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

[Fangrui Song]

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".