Messages in this thread | | | Date | Thu, 1 Oct 2015 09:09:52 +0200 | From | Ingo Molnar <> | Subject | Re: rwx mapping between ex_table and rodata |
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* Kees Cook <keescook@chromium.org> wrote:
> On Mon, Sep 28, 2015 at 7:11 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote: > > On 09/24/2015 06:25 PM, Kees Cook wrote: > >> On Thu, Sep 24, 2015 at 1:26 PM, Stephen Smalley <sds@tycho.nsa.gov> wrote: > >>> Hi, > >>> > >>> With the attached config and 4.3-rc2 on x86_64, I see the following in /sys/kernel/debug/kernel_page_tables: > >>> ... > >>> ---[ High Kernel Mapping ]--- > >>> 0xffffffff80000000-0xffffffff81000000 16M pmd > >>> 0xffffffff81000000-0xffffffff81600000 6M ro PSE GLB x pmd > >>> 0xffffffff81600000-0xffffffff81775000 1492K ro GLB x pte > >>> 0xffffffff81775000-0xffffffff81800000 556K RW GLB x pte > >>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > >>> 0xffffffff81800000-0xffffffff81a00000 2M ro PSE GLB NX pmd > >>> 0xffffffff81a00000-0xffffffff81b43000 1292K ro GLB NX pte > >>> 0xffffffff81b43000-0xffffffff82000000 4852K RW GLB NX pte > >>> 0xffffffff82000000-0xffffffff82200000 2M RW PSE GLB NX pmd > >>> 0xffffffff82200000-0xffffffffa0000000 478M pmd > >>> ... > >>> > >>> This region seems to be between the end of ex_table and the start of rodata, > >>> $ objdump -x vmlinux | sort > >>> ... > >>> ffffffff817728b0 g __ex_table 0000000000000000 __start___ex_table > >>> ffffffff817728b0 l d __ex_table 0000000000000000 __ex_table > >>> ffffffff81774998 g __ex_table 0000000000000000 __stop___ex_table > >>> ffffffff81800000 g .rodata 0000000000000000 __start_rodata > >>> ffffffff81800000 l d .rodata 0000000000000000 .rodata > >>> ... > >>> > >>> $ readelf -a vmlinux > >>> ... > >>> Section Headers: > >>> [Nr] Name Type Address Offset > >>> Size EntSize Flags Link Info Align > >>> ... > >>> [ 3] __ex_table PROGBITS ffffffff817728b0 009728b0 > >>> 00000000000020e8 0000000000000000 A 0 0 8 > >>> [ 4] .rodata PROGBITS ffffffff81800000 00a00000 > >>> 00000000002eefd2 0000000000000000 A 0 0 64 > >>> ... > >>> > >>> I see a similar rwx mapping with the stock Fedora kernels (e.g. 4.1.6), so it isn't new to 4.3. > >> > >> To me it looks like another alignment/padding issue like got fixed > >> before. The space between __ex_table and rodata is (seems?) unused, so > >> the default page table permissions end up being W+X. Can we fix the > >> default to be NX instead? It'll make these bugs stay gone. > > > > Not sure where that would get fixed (or the ramifications), but is there > > a reason we can't just do the following to fix this particular case? > > > > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c > > index 30564e2..df48430 100644 > > --- a/arch/x86/mm/init_64.c > > +++ b/arch/x86/mm/init_64.c > > @@ -1132,7 +1132,7 @@ void mark_rodata_ro(void) > > * has been zapped already via cleanup_highmem(). > > */ > > all_end = roundup((unsigned long)_brk_end, PMD_SIZE); > > - set_memory_nx(rodata_start, (all_end - rodata_start) >> PAGE_SHIFT); > > + set_memory_nx(text_end, (all_end - text_end) >> PAGE_SHIFT); > > > > rodata_test(); > > > > > > That should work, yeah. I'd still like to find the default permissions > and make them W+nx, though. Regardless, let's get the above added.
Ok, could someone please send a changelogged, signed off patch for this?
Thanks,
Ingo
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