From: Ard Biesheuvel <ardb@kernel.org>
To: Jessica Yu <jeyu@kernel.org>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Kees Cook <keescook@chromium.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Miroslav Benes <mbenes@suse.cz>,
Mark Rutland <mark.rutland@arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX
Date: Wed, 12 Aug 2020 18:37:57 +0200 [thread overview]
Message-ID: <CAMj1kXFfSLvujJYk4Em6T+UvAUDW3VX0BibsD43z30Q_TSsehg@mail.gmail.com> (raw)
In-Reply-To: <20200812160017.GA30302@linux-8ccs>
module_frob_arch_sections
On Wed, 12 Aug 2020 at 18:00, Jessica Yu <jeyu@kernel.org> wrote:
>
> +++ Szabolcs Nagy [12/08/20 15:15 +0100]:
> >The 08/12/2020 13:56, Will Deacon wrote:
> >> On Wed, Aug 12, 2020 at 12:40:05PM +0200, peterz@infradead.org wrote:
> >> > On Wed, Aug 12, 2020 at 10:56:56AM +0200, Ard Biesheuvel wrote:
> >> > > The module .lds has BYTE(0) in the section contents to prevent the
> >> > > linker from pruning them entirely. The (NOLOAD) is there to ensure
> >> > > that this byte does not end up in the .ko, which is more a matter of
> >> > > principle than anything else, so we can happily drop that if it helps.
> >> > >
> >> > > However, this should only affect the PROGBITS vs NOBITS designation,
> >> > > and so I am not sure whether it makes a difference.
> >> > >
> >> > > Depending on where the w^x check occurs, we might simply override the
> >> > > permissions of these sections, and strip the writable permission if it
> >> > > is set in the PLT handling init code, which manipulates the metadata
> >> > > of all these 3 sections before the module space is vmalloc'ed.
> >> >
> >> > What's curious is that this seems the result of some recent binutils
> >> > change. Every build with binutils-2.34 (or older) does not seem to
> >> > generate these as WAX, but has the much more sensible WA.
> >> >
> >> > I suppose we can change the kernel check and 'allow' W^X for 0 sized
> >> > sections, but I think we should still figure out why binutils-2.35 is
> >> > now generating WAX sections all of a sudden, it might come bite us
> >> > elsewhere.
> >>
> >> Agreed, I think it's important to figure out what's going on here before we
> >> try to bodge around it.
> >>
> >> Adding Szabolcs, in case he has any ideas.
> >>
> >> To save him reading the whole thread, here's a summary:
> >>
> >> AArch64 kernel modules built with binutils 2.35 end up with a couple of
> >> ELF sections marked as SHF_WRITE | SHF_ALLOC | SHF_EXECINSTR:
> >>
> >> [ 5] .plt PROGBITS 0000000000000388 01d000 000008 00 WAX 0 0 1
> >> [ 6] .init.plt NOBITS 0000000000000390 01d008 000008 00 WA 0 0 1
> >> [ 7] .text.ftrace_trampoline PROGBITS 0000000000000398 01d008 000008 00 WAX 0 0 1
> >>
> >> This results in the module being rejected by our loader, because we don't
> >> permit writable, executable mappings.
> >>
> >> Our linker script for these entries uses NOLOAD, so it's odd to see PROGBITS
> >> appearing in the readelf output above (and older binutils emits NOBITS
> >> sections). Anyway, here's the linker script:
> >>
> >> SECTIONS {
> >> .plt (NOLOAD) : { BYTE(0) }
> >> .init.plt (NOLOAD) : { BYTE(0) }
> >> .text.ftrace_trampoline (NOLOAD) : { BYTE(0) }
> >> }
> >>
> >> It appears that the name of the section influences the behaviour, as
> >> Jessica observed [1] that sections named .text.* end up with PROGBITS,
> >> whereas random naming such as ".test" ends up with NOBITS, as before.
> >>
> >> We've looked at the changelog between binutils 2.34 and 2.35, but nothing
> >> stands out. Any clues? Is this intentional binutils behaviour?
> >
> >for me it bisects to
> >
> >https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=8c803a2dd7d3d742a3d0071914f557ef465afe71
> >
> >i will have to investigate further what's going on.
>
> Thanks for the hint. I'm almost certain it's due to this excerpt from
> the changelog: "we now init sh_type and sh_flags for all known ABI sections
> in _bfd_elf_new_section_hook."
>
> Indeed, .plt and .text.* are listed as special sections in bfd/elf.c.
> The former requires an exact match and the latter only has to match
> the prefix ".text." Since the code considers ".plt" and
> ".text.ftrace_trampoline" special sections, their sh_type and sh_flags
> are now set by default. Now I guess the question is whether this can
> be overriden by a linker script..
>
If this is even possible to begin with, doing this in a way that is
portable across the various linkers that we claim to support is going
to be tricky.
I suppose this is the downside of using partially linked objects as
our module format - using ordinary shared libraries (along with the
appropriate dynamic relocations which are mostly portable across
architectures) would get rid of most of the PLT and trampoline issues,
and of a lot of complex static relocation processing code.
I know there is little we can do at this point, apart from ignoring
the permissions - perhaps we should just defer the w^x check until
after calling module_frob_arch_sections()?
next prev parent reply other threads:[~2020-08-12 16:38 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-03 17:13 [PATCH v2] module: Harden STRICT_MODULE_RWX Peter Zijlstra
2020-04-03 20:31 ` Kees Cook
2020-04-08 15:32 ` Jessica Yu
2020-04-08 15:43 ` [PATCH] module: break nested ARCH_HAS_STRICT_MODULE_RWX and STRICT_MODULE_RWX #ifdefs Jessica Yu
2020-04-08 15:57 ` [PATCH v2] module: Harden STRICT_MODULE_RWX Peter Zijlstra
2020-04-08 16:20 ` Jessica Yu
2020-08-08 8:12 ` Mauro Carvalho Chehab
2020-08-10 9:25 ` Jessica Yu
2020-08-10 15:06 ` Jessica Yu
2020-08-11 14:34 ` Mauro Carvalho Chehab
2020-08-11 14:55 ` peterz
2020-08-11 15:27 ` Mauro Carvalho Chehab
2020-08-11 16:01 ` Jessica Yu
2020-08-11 16:57 ` Will Deacon
2020-08-11 17:59 ` peterz
2020-08-11 21:29 ` Peter Zijlstra
2020-08-12 8:56 ` Ard Biesheuvel
2020-08-12 10:40 ` peterz
2020-08-12 11:41 ` Jessica Yu
2020-08-12 13:14 ` H.J. Lu
2020-08-12 12:56 ` Will Deacon
2020-08-12 14:15 ` Szabolcs Nagy
2020-08-12 16:00 ` Jessica Yu
2020-08-12 16:37 ` Ard Biesheuvel [this message]
2020-08-12 16:42 ` Szabolcs Nagy
2020-08-13 9:00 ` Will Deacon
2020-08-12 20:00 ` Peter Zijlstra
2020-08-13 8:36 ` Ard Biesheuvel
2020-08-13 13:04 ` Jessica Yu
2020-08-13 13:07 ` Ard Biesheuvel
2020-08-21 12:20 ` Will Deacon
2020-08-21 12:27 ` Ard Biesheuvel
2020-08-21 12:30 ` Will Deacon
2020-08-22 13:47 ` Ard Biesheuvel
2020-08-24 15:24 ` Jessica Yu
2020-08-25 1:54 ` Masahiro Yamada
2020-08-31 9:46 ` Jessica Yu
2020-08-31 10:42 ` Masahiro Yamada
2020-08-31 13:25 ` Ard Biesheuvel
2020-08-31 15:31 ` Jessica Yu
2020-08-31 15:46 ` Masahiro Yamada
2020-09-03 12:37 ` Jessica Yu
2020-09-01 12:51 ` Will Deacon
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAMj1kXFfSLvujJYk4Em6T+UvAUDW3VX0BibsD43z30Q_TSsehg@mail.gmail.com \
--to=ardb@kernel.org \
--cc=jeyu@kernel.org \
--cc=jpoimboe@redhat.com \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mbenes@suse.cz \
--cc=mchehab+huawei@kernel.org \
--cc=nd@arm.com \
--cc=peterz@infradead.org \
--cc=szabolcs.nagy@arm.com \
--cc=tglx@linutronix.de \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).