linux-hardening - Re: [PATCH v4 0/6] binfmt_elf: Support segments with 0 filesz and misaligned starts

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Message-ID: <CAKbZUD3dxYqb4RSnXFs9ehWymXe15pt8ra232WAD_msJsBF_BQ@mail.gmail.com>
Date: Fri, 29 Sep 2023 12:58:18 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: Eric Biederman <ebiederm@...ssion.com>, Sebastian Ott <sebott@...hat.com>, 
	Thomas Weißschuh <linux@...ssschuh.net>, 
	Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 0/6] binfmt_elf: Support segments with 0 filesz and
 misaligned starts

On Fri, Sep 29, 2023 at 4:24 AM Kees Cook <keescook@...omium.org> wrote:
>
> Hi,
>
> This is the continuation of the work Eric started for handling
> "p_memsz > p_filesz" in arbitrary segments (rather than just the last,
> BSS, segment). I've added the suggested changes:
>
>  - drop unused "elf_bss" variable
>  - refactor load_elf_interp() to use elf_load()
>  - refactor load_elf_library() to use elf_load()
>  - report padzero() errors when PROT_WRITE is present
>  - drop vm_brk()
>
> Thanks!
>
> -Kees
>
> v4:
>  - refactor load_elf_library() too
>  - don't refactor padzero(), just test in the only remaining caller
>  - drop now-unused vm_brk()
> v3: https://lore.kernel.org/all/20230927033634.make.602-kees@kernel.org
> v2: https://lore.kernel.org/lkml/87sf71f123.fsf@email.froward.int.ebiederm.org
> v1: https://lore.kernel.org/lkml/87jzsemmsd.fsf_-_@email.froward.int.ebiederm.org
>
> Eric W. Biederman (1):
>   binfmt_elf: Support segments with 0 filesz and misaligned starts
>
> Kees Cook (5):
>   binfmt_elf: elf_bss no longer used by load_elf_binary()
>   binfmt_elf: Use elf_load() for interpreter
>   binfmt_elf: Use elf_load() for library
>   binfmt_elf: Only report padzero() errors when PROT_WRITE
>   mm: Remove unused vm_brk()
>
>  fs/binfmt_elf.c    | 214 ++++++++++++++++-----------------------------
>  include/linux/mm.h |   3 +-
>  mm/mmap.c          |   6 --
>  mm/nommu.c         |   5 --
>  4 files changed, 76 insertions(+), 152 deletions(-)

Sorry for taking so long to take a look at this.
While I didn't test PPC64 (I don't own PPC64 hardware, and I wasn't
the original reporter), I did manage to craft a reduced test case of:

a.out:

Program Headers:
 Type           Offset             VirtAddr           PhysAddr
                FileSiz            MemSiz              Flags  Align
 PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                0x00000000000001f8 0x00000000000001f8  R      0x8
 INTERP         0x0000000000000238 0x0000000000000238 0x0000000000000238
                0x0000000000000020 0x0000000000000020  R      0x1
     [Requesting program interpreter: /home/pfalcato/musl/lib/libc.so]
 LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                0x0000000000000428 0x0000000000000428  R      0x1000
 LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
                0x00000000000000cd 0x00000000000000cd  R E    0x1000
 LOAD           0x0000000000002000 0x0000000000002000 0x0000000000002000
                0x0000000000000084 0x0000000000000084  R      0x1000
 LOAD           0x0000000000002e50 0x0000000000003e50 0x0000000000003e50
                0x00000000000001c8 0x00000000000001c8  RW     0x1000
 DYNAMIC        0x0000000000002e50 0x0000000000003e50 0x0000000000003e50
                0x0000000000000180 0x0000000000000180  RW     0x8
 GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                0x0000000000000000 0x0000000000000000  RW     0x10
 GNU_RELRO      0x0000000000002e50 0x0000000000003e50 0x0000000000003e50
                0x00000000000001b0 0x00000000000001b0  R      0x1

/home/pfalcato/musl/lib/libc.so:
Program Headers:
 Type           Offset             VirtAddr           PhysAddr
                FileSiz            MemSiz              Flags  Align
 PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                0x0000000000000230 0x0000000000000230  R      0x8
 LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                0x0000000000049d9c 0x0000000000049d9c  R      0x1000
 LOAD           0x0000000000049da0 0x000000000004ada0 0x000000000004ada0
                0x0000000000057d30 0x0000000000057d30  R E    0x1000
 LOAD           0x00000000000a1ad0 0x00000000000a3ad0 0x00000000000a3ad0
                0x00000000000005f0 0x00000000000015f0  RW     0x1000
 LOAD           0x00000000000a20c0 0x00000000000a60c0 0x00000000000a60c0
                0x0000000000000428 0x0000000000002f80  RW     0x1000
 DYNAMIC        0x00000000000a1f38 0x00000000000a3f38 0x00000000000a3f38
                0x0000000000000110 0x0000000000000110  RW     0x8
 GNU_RELRO      0x00000000000a1ad0 0x00000000000a3ad0 0x00000000000a3ad0
                0x00000000000005f0 0x0000000000002530  R      0x1
 GNU_EH_FRAME   0x0000000000049d10 0x0000000000049d10 0x0000000000049d10
                0x0000000000000024 0x0000000000000024  R      0x4
 GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                0x0000000000000000 0x0000000000000000  RW     0x0
 NOTE           0x0000000000000270 0x0000000000000270 0x0000000000000270
                0x0000000000000018 0x0000000000000018  R      0x4

Section to Segment mapping:
 Segment Sections...
  00
  01     .note.gnu.build-id .dynsym .gnu.hash .hash .dynstr .rela.dyn
.rela.plt .rodata .eh_frame_hdr .eh_frame
  02     .text .plt
  03     .data.rel.ro .dynamic .got .toc
  04     .data .got.plt .bss
  05     .dynamic
  06     .data.rel.ro .dynamic .got .toc
  07     .eh_frame_hdr
  08
  09     .note.gnu.build-id


So on that end, you can take my

Tested-by: Pedro Falcato <pedro.falcato@...il.com>

Although this still doesn't address the other bug I found
(https://github.com/heatd/elf-bug-questionmark), where segments can
accidentally overwrite cleared BSS if we end up in a situation where
e.g we have the following segments:

Program Headers:
 Type           Offset             VirtAddr           PhysAddr
                FileSiz            MemSiz              Flags  Align
 LOAD           0x0000000000001000 0x0000000000400000 0x0000000000400000
                0x0000000000000045 0x0000000000000045  R E    0x1000
 LOAD           0x0000000000002000 0x0000000000401000 0x0000000000401000
                0x000000000000008c 0x000000000000008c  R      0x1000
 LOAD           0x0000000000000000 0x0000000000402000 0x0000000000402000
                0x0000000000000000 0x0000000000000801  RW     0x1000
 LOAD           0x0000000000002801 0x0000000000402801 0x0000000000402801
                0x0000000000000007 0x0000000000000007  RW     0x1000
 NOTE           0x0000000000002068 0x0000000000401068 0x0000000000401068
                0x0000000000000024 0x0000000000000024         0x4

Section to Segment mapping:
 Segment Sections...
  00     .text
  01     .rodata .note.gnu.property .note.gnu.build-id
  02     .bss
  03     .data
  04     .note.gnu.build-id

since the mmap of .data will end up happening over .bss.

-- 
Pedro