[PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

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[PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

Bhupesh Sharma
Since 7e60d1b427c51cf2525e5d736a71780978cfb828, the ELF_ET_DYN_BASE
for powerpc applications has been set to 512MB.

Recently there have been several reports of applications SEGV'ing
and newer version of glibc also SEGV'ing (while testing) when using the following
test method:

    LD_LIBRARY_PATH=/XXX/lib /XXX/lib/ld-2.24.so <binary>

For reproducing the above, consider the following test application which
uses a larger bss:

    1.
    # cat large-bss-test-app.c
 
    #include <stdio.h>
    #include <unistd.h>
    #define VECSIZE (1024 * 1024 * 100)
   
    float p[VECSIZE], v1[VECSIZE], v2[VECSIZE];
   
    void vec_mult(long int N) {
    long int i;
    for (i = 0; i < N; i++)
    p[i] = v1[i] * v2[i];
    }
   
    int main() {
        char command[1024];
    sprintf(command,"cat /proc/%d/maps",getpid());
    system(command);
   
        vec_mult(VECSIZE/100);
        printf ("Done\n");
    return 0;
    }
   
    2. Compile it using gcc (I am using gcc-6.3.1):
    # gcc -g -o large-bss-test-app large-bss-test-app.c
   
    3. Running the compiled application with ld.so directly is enough to trigger the SEGV
       on ppc64le/ppc64:
    # /lib64/ld-2.24.so ./large-bss-test-app
    Segmentation fault (core dumped)
   
    4. Notice it's random depending on the layout changes, so it passes on some occassions as well:
    # /lib64/ld-2.24.so ./large-bss-test-app
    10000000-10010000 r-xp 00000000 fd:00 2883597                            /root/large-bss-test-app
    10010000-10020000 r--p 00000000 fd:00 2883597                            /root/large-bss-test-app
    10020000-10030000 rw-p 00010000 fd:00 2883597                            /root/large-bss-test-app
    10030000-5b030000 rw-p 00000000 00:00 0
    5e950000-5e990000 r-xp 00000000 fd:00 1180301                            /usr/lib64/ld-2.24.so
    5e990000-5e9a0000 r--p 00030000 fd:00 1180301                            /usr/lib64/ld-2.24.so
    5e9a0000-5e9b0000 rw-p 00040000 fd:00 1180301                            /usr/lib64/ld-2.24.so
    3fffa3680000-3fffa3860000 r-xp 00000000 fd:00 1180308                    /usr/lib64/libc-2.24.so
    3fffa3860000-3fffa3870000 r--p 001d0000 fd:00 1180308                    /usr/lib64/libc-2.24.so
    3fffa3870000-3fffa3880000 rw-p 001e0000 fd:00 1180308                    /usr/lib64/libc-2.24.so
    3fffa3890000-3fffa38b0000 r-xp 00000000 00:00 0                          [vdso]
    3fffc6740000-3fffc6770000 rw-p 00000000 00:00 0                          [stack]
    Done

One way to fix this is to move ELF_ET_DYN_BASE from 0x20000000 (512MB) to 0x10000000000 (1TB),
atleast for 64-bit applications. This allows hopefully enough space for most of the
applications without causing them to trample upon the ld.so, leading to a SEGV.

ELF_ET_DYN_BASE is still kept as 0x20000000 (512MB) for 32-bit applications to preserve their
compatibility.

After this change, the layout for the 'large-bss-test-app' changes as shown below:
    # /lib64/ld-2.24.so ./large-bss-test-app
    10000000-10010000 r-xp 00000000 fd:00 2107527                            /root/large-bss-test-app
    10010000-10020000 r--p 00000000 fd:00 2107527                            /root/large-bss-test-app
    10020000-10030000 rw-p 00010000 fd:00 2107527                            /root/large-bss-test-app
    10030000-5b030000 rw-p 00000000 00:00 0
    100283b0000-100283f0000 r-xp 00000000 fd:00 1835645                      /usr/lib64/ld-2.24.so
    100283f0000-10028400000 r--p 00030000 fd:00 1835645                      /usr/lib64/ld-2.24.so
    10028400000-10028410000 rw-p 00040000 fd:00 1835645                      /usr/lib64/ld-2.24.so
    7fff8a470000-7fff8a650000 r-xp 00000000 fd:00 1835652                    /usr/lib64/libc-2.24.so
    7fff8a650000-7fff8a660000 r--p 001d0000 fd:00 1835652                    /usr/lib64/libc-2.24.so
    7fff8a660000-7fff8a670000 rw-p 001e0000 fd:00 1835652                    /usr/lib64/libc-2.24.so
    7fff8a680000-7fff8a6a0000 r-xp 00000000 00:00 0                          [vdso]
    7fffc6d90000-7fffc6dc0000 rw-p 00000000 00:00 0                          [stack]
    Done

Cc: Anton Blanchard <[hidden email]>
Cc: Daniel Cashman <[hidden email]>
Cc: Kees Cook <[hidden email]>
Cc: Michael Ellerman <[hidden email]>
Cc: Benjamin Herrenschmidt <[hidden email]>
Signed-off-by: Bhupesh Sharma <[hidden email]>
---
 arch/powerpc/include/asm/elf.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/elf.h b/arch/powerpc/include/asm/elf.h
index 09bde6e..683230c 100644
--- a/arch/powerpc/include/asm/elf.h
+++ b/arch/powerpc/include/asm/elf.h
@@ -28,7 +28,9 @@
    the loader.  We need to make sure that it is out of the way of the program
    that it will "exec", and that there is sufficient room for the brk.  */
 
-#define ELF_ET_DYN_BASE 0x20000000
+/* Keep this as 512MB for 32-bit applications and 1TB for 64-bit ones */
+#define ELF_ET_DYN_BASE (test_thread_flag(TIF_32BIT) ? \
+ 0x20000000 : 0x10000000000)
 
 #define ELF_CORE_EFLAGS (is_elf2_task() ? 2 : 0)
 
--
2.7.4

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Re: [kernel-hardening] [PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

Daniel Micay
Rather than doing this, the base should just be split for an ELF
interpreter like PaX. It makes sense for a standalone executable to be
as low in the address space as possible. Doing that ASAP fixes issues
like this and opens up the possibility of fixing stack mapping ASLR
entropy on various architectures. It should be a pretty small change.
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Re: [kernel-hardening] [PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

Michael Ellerman-2
Daniel Micay <[hidden email]> writes:

> Rather than doing this, the base should just be split for an ELF
> interpreter like PaX.

I don't quite parse that, I think you mean PaX uses a different base for
an ELF interpreter vs a regular ET_DYN?

That would be cool. How do you know that it's an ELF interpreter you're
loading? Is it just something that's PIE but doesn't request an
interpreter?

Is the PaX code somewhere I can look at?

> It makes sense for a standalone executable to be as low in the address
> space as possible.

More or less. There are performance reasons why 1T could be good for us,
but I want to see some performance numbers to justify that change. And
it does mean you have a bit less address space to play with.

cheers
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Re: [kernel-hardening] [PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

Bhupesh SHARMA-2
On Wed, Jun 7, 2017 at 2:59 PM, Michael Ellerman <[hidden email]> wrote:
> Daniel Micay <[hidden email]> writes:
>
>> Rather than doing this, the base should just be split for an ELF
>> interpreter like PaX.
>
> I don't quite parse that, I think you mean PaX uses a different base for
> an ELF interpreter vs a regular ET_DYN?

I am also not very conversant with PaX. AFAIU, we can use the
following methods to print the shared object dependencies instead of
ldd:

1. One can load the binary directly with LD_TRACE_LOADED_OBJECTS=1.

So, instead of:

# /lib64/ld-2.24.so ./large-bss-test-app
Segmentation fault (core dumped)

One can do:
# LD_TRACE_LOADED_OBJECTS=1 ./large-bss-test-app
        linux-vdso64.so.1 (0x00007fffa67a0000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fffa6590000)
        /lib64/ld64.so.2 (0x00007fffa67c0000)

2. There are other utils like pax-utils etc that we can use.

But, we generally cannot force a user to not use ldd to determine the
shared object dependencies, especially when all the documentation
points to it and it works well on the other archs like x86 and arm64.

> That would be cool. How do you know that it's an ELF interpreter you're
> loading? Is it just something that's PIE but doesn't request an
> interpreter?
>
> Is the PaX code somewhere I can look at?
>
>> It makes sense for a standalone executable to be as low in the address
>> space as possible.
>
> More or less. There are performance reasons why 1T could be good for us,
> but I want to see some performance numbers to justify that change. And
> it does mean you have a bit less address space to play with.

Do you have any specific performance test(s) in mind which I can run
to see how the 1TB impacts them? I am trying to run ltp after this
change and will be able to share the results shortly, but I am not
sure it provides the right data to validate such a change.

Regards,
Bhupesh
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Re: [kernel-hardening] [PATCH] powerpc: Increase ELF_ET_DYN_BASE to 1TB for 64-bit applications

Kees Cook
In reply to this post by Michael Ellerman-2
On Wed, Jun 7, 2017 at 2:29 AM, Michael Ellerman <[hidden email]> wrote:

> Daniel Micay <[hidden email]> writes:
>
>> Rather than doing this, the base should just be split for an ELF
>> interpreter like PaX.
>
> I don't quite parse that, I think you mean PaX uses a different base for
> an ELF interpreter vs a regular ET_DYN?
>
> That would be cool. How do you know that it's an ELF interpreter you're
> loading? Is it just something that's PIE but doesn't request an
> interpreter?

I talk a bit about the situation here:
http://www.openwall.com/lists/kernel-hardening/2017/06/03/6

> Is the PaX code somewhere I can look at?

It's near here:
https://github.com/linux-scraping/linux-grsecurity/blob/grsec-test/fs/binfmt_elf.c#L1362

(Note the "&& elf_interpreter" part.) It's replacing the
arch_rnd_mmap() result with its own under some situations, etc.

To do something like this in upstream, we need to be sure we've sanely
dealt with the brk region, which follows the first loaded ELF, and if
it's the interpreter, that means brk ends up in mmap area, up near the
executable area (near what would become the misnomer of
ELF_ET_DYN_BASE -- should be ELF_PIE_BASE).

-Kees

--
Kees Cook
Pixel Security