linux-kernel - Re: [syzbot] [arm?] upstream test error: KASAN: invalid-access Write in setup

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Message-ID: <CAMj1kXHHRvWBPOoxOU=S0BY6fkEhs1j1qBvrygBH7pKwnbcJXA@mail.gmail.com>
Date: Thu, 5 Sep 2024 16:25:57 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>, 
	syzbot <syzbot+908886656a02769af987@...kaller.appspotmail.com>, 
	catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [syzbot] [arm?] upstream test error: KASAN: invalid-access Write
 in setup_arch

On Thu, 5 Sept 2024 at 16:03, Mark Rutland <mark.rutland@....com> wrote:
>
> [adding Ard and LLVM folk; there's a question right at the end after
> some context]
>
> On Sat, Aug 31, 2024 at 06:52:52PM +0100, Marc Zyngier wrote:
> > On Fri, 30 Aug 2024 10:52:54 +0100,
> > Will Deacon <will@...nel.org> wrote:
> > >
> > > On Fri, Aug 30, 2024 at 01:35:24AM -0700, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    33faa93bc856 Merge branch kvmarm-master/next into kvmarm-m..
> > > > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git fuzzme
> > >
> > > +Marc, as this is his branch.
> > >
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1398420b980000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=2b7b31c9aa1397ca
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=908886656a02769af987
> > > > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > > userspace arch: arm64
> >
> > As it turns out, this isn't specific to this branch. I can reproduce
> > it with this config on a vanilla 6.10 as a KVM guest. Even worse,
> > compiling with clang results in an unbootable kernel (without any
> > output at all).
> >
> > Mind you, the binary is absolutely massive (130MB with gcc, 156MB with
> > clang), and I wouldn't be surprised if we were hitting some kind of
> > odd limit.
>
> Putting the KASAN issue aside (which I'll handle in a separate thread),
> I think there is a real issue here with LLVM.
>
> What's going on here is that .idmap.text ends up more than 128M away
> from .head.text, so the 'b primary_entry' at the start of the Image
> isn't in range:
>
> | [mark@...rids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> | ffff800080000000 g       .head.text     0000000000000000 _text
> | [mark@...rids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry
> | ffff8000889df0e0 g       .rodata.text   000000000000006c primary_entry
>
> ... as those are ~128MiB apart.
>
> When building with GCC those end up ~101MiB apart:
>
> | [mark@...rids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> | ffff800080000000 g       .head.text     0000000000000000 _text
> | [mark@...rids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry
> | ffff8000865ae0e0 g       .rodata.text   000000000000006c primary_entry
>
> When that happens, LLD makes the header branch to a veneer/thunk:
>
> | ffff800080000000 <_text>:
> | ffff800080000000:       fa405a4d        ccmp    x18, #0x0, #0xd, pl     // pl = nfrst
> | ffff800080000004:       14003fff        b       ffff800080010000 <__AArch64AbsLongThunk_primary_entry>
>
> ... and unfortunately, that veneer/thunk uses a literal with the
> statically-linked TTBR1 address of primary_entry:
>
> | ffff800080010000 <__AArch64AbsLongThunk_primary_entry>:
> | ffff800080010000:       58000050        ldr     x16, ffff800080010008 <__AArch64AbsLongThunk_primary_entry+0x8>
> | ffff800080010004:       d61f0200        br      x16
> | ffff800080010008:       889df0e0        .word   0x889df0e0
> | ffff80008001000c:       ffff8000        .word   0xffff8000
>
...
> LLVM folk, is there any existing option to ask LLD to use ADRP+ADD for
> the veneer/thunk? ... and if not, would it be possible to add an option
> for that?
>

ld.lld takes --pic-veneer, which (from looking at the llvm sources)
appears to do what we need here.