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SubjectRe: [PATCH v4 00/10] Function Granular KASLR
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.
>> >
>> >
>> >
>> > 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 (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,

# 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.
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".

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