Date: Sat, 23 Jan 2021 14:59:28 -0800 From: Fangrui Song <maskray@...gle.com> To: Kristen Carlson Accardi <kristen@...ux.intel.com> Cc: Miroslav Benes <mbenes@...e.cz>, Josh Poimboeuf <jpoimboe@...hat.com>, Kees Cook <keescook@...omium.org>, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, arjan@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com, live-patching@...r.kernel.org, Hongjiu Lu <hongjiu.lu@...el.com>, joe.lawrence@...hat.com Subject: Re: [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. >> > >> > 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"), findall("foo"), findall("foo")) ? * 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".
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