Messages in this thread | | | Subject | Re: [PATCH 2/2] ARM: mm: keep rodata non-executable | From | "Jon Medhurst (Tixy)" <> | Date | Mon, 24 Mar 2014 10:47:44 +0000 |
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On Sun, 2014-03-23 at 16:21 -0600, Kees Cook wrote: > On Sun, Mar 23, 2014 at 12:47 PM, Laura Abbott <lauraa@codeaurora.org> 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? > >>> > >>>> Possibly this should be in .rodata, not .text. > >>> > >>> Well, rodata should be neither writable nor executable. > >> > >> We're not writing into code exactly. > >> > >> This code is never executed in-place in vmlinux. It gets copied, and > >> only copies are ever executed. > >> > >> Some pointers and offsets get poked into the code to configure it. > >> > >> I think it would be better simply to put the code in .rodata, and > >> poke paramaters into the copy, not the original -- but that's a bit > >> more awkward to code up, since the values can't be poked simply by > >> writing global variables. > >> > >>> > >>>> There may be a few other instances of this kind of thing. > >>> > >>> This config will certainly find them! :) But, that's why it's behind a config. > >> > >> I haven't tested exhaustively, but it think this is sufficient for a > >> Tested-by. The patch does seem to be doing what it is intended to > >> do, and doesn't seem to be triggering false positives all over the > >> place. > >> > >>> > >>>> Are you aware of similar situations on other arches? > >>> > >>> I think there were some problems a long time ago on x86 for rodata too. > >> > >> It would be good to get this kexec case fixed -- I'll try to hack up > >> a separate patch. > >> > > > > FWIW, we've hit issues not just with kexec but kprobes as well. The same > > problems exist with this series: > > For this stage, how about I make this "depends on KEXEC=n && > KPROBES=n"?
There's also ftrace (CONFIG_DYNAMIC_FTRACE I believe) which modifies kernel code with a call to probe_kernel_write(), which GDB uses as well.
And grepping for the patch_text() function also shows __arch_jump_label_transform() modifies kernel code. Not sure how and when that gets used.
-- Tixy
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