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Date: Mon, 28 Sep 2015 11:27:33 -0700 From: Kees Cook <keescook@...omium.org> To: Stephen Smalley <sds@...ho.nsa.gov> Cc: "x86@...nel.org" <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org> Subject: Re: rwx mapping between ex_table and rodata On Mon, Sep 28, 2015 at 7:11 AM, Stephen Smalley <sds@...ho.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@...ho.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. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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