different matter: getting it to work with strict permissions is likely to be complex and costly. kprobes is already unavoidably complex and costly, so that is probably better viewed as a kprobes problem to be solved later, rather than a strict permissions problem.
Cheers ---Dave
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