Re: [PATCH 5/5] copy_to_user, copy_from_user: Use generic instrumented.h

[Marco Elver]

On Mon, 20 Jan 2020 at 15:52, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Mon, Jan 20, 2020 at 3:19 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > This replaces the KASAN instrumentation with generic instrumentation,
> > implicitly adding KCSAN instrumentation support.
> >
> > For KASAN no functional change is intended.
> >
> > Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > ---
> >  include/linux/uaccess.h | 46 +++++++++++++++++++++++++++++------------
> >  lib/usercopy.c          | 14 ++++++++-----
> >  2 files changed, 42 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> > index 67f016010aad..d3f2d9a8cae3 100644
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > @@ -2,9 +2,9 @@
> >  #ifndef __LINUX_UACCESS_H__
> >  #define __LINUX_UACCESS_H__
> >
> > +#include <linux/instrumented.h>
> >  #include <linux/sched.h>
> >  #include <linux/thread_info.h>
> > -#include <linux/kasan-checks.h>
> >
> >  #define uaccess_kernel() segment_eq(get_fs(), KERNEL_DS)
> >
> > @@ -58,18 +58,26 @@
> >  static __always_inline __must_check unsigned long
> >  __copy_from_user_inatomic(void *to, const void __user *from, unsigned long n)
> >  {
> > -       kasan_check_write(to, n);
> > +       unsigned long res;
> > +
> >         check_object_size(to, n, false);
> > -       return raw_copy_from_user(to, from, n);
> > +       instrument_copy_from_user_pre(to, n);
> > +       res = raw_copy_from_user(to, from, n);
> > +       instrument_copy_from_user_post(to, n, res);
> > +       return res;
> >  }
>
> There is also something called strncpy_from_user() that has kasan
> instrumentation now:
> https://elixir.bootlin.com/linux/v5.5-rc6/source/lib/strncpy_from_user.c#L117

Yes, however, I think it's a special case for KASAN. The
implementation is already instrumented by the compiler. In the
original commit it says (1771c6e1a567e):

"Note: Unlike others strncpy_from_user() is written mostly in C and KASAN
    sees memory accesses in it.  However, it makes sense to add explicit
    check for all @count bytes that *potentially* could be written to the
    kernel."

I don't think we want unconditional double-instrumentation here. Let
me know if you think otherwise.

Thanks,
-- Marco

> >  static __always_inline __must_check unsigned long
> >  __copy_from_user(void *to, const void __user *from, unsigned long n)
> >  {
> > +       unsigned long res;
> > +
> >         might_fault();
> > -       kasan_check_write(to, n);
> >         check_object_size(to, n, false);
> > -       return raw_copy_from_user(to, from, n);
> > +       instrument_copy_from_user_pre(to, n);
> > +       res = raw_copy_from_user(to, from, n);
> > +       instrument_copy_from_user_post(to, n, res);
> > +       return res;
> >  }
> >
> >  /**
> > @@ -88,18 +96,26 @@ __copy_from_user(void *to, const void __user *from, unsigned long n)
> >  static __always_inline __must_check unsigned long
> >  __copy_to_user_inatomic(void __user *to, const void *from, unsigned long n)
> >  {
> > -       kasan_check_read(from, n);
> > +       unsigned long res;
> > +
> >         check_object_size(from, n, true);
> > -       return raw_copy_to_user(to, from, n);
> > +       instrument_copy_to_user_pre(from, n);
> > +       res = raw_copy_to_user(to, from, n);
> > +       instrument_copy_to_user_post(from, n, res);
> > +       return res;
> >  }
> >
> >  static __always_inline __must_check unsigned long
> >  __copy_to_user(void __user *to, const void *from, unsigned long n)
> >  {
> > +       unsigned long res;
> > +
> >         might_fault();
> > -       kasan_check_read(from, n);
> >         check_object_size(from, n, true);
> > -       return raw_copy_to_user(to, from, n);
> > +       instrument_copy_to_user_pre(from, n);
> > +       res = raw_copy_to_user(to, from, n);
> > +       instrument_copy_to_user_post(from, n, res);
> > +       return res;
> >  }
> >
> >  #ifdef INLINE_COPY_FROM_USER
> > @@ -109,8 +125,9 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
> >         unsigned long res = n;
> >         might_fault();
> >         if (likely(access_ok(from, n))) {
> > -               kasan_check_write(to, n);
> > +               instrument_copy_from_user_pre(to, n);
> >                 res = raw_copy_from_user(to, from, n);
> > +               instrument_copy_from_user_post(to, n, res);
> >         }
> >         if (unlikely(res))
> >                 memset(to + (n - res), 0, res);
> > @@ -125,12 +142,15 @@ _copy_from_user(void *, const void __user *, unsigned long);
> >  static inline __must_check unsigned long
> >  _copy_to_user(void __user *to, const void *from, unsigned long n)
> >  {
> > +       unsigned long res = n;
> > +
> >         might_fault();
> >         if (access_ok(to, n)) {
> > -               kasan_check_read(from, n);
> > -               n = raw_copy_to_user(to, from, n);
> > +               instrument_copy_to_user_pre(from, n);
> > +               res = raw_copy_to_user(to, from, n);
> > +               instrument_copy_to_user_post(from, n, res);
> >         }
> > -       return n;
> > +       return res;
> >  }
> >  #else
> >  extern __must_check unsigned long
> > diff --git a/lib/usercopy.c b/lib/usercopy.c
> > index cbb4d9ec00f2..1c20d4423b86 100644
> > --- a/lib/usercopy.c
> > +++ b/lib/usercopy.c
> > @@ -1,6 +1,7 @@
> >  // SPDX-License-Identifier: GPL-2.0
> > -#include <linux/uaccess.h>
> >  #include <linux/bitops.h>
> > +#include <linux/instrumented.h>
> > +#include <linux/uaccess.h>
> >
> >  /* out-of-line parts */
> >
> > @@ -10,8 +11,9 @@ unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n
> >         unsigned long res = n;
> >         might_fault();
> >         if (likely(access_ok(from, n))) {
> > -               kasan_check_write(to, n);
> > +               instrument_copy_from_user_pre(to, n);
> >                 res = raw_copy_from_user(to, from, n);
> > +               instrument_copy_from_user_post(to, n, res);
> >         }
> >         if (unlikely(res))
> >                 memset(to + (n - res), 0, res);
> > @@ -23,12 +25,14 @@ EXPORT_SYMBOL(_copy_from_user);
> >  #ifndef INLINE_COPY_TO_USER
> >  unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
> >  {
> > +       unsigned long res = n;
> >         might_fault();
> >         if (likely(access_ok(to, n))) {
> > -               kasan_check_read(from, n);
> > -               n = raw_copy_to_user(to, from, n);
> > +               instrument_copy_to_user_pre(from, n);
> > +               res = raw_copy_to_user(to, from, n);
> > +               instrument_copy_to_user_post(from, n, res);
> >         }
> > -       return n;
> > +       return res;
> >  }
> >  EXPORT_SYMBOL(_copy_to_user);
> >  #endif
> > --
> > 2.25.0.341.g760bfbb309-goog
> >