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

Ard Biesheuvel
2024-09-05T14:31:13Z

On Thu, 5 Sept 2024 at 16:03, Mark Rutland <mark.rutland@arm.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@kernel.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@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> | ffff800080000000 g .head.text 0000000000000000 _text
> | [mark@lakrids:~/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@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> | ffff800080000000 g .head.text 0000000000000000 _text
> | [mark@lakrids:~/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.