Linux + Clang + execstack
Magnus Ihse Bursie
magnus.ihse.bursie at oracle.com
Wed Sep 5 06:53:02 UTC 2018
On 2018-09-05 04:06, Martin Buchholz wrote:
> Here's Arthur's patch:
>
> 8205457: gcc and clang should use the same ld flags
> http://cr.openjdk.java.net/~martin/webrevs/jdk/noexecstack/
> <http://cr.openjdk.java.net/%7Emartin/webrevs/jdk/noexecstack/>
> https://bugs.openjdk.java.net/browse/JDK-8205457
>
> This applies -Wl,-z,noexecstack uniformly to all linker invocations
> where applicable.
>
> TODO:
> All ld flags on Linux should be treated equally by gcc and clang
Just FYI: I'm on a long-term mission to clean up the flag handling in
the entire build. There's a lot of weird stuff going on with the flags,
like "why do we do like this on this platform, and like that on that
platform? shouldn't they be the same?". And most of the time, yes, they
probably should be the same. But at other times, there's a reason they
are different and the difference is a deliberate bug fix. It takes time,
effort and a lot of repository archeology to figure that out.
/Magnus
> The test TestCheckJDK and supporting infrastructure should stop
> advertising itself as only dealing with libraries.
> Maybe add GNU-stack annotations to all the Linux .s files as well?
>
>
> On Tue, Sep 4, 2018 at 4:01 PM, Martin Buchholz <martinrb at google.com
> <mailto:martinrb at google.com>> wrote:
>
> I think we can all agree that passing flags to the linker to
> ensure non-executable stack is the right thing to do. But there's
> a question whether *also* adding something to our assembly
> language source files will be worth doing. Neither mechanism is
> sure to work. For the linker flag, we need to be aware of and
> test for the presence of the linker flag, but we might be using
> some other linker... Similarly, we might end up using some other
> assembler, or we might need to mark the assembly source file in a
> different way than "GNU-stack".
>
> On Tue, Aug 21, 2018 at 4:14 AM, Magnus Ihse Bursie
> <magnus.ihse.bursie at oracle.com
> <mailto:magnus.ihse.bursie at oracle.com>> wrote:
>
> On 2018-08-21 02:03, David Holmes wrote:
>
> On 21/08/2018 9:39 AM, Arthur Eubanks wrote:
>
> On Mon, Aug 20, 2018 at 4:18 PM David Holmes
> <david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>>> wrote:
>
> Hi Arthur,
>
> cc'ing build-dev as this is currently a build issue.
>
> On 21/08/2018 3:11 AM, Arthur Eubanks wrote:
> > Hi,
> >
> > At Google we're trying to build hotspot on
> Linux with clang. One
> thing that
> > happens is that the resulting libjvm.so is
> stack executable. When
> running
> > hotspot we get warnings about the stack being
> executable.
> >
> > Compiling an assembly file into the final .so
> results in the
> stack being
> > executable. In this case the file is
> linux_x86_64.s. This doesn't
> happen
> > with gcc because "-Wl,-z,noexecstack" is passed
> as a hotspot
> linker flag
> > with gcc in flags-ldflags.m4. When using clang
> that linker flag isn't
> > passed.
> >
> > Doing something like the solution in
> >
> https://wiki.ubuntu.com/SecurityTeam/Roadmap/ExecutableStacks
> <https://wiki.ubuntu.com/SecurityTeam/Roadmap/ExecutableStacks>
> > fixes the problem without the use of linker flags.
>
> You mean the source code directives for the linker?
>
> Sorry, I wasn't specific enough, I meant the flags for
> the assembler.
> #if defined(__linux__) && defined(__ELF__)
> .section .note.GNU-stack, "", %progbits
> #endif
>
>
> I think I prefer to see this handled explicitly in
> the build as is
> currently done. Can we just adjust
> ./make/autoconf/flags-ldflags.m4 to
> pass the linker flags for gcc and clang?
>
> I don't mind this solution, but it seems like the
> right thing to do is to fix things at the source level
> and remove extra unnecessary linker flags.
>
>
> Personally I see this as source code pollution. The
> concept of executable stacks has nothing to do with what
> is being expressed by the source code, or the language
> used for it.
>
> Just my 2c. I'll defer to build folk ... though they are
> still on vacation at the moment.
>
>
> I agree with David. The executable stack is a build option.
> Even if you change the source code so the compiler/assember
> does the right thing, we would still want to keep the compiler
> option. (Otherwise one day you'll end up with executable
> stacks due to someone adding a new asm file and forgetting the
> "magic incantation".)
>
> And, since we will keep the compiler option, there seems
> little point in also adding this stuff to the asm files.
>
> To address your concerns on clang: we should reasonably be
> giving the same options to clang. There is no good reason,
> except for oversight, that this is not done already. (Cleaning
> up and unifying the compiler flags is an ongoing, but slowly
> moving, process.) So the correct fix is to update
> flags-ldflags.m4.
>
> /Magnus
>
>
>
>
>
> I removed "-Wl,-z,noexecstack" from the flags after
> adding the above assembler flags and libjvm.so is
> still correctly not stack executable. I don't really
> mind either way though. Maybe it's good to have an
> extra safeguard in the linker flags.
>
>
> > The jtreg test
> test/hotspot/jtreg/runtime/execstack/TestCheckJDK.java
> > checks for the stack being executable.
> >
> > Any thoughts? If there are no objections, I can
> propose a patch
> that works
> > for both gcc and clang on Linux. Also, I'm not
> sure how/if macOS
> handles
> > this problem given that it uses clang.
>
> We don't seem to handle it at all on OS X. Does OS
> X prevent executable
> stacks itself?
>
> A quick search, according to Wikipedia
> (https://en.wikipedia.org/wiki/Executable_space_protection#macOS
> <https://en.wikipedia.org/wiki/Executable_space_protection#macOS>),
> 64-bit executables on macOS aren't stack or heap
> executable. Not sure if that information is accurate
> though.
>
>
> Seems to be:
>
> https://developer.apple.com/library/archive/documentation/Security/Conceptual/SecureCodingGuide/Articles/BufferOverflows.html
> <https://developer.apple.com/library/archive/documentation/Security/Conceptual/SecureCodingGuide/Articles/BufferOverflows.html>
>
>
> "macOS and iOS provide two features that can make it
> harder to exploit stack and buffer overflows: address
> space layout randomization (ASLR) and a non-executable
> stack and heap."
>
> Cheers,
> David
>
>
> David
>
>
>
>
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