mm/usercopy: get rid of CONFIG_DEBUG_STRICT_USER_COPY_CHECKS

There are three usercopy warnings which are currently being silenced for
gcc 4.6 and newer:

1) "copy_from_user() buffer size is too small" compile warning/error

   This is a static warning which happens when object size and copy size
   are both const, and copy size > object size.  I didn't see any false
   positives for this one.  So the function warning attribute seems to
   be working fine here.

   Note this scenario is always a bug and so I think it should be
   changed to *always* be an error, regardless of
   CONFIG_DEBUG_STRICT_USER_COPY_CHECKS.

2) "copy_from_user() buffer size is not provably correct" compile warning

   This is another static warning which happens when I enable
   __compiletime_object_size() for new compilers (and
   CONFIG_DEBUG_STRICT_USER_COPY_CHECKS).  It happens when object size
   is const, but copy size is *not*.  In this case there's no way to
   compare the two at build time, so it gives the warning.  (Note the
   warning is a byproduct of the fact that gcc has no way of knowing
   whether the overflow function will be called, so the call isn't dead
   code and the warning attribute is activated.)

   So this warning seems to only indicate "this is an unusual pattern,
   maybe you should check it out" rather than "this is a bug".

   I get 102(!) of these warnings with allyesconfig and the
   __compiletime_object_size() gcc check removed.  I don't know if there
   are any real bugs hiding in there, but from looking at a small
   sample, I didn't see any.  According to Kees, it does sometimes find
   real bugs.  But the false positive rate seems high.

3) "Buffer overflow detected" runtime warning

   This is a runtime warning where object size is const, and copy size >
   object size.

All three warnings (both static and runtime) were completely disabled
for gcc 4.6 with the following commit:

  2fb0815c9ee6 ("gcc4: disable __compiletime_object_size for GCC 4.6+")

That commit mistakenly assumed that the false positives were caused by a
gcc bug in __compiletime_object_size().  But in fact,
__compiletime_object_size() seems to be working fine.  The false
positives were instead triggered by #2 above.  (Though I don't have an
explanation for why the warnings supposedly only started showing up in
gcc 4.6.)

So remove warning #2 to get rid of all the false positives, and re-enable
warnings #1 and #3 by reverting the above commit.

Furthermore, since #1 is a real bug which is detected at compile time,
upgrade it to always be an error.

Having done all that, CONFIG_DEBUG_STRICT_USER_COPY_CHECKS is no longer
needed.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 61 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c580d8c33562..2a1f0ce7c59a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -24,7 +24,6 @@ config X86
 	select ARCH_DISCARD_MEMBLOCK
 	select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
-	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FAST_MULTIPLIER
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a0ae610b9280..c3f291195294 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -697,43 +697,14 @@ unsigned long __must_check _copy_from_user(void *to, const void __user *from,
 unsigned long __must_check _copy_to_user(void __user *to, const void *from,
 					 unsigned n);
 
-#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
-# define copy_user_diag __compiletime_error
-#else
-# define copy_user_diag __compiletime_warning
-#endif
-
-extern void copy_user_diag("copy_from_user() buffer size is too small")
-copy_from_user_overflow(void);
-extern void copy_user_diag("copy_to_user() buffer size is too small")
-copy_to_user_overflow(void) __asm__("copy_from_user_overflow");
-
-#undef copy_user_diag
-
-#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
-
-extern void
-__compiletime_warning("copy_from_user() buffer size is not provably correct")
-__copy_from_user_overflow(void) __asm__("copy_from_user_overflow");
-#define __copy_from_user_overflow(size, count) __copy_from_user_overflow()
-
-extern void
-__compiletime_warning("copy_to_user() buffer size is not provably correct")
-__copy_to_user_overflow(void) __asm__("copy_from_user_overflow");
-#define __copy_to_user_overflow(size, count) __copy_to_user_overflow()
-
-#else
+extern void __compiletime_error("usercopy buffer size is too small")
+__bad_copy_user(void);
 
-static inline void
-__copy_from_user_overflow(int size, unsigned long count)
+static inline void copy_user_overflow(int size, unsigned long count)
 {
 	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
 }
 
-#define __copy_to_user_overflow __copy_from_user_overflow
-
-#endif
-
 static inline unsigned long __must_check
 copy_from_user(void *to, const void __user *from, unsigned long n)
 {
@@ -743,31 +714,13 @@ copy_from_user(void *to, const void __user *from, unsigned long n)
 
 	kasan_check_write(to, n);
 
-	/*
-	 * While we would like to have the compiler do the checking for us
-	 * even in the non-constant size case, any false positives there are
-	 * a problem (especially when DEBUG_STRICT_USER_COPY_CHECKS, but even
-	 * without - the [hopefully] dangerous looking nature of the warning
-	 * would make people go look at the respecitive call sites over and
-	 * over again just to find that there's no problem).
-	 *
-	 * And there are cases where it's just not realistic for the compiler
-	 * to prove the count to be in range. For example when multiple call
-	 * sites of a helper function - perhaps in different source files -
-	 * all doing proper range checking, yet the helper function not doing
-	 * so again.
-	 *
-	 * Therefore limit the compile time checking to the constant size
-	 * case, and do only runtime checking for non-constant sizes.
-	 */
-
 	if (likely(sz < 0 || sz >= n)) {
 		check_object_size(to, n, false);
 		n = _copy_from_user(to, from, n);
-	} else if (__builtin_constant_p(n))
-		copy_from_user_overflow();
+	} else if (!__builtin_constant_p(n))
+		copy_user_overflow(sz, n);
 	else
-		__copy_from_user_overflow(sz, n);
+		__bad_copy_user();
 
 	return n;
 }
@@ -781,21 +734,17 @@ copy_to_user(void __user *to, const void *from, unsigned long n)
 
 	might_fault();
 
-	/* See the comment in copy_from_user() above. */
 	if (likely(sz < 0 || sz >= n)) {
 		check_object_size(from, n, true);
 		n = _copy_to_user(to, from, n);
-	} else if (__builtin_constant_p(n))
-		copy_to_user_overflow();
+	} else if (!__builtin_constant_p(n))
+		copy_user_overflow(sz, n);
 	else
-		__copy_to_user_overflow(sz, n);
+		__bad_copy_user();
 
 	return n;
 }
 
-#undef __copy_from_user_overflow
-#undef __copy_to_user_overflow
-
 /*
  * We rely on the nested NMI work to allow atomic faults from the NMI path; the
  * nested NMI paths are careful to preserve CR2.