Re: [PATCH v12 20/25] kasan, arm64: add brk handler for inline instrumentation

[Andrey Konovalov]

On Thu, Nov 29, 2018 at 7:01 PM Will Deacon <will.deacon@xxxxxxx> wrote:
>
> On Tue, Nov 27, 2018 at 05:55:38PM +0100, Andrey Konovalov wrote:
> > Tag-based KASAN inline instrumentation mode (which embeds checks of shadow
> > memory into the generated code, instead of inserting a callback) generates
> > a brk instruction when a tag mismatch is detected.
> >
> > This commit adds a tag-based KASAN specific brk handler, that decodes the
> > immediate value passed to the brk instructions (to extract information
> > about the memory access that triggered the mismatch), reads the register
> > values (x0 contains the guilty address) and reports the bug.
> >
> > Reviewed-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/brk-imm.h |  2 +
> >  arch/arm64/kernel/traps.c        | 68 +++++++++++++++++++++++++++++++-
> >  include/linux/kasan.h            |  3 ++
> >  3 files changed, 71 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
> > index ed693c5bcec0..2945fe6cd863 100644
> > --- a/arch/arm64/include/asm/brk-imm.h
> > +++ b/arch/arm64/include/asm/brk-imm.h
> > @@ -16,10 +16,12 @@
> >   * 0x400: for dynamic BRK instruction
> >   * 0x401: for compile time BRK instruction
> >   * 0x800: kernel-mode BUG() and WARN() traps
> > + * 0x9xx: tag-based KASAN trap (allowed values 0x900 - 0x9ff)
> >   */
> >  #define FAULT_BRK_IMM                        0x100
> >  #define KGDB_DYN_DBG_BRK_IMM         0x400
> >  #define KGDB_COMPILED_DBG_BRK_IMM    0x401
> >  #define BUG_BRK_IMM                  0x800
> > +#define KASAN_BRK_IMM                        0x900
> >
> >  #endif
> > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > index 5f4d9acb32f5..04bdc53716ef 100644
> > --- a/arch/arm64/kernel/traps.c
> > +++ b/arch/arm64/kernel/traps.c
> > @@ -35,6 +35,7 @@
> >  #include <linux/sizes.h>
> >  #include <linux/syscalls.h>
> >  #include <linux/mm_types.h>
> > +#include <linux/kasan.h>
> >
> >  #include <asm/atomic.h>
> >  #include <asm/bug.h>
> > @@ -284,10 +285,14 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
> >       }
> >  }
> >
> > -void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
> > +void __arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
> >  {
> >       regs->pc += size;
> > +}
> >
> > +void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
> > +{
> > +     __arm64_skip_faulting_instruction(regs, size);
> >       /*
> >        * If we were single stepping, we want to get the step exception after
> >        * we return from the trap.
> > @@ -959,7 +964,7 @@ static int bug_handler(struct pt_regs *regs, unsigned int esr)
> >       }
> >
> >       /* If thread survives, skip over the BUG instruction and continue: */
> > -     arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
> > +     __arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
>
> Why do you want to avoid the single-step logic here? Given that we're
> skipping over the brk instruction, why wouldn't you want that to trigger
> a step exception if single-step is enabled?

I was asked to do that, see the discussion here:

https://www.spinics.net/lists/linux-mm/msg146575.html
https://www.spinics.net/lists/linux-mm/msg148215.html
https://www.spinics.net/lists/linux-mm/msg148367.html