Re: [PATCH v4 00/10] Function Granular KASLR

[Fangrui Song <maskray@google.com>]

On 2020-08-28, Josh Poimboeuf wrote:
>On Fri, Aug 28, 2020 at 12:21:13PM +0200, Miroslav Benes wrote:
>> > Hi there! I was trying to find a super easy way to address this, so I
>> > thought the best thing would be if there were a compiler or linker
>> > switch to just eliminate any duplicate symbols at compile time for
>> > vmlinux. I filed this question on the binutils bugzilla looking to see
>> > if there were existing flags that might do this, but H.J. Lu went ahead
>> > and created a new one "-z unique", that seems to do what we would need
>> > it to do.
>> >
>> > https://sourceware.org/bugzilla/show_bug.cgi?id=26391
>> >
>> > When I use this option, it renames any duplicate symbols with an
>> > extension - for example duplicatefunc.1 or duplicatefunc.2. You could
>> > either match on the full unique name of the specific binary you are
>> > trying to patch, or you match the base name and use the extension to
>> > determine original position. Do you think this solution would work?
>>
>> Yes, I think so (thanks, Joe, for testing!).
>>
>> It looks cleaner to me than the options above, but it may just be a matter
>> of taste. Anyway, I'd go with full name matching, because -z unique-symbol
>> would allow us to remove sympos altogether, which is appealing.
>>
>> > If
>> > so, I can modify livepatch to refuse to patch on duplicated symbols if
>> > CONFIG_FG_KASLR and when this option is merged into the tool chain I
>> > can add it to KBUILD_LDFLAGS when CONFIG_FG_KASLR and livepatching
>> > should work in all cases.
>>
>> Ok.
>>
>> Josh, Petr, would this work for you too?
>
>Sounds good to me.  Kristen, thanks for finding a solution!

(I am not subscribed. I came here via https://sourceware.org/bugzilla/show_bug.cgi?id=26391 (ld -z unique-symbol))

> This works great after randomization because it always receives the
> current address at runtime rather than relying on any kind of
> buildtime address. The issue with with the live-patching code's
> algorithm for resolving duplicate symbol names. If they request a
> symbol by name from the kernel and there are 3 symbols with the same
> name, they use the symbol's position in the built binary image to
> select the correct symbol.

If a.o, b.o and c.o define local symbol 'foo'.
By position, do you mean that

* the live-patching code uses something like (findall("foo")[0], findall("foo")[1], findall("foo")[2]) ?
* shuffling a.o/b.o/c.o will make the returned triple different

Local symbols are not required to be unique. Instead of patching the toolchain,
have you thought about making the live-patching code smarter?
(Depend on the duplicates, such a linker option can increase the link time/binary size considerably
AND I don't know in what other cases such an option will be useful)

For the following example, 

https://sourceware.org/bugzilla/show_bug.cgi?id=26822

   # RUN: split-file %s %t
   # RUN: gcc -c %t/a.s -o %t/a.o
   # RUN: gcc -c %t/b.s -o %t/b.o
   # RUN: gcc -c %t/c.s -o %t/c.o
   # RUN: ld-new %t/a.o %t/b.o %t/c.o -z unique-symbol -o %t.exe
   
   #--- a.s
   a: a.1: a.2: nop
   #--- b.s
   a: nop
   #--- c.s
   a: nop

readelf -Ws output:

Symbol table '.symtab' contains 13 entries:
    Num:    Value          Size Type    Bind   Vis      Ndx Name
      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
      1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS a.o
      2: 0000000000401000     0 NOTYPE  LOCAL  DEFAULT    1 a
      3: 0000000000401000     0 NOTYPE  LOCAL  DEFAULT    1 a.1
      4: 0000000000401000     0 NOTYPE  LOCAL  DEFAULT    1 a.2
      5: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS b.o
      6: 0000000000401001     0 NOTYPE  LOCAL  DEFAULT    1 a.1
      7: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS c.o
      8: 0000000000401002     0 NOTYPE  LOCAL  DEFAULT    1 a.2
      9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _start
     10: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    1 __bss_start
     11: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    1 _edata
     12: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    1 _end

Note that you have STT_FILE SHN_ABS symbols.
If the compiler does not produce them, they will be synthesized by GNU ld.

   https://sourceware.org/bugzilla/show_bug.cgi?id=26822
   ld.bfd copies non-STT_SECTION local symbols from input object files.  If an
   object file does not have STT_FILE symbols (no .file directive) but has
   non-STT_SECTION local symbols, ld.bfd synthesizes a STT_FILE symbol

The filenames are usually base names, so "a.o" and "a.o" in two directories will
be indistinguishable.  The live-patching code can possibly work around this by
not changing the relative order of the two "a.o".