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

From: Will Deacon <[email-masked]>
Date: Thu, 6 Dec 2018 11:11:07 +0000

On Thu, Dec 06, 2018 at 11:31:43AM +0100, Andrey Konovalov wrote:
> On Thu, Nov 29, 2018 at 7:01 PM Will Deacon <[email-masked]> 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 <[email-masked]>
> > > Reviewed-by: Dmitry Vyukov <[email-masked]>
> > > Signed-off-by: Andrey Konovalov <[email-masked]>
> > > ---
> > >  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

Aha, but we subsequently fixed the underlying problem in commit
9478f1927e6e ("arm64: only advance singlestep for user instruction traps").
You were on cc, but I appreciate it's not clear that it was related to this.

Anyway, you can just call arm64_skip_faulting_instruction() as you were
doing and there's no need for this refactoring.

Please could you spin a new version so that akpm can replace the one which
he has queued?

Thanks,

Will

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