[Xen-devel] [PATCH v8 00/23] XSA55 libelf fixes for unstable

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

This is version 8 of my series to try to fix libelf and the domain
loader.  I hope this will be the final version.

Barring further showstoppers, and in the absence of objections, I
intend to push this to xen.git#staging tomorrow, and include it in a
revised version of the XSA-55 advisory.

I think further problems with this code should be treated, as far as
feasible, as new problems.  In particular, further security bugs
should be treated as new security bugs and go only to
security@xxxxxxx, and be handled with a predisclosure period.


This is available via git:
  http://xenbits.xen.org/gitweb/?p=people/iwj/xen-unstable.git;a=summary
  git://xenbits.xen.org/people/iwj/xen-unstable.git
in the commits
  xsa55-unstable-base-rebasing..xsa55-unstable-rebasing

Here is a summary of the state of series:

A  01 libelf: abolish libelf-relocate.c
A  02 libxc: introduce xc_dom_seg_to_ptr_pages
 a 03 libxc: Fix range checking in xc_dom_pfn_to_ptr etc.
A  04 libelf: add `struct elf_binary*' parameter to elf_load_image
A  05 libelf: abolish elf_sval and elf_access_signed
A  06 libelf: move include of <asm/guest_access.h> to top of file
A  07 libelf/xc_dom_load_elf_symtab: Do not use "syms" uninitialised
A  08 libxl: introduce macros for memory access and pointer handling
A  09 tools/xcutils/readnotes: adjust print_l1_mfn_valid_note
A  10 libelf: check nul-terminated strings properly
oa 11 libxl: check all pointer accesses
A  12 libxl: Check pointer references in elf_is_elfbinary
A  13 libelf: Make all callers call elf_check_broken
A  14 libelf: use C99 bool for booleans
A -15 libelf: use only unsigned integers
 a*16 libelf: check loops for running away
A  17 libelf: abolish obsolete macros
 a 18 libxc: Add range checking to xc_dom_binloader
 a/19 libxc: check failure of xc_dom_*_to_ptr, xc_map_foreign_range
 a/20 libxc: check return values from malloc
A  21 libxc: range checks in xc_dom_p2m_host and _guest
A -22 libxc: check blob size before proceeding in xc_dom_check_gzip
 a+23  libxc: Better range check in xc_dom_alloc_segment

Key to symbols:
 *   Updated in this version of the series.
 +   New patch in this version.
 /   Updated but only to move changes between patches.
 -   Updated with style, comment or commit message changes only.
 a   Acked/reviwed by one reviewer.
 o   Older version also acked (by other reviewers if applicable).
 A   Acked/reviwed by more than one reviewer.
Also in every patch:
Updated commit msgs to correct email address for me.


libelf, and some of its callers, did not do nearly enough checking on
their input.  Invalid inputs could cause signed integer arithmetic
overflows and wild pointer dereferences.

In this series we try to systematically eliminate this problem in a
way which has a reasonable chance of (i) still accepting all
previously-accepted ELF images (ii) not having remaining security
bugs, in a form which can be reviewied to verify (i) and (ii).

Additionally, we fix some related security bugs in the domain loading
code.

The approach is:

(i) Remove all uses of signed integers (of any kind).  That
    elmininates all integer overflows as sources of undefined
    behaviour.  Of course it still means that we can get incorrect
    values throughout the code.

(ii) Replace all uses of pointers, both pointers into the supplied
    ELF image, and pointers into the output (where we are loading)
    by uintptr_t.  That eliminates all pointer arithmetic overflows as
    sources of undefined behaviour.  Of course it still means that we
    can get incorrect and unreasonable "pointer" values.

(iii) But these pointer values will be in uintptr_t, which cannot be
    simply dereferenced by mistake.  We will replace all dereferences
    by macros which do range checking; out of range reads will read 0
    and out of range writes will be ignored.  Happily most (but not
    all) of the reads in the code already go through macros which
    abstract endianness[1] and/or 32/64bitness.

    [1] Although not all the accesses use endian-aware techniques so
    in fact the code can't cope with foreign-endian ELFs.  This is a
    problem for another day.

(iv) Look for all loops and check that they are guaranteed to
    terminate.

To enable verification of correctness of these changes I provide them
as a series roughly as follows:

1-6:
   Pre-patches which make a few semantically neutral or semantically
   correct changes.  For human review.

7: Introduces a set of macros to abstract away pointer arithmetic and
   input and output image memory accesses in libelf.  Use these macros
   everwhere they are applicable.  However, define the macros in a way
   that corresponds to the existing code.  That this patch has no
   functional change can be verified by comparing the before-and-after
   assembler output from the compiler.

9. Introduce some macros for dealing with nul-terminated strings,
   defined so as not to have any functional change at this stage.

10. Change the macro definitions, and introduce the new pseudopointer
   types, pointer range checking, etc.  For close human review.  Each
   macro change is justified in the commit message.  This patch
   eliminates most of the potential wild pointer accesses.

8,11-12,14-15:
   Smaller patches for human review, fixing some leftover bugs,
   including ensuring that all loops terminate.

13: Eliminate signed integers.  Replace every "int", "int*_t",
   "long" and most "char"s by corresponding unsigned types.  This
    eliminates all integer arithmetic overflows.

After this patch, libelf should be safe against hostile input:

 * All arithmetic operations on values from the input file use
   unsigned arithmetic which is guaranteed to be defined (although
   it may of course result in wrong answers);

 * All pointer accesses based on pointers to locations which depend on
   the input file go via our range-checking accessors; accesses which
   are not to the input or output regions are ignored (reads returning
   0).

 * The loops have been checked to ensure that they terminate and are at
   worst O(image_size).

 * Whenever an array variable was declared, the code has been manually
   reviewed looking for possible out-of-bounds accesses.

This is XSA-55.

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