Re: [PATCH] x86/traps: Enable UBSAN traps on x86

[Marco Elver]

On Wed, 29 May 2024 at 20:17, Gatlin Newhouse <gatlin.newhouse@xxxxxxxxx> wrote:
>
> On Wed, May 29, 2024 at 09:25:21AM UTC, Marco Elver wrote:
> > On Wed, 29 May 2024 at 04:20, Gatlin Newhouse <gatlin.newhouse@xxxxxxxxx> wrote:
> > [...]
> > >         if (regs->flags & X86_EFLAGS_IF)
> > >                 raw_local_irq_enable();
> > > -       if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
> > > -           handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
> > > -               regs->ip += LEN_UD2;
> > > -               handled = true;
> > > +
> > > +       if (insn == INSN_UD2) {
> > > +               if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
> > > +               handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
> > > +                       regs->ip += LEN_UD2;
> > > +                       handled = true;
> > > +               }
> > > +       } else {
> > > +               if (handle_ubsan_failure(regs, insn) == BUG_TRAP_TYPE_WARN) {
> >
> > handle_ubsan_failure currently only returns BUG_TRAP_TYPE_NONE?
> >
> > > +                       if (insn == INSN_REX)
> > > +                               regs->ip += LEN_REX;
> > > +                       regs->ip += LEN_UD1;
> > > +                       handled = true;
> > > +               }
> > >         }
> > >         if (regs->flags & X86_EFLAGS_IF)
> > >                 raw_local_irq_disable();
> > > diff --git a/arch/x86/kernel/ubsan.c b/arch/x86/kernel/ubsan.c
> > > new file mode 100644
> > > index 000000000000..6cae11f4fe23
> > > --- /dev/null
> > > +++ b/arch/x86/kernel/ubsan.c
> > > @@ -0,0 +1,32 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * Clang Undefined Behavior Sanitizer trap mode support.
> > > + */
> > > +#include <linux/bug.h>
> > > +#include <linux/string.h>
> > > +#include <linux/printk.h>
> > > +#include <linux/ubsan.h>
> > > +#include <asm/ptrace.h>
> > > +#include <asm/ubsan.h>
> > > +
> > > +/*
> > > + * Checks for the information embedded in the UD1 trap instruction
> > > + * for the UB Sanitizer in order to pass along debugging output.
> > > + */
> > > +enum bug_trap_type handle_ubsan_failure(struct pt_regs *regs, int insn)
> > > +{
> > > +       u32 type = 0;
> > > +
> > > +       if (insn == INSN_REX) {
> > > +               type = (*(u16 *)(regs->ip + LEN_REX + LEN_UD1));
> > > +               if ((type & 0xFF) == 0x40)
> > > +                       type = (type >> 8) & 0xFF;
> > > +       } else {
> > > +               type = (*(u16 *)(regs->ip + LEN_UD1));
> > > +               if ((type & 0xFF) == 0x40)
> > > +                       type = (type >> 8) & 0xFF;
> > > +       }
> > > +       pr_crit("%s at %pS\n", report_ubsan_failure(regs, type), (void *)regs->ip);
> > > +
> > > +       return BUG_TRAP_TYPE_NONE;
> > > +}
> >
> > Shouldn't this return BUG_TRAP_TYPE_WARN?
>
> So as far as I understand, UBSAN trap mode never warns. Perhaps it does on
> arm64, although it calls die() so I am unsure. Maybe the condition in
> handle_bug() should be rewritten in the case of UBSAN ud1s? Do you have any
> suggestions?

AFAIK on arm64 it's basically a kernel OOPS.

The main thing I just wanted to point out though is that your newly added branch

> if (handle_ubsan_failure(regs, insn) == BUG_TRAP_TYPE_WARN) {

will never be taken, because I don't see where handle_ubsan_failure()
returns BUG_TRAP_TYPE_WARN.

That means 'handled' will be false, and the code in exc_invalid_op
will proceed to call handle_invalid_op() which is probably not what
you intended - i.e. it's definitely not BUG_TRAP_TYPE_NONE, but one of
TYPE_WARN of TYPE_BUG.

Did you test it and you got the behaviour you expected?