[PATCH v2] mm/usercopy: get rid of CONFIG_DEBUG_STRICT_USER_COPY_CHECKS

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Aug 29, 2016 at 08:36:46AM -0700, Linus Torvalds wrote:
> On Mon, Aug 29, 2016 at 7:48 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > So I *think* your patch fixes the wrong problem.  That's probably at
> > least somewhat my fault because I misunderstood the issue before and may
> > have described it wrong at some point.
> >
> > AFAICT, gcc isn't doing anything wrong, and the false positives are
> > "intentional".
> >
> > There are in fact two static warnings (which are being silenced for new
> > versions of gcc):
> 
> [ snip snip details ]
> 
> Ok.
> 
> Color me convinced, I never even looked at the two different cases, I
> thought it was just one issue.
> 
> Let's just remove the spurious false positive warning then, in order
> to re-instate the *actual* warning that right now is disabled entirely
> due to the unrelated false positives.
> 
> Thanks for looking into this. Would you happen to also have a patch
> that can be applied? Hint hint..

How about something like this?  I can split it up if needed...

---

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH v2] 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>
---
 arch/parisc/Kconfig                         |  1 -
 arch/parisc/configs/c8000_defconfig         |  1 -
 arch/parisc/configs/generic-64bit_defconfig |  1 -
 arch/parisc/include/asm/uaccess.h           | 22 ++++-----
 arch/s390/Kconfig                           |  1 -
 arch/s390/configs/default_defconfig         |  1 -
 arch/s390/configs/gcov_defconfig            |  1 -
 arch/s390/configs/performance_defconfig     |  1 -
 arch/s390/defconfig                         |  1 -
 arch/s390/include/asm/uaccess.h             | 19 +++++---
 arch/tile/Kconfig                           |  1 -
 arch/tile/include/asm/uaccess.h             | 19 ++++----
 arch/x86/Kconfig                            |  1 -
 arch/x86/include/asm/uaccess.h              | 69 ++++-------------------------
 include/asm-generic/uaccess.h               |  1 +
 include/linux/compiler-gcc.h                |  2 +-
 lib/Kconfig.debug                           | 18 --------
 lib/Makefile                                |  1 -
 lib/usercopy.c                              |  9 ----
 19 files changed, 45 insertions(+), 125 deletions(-)
 delete mode 100644 lib/usercopy.c

diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index cd87781..af12c2d 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -1,6 +1,5 @@
 config PARISC
 	def_bool y
-	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select HAVE_IDE
 	select HAVE_OPROFILE
diff --git a/arch/parisc/configs/c8000_defconfig b/arch/parisc/configs/c8000_defconfig
index 1a8f6f95..f6a4c01 100644
--- a/arch/parisc/configs/c8000_defconfig
+++ b/arch/parisc/configs/c8000_defconfig
@@ -245,7 +245,6 @@ CONFIG_DEBUG_RT_MUTEXES=y
 CONFIG_PROVE_RCU_DELAY=y
 CONFIG_DEBUG_BLOCK_EXT_DEVT=y
 CONFIG_LATENCYTOP=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_KEYS=y
 # CONFIG_CRYPTO_HW is not set
 CONFIG_FONTS=y
diff --git a/arch/parisc/configs/generic-64bit_defconfig b/arch/parisc/configs/generic-64bit_defconfig
index 7e07926..c564e6e 100644
--- a/arch/parisc/configs/generic-64bit_defconfig
+++ b/arch/parisc/configs/generic-64bit_defconfig
@@ -291,7 +291,6 @@ CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC=y
 CONFIG_BOOTPARAM_HUNG_TASK_PANIC=y
 # CONFIG_SCHED_DEBUG is not set
 CONFIG_TIMER_STATS=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_CRYPTO_MANAGER=y
 CONFIG_CRYPTO_ECB=m
 CONFIG_CRYPTO_PCBC=m
diff --git a/arch/parisc/include/asm/uaccess.h b/arch/parisc/include/asm/uaccess.h
index 0f59fd9..736c0c1 100644
--- a/arch/parisc/include/asm/uaccess.h
+++ b/arch/parisc/include/asm/uaccess.h
@@ -208,13 +208,13 @@ unsigned long copy_in_user(void __user *dst, const void __user *src, unsigned lo
 #define __copy_to_user_inatomic __copy_to_user
 #define __copy_from_user_inatomic __copy_from_user
 
-extern void copy_from_user_overflow(void)
-#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
-        __compiletime_error("copy_from_user() buffer size is not provably correct")
-#else
-        __compiletime_warning("copy_from_user() buffer size is not provably correct")
-#endif
-;
+extern void __compiletime_error("usercopy buffer size is too small")
+__bad_copy_user(void);
+
+static inline void copy_user_overflow(int size, unsigned long count)
+{
+	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
+}
 
 static inline unsigned long __must_check copy_from_user(void *to,
                                           const void __user *from,
@@ -223,10 +223,12 @@ static inline unsigned long __must_check copy_from_user(void *to,
         int sz = __compiletime_object_size(to);
         int ret = -EFAULT;
 
-        if (likely(sz == -1 || !__builtin_constant_p(n) || sz >= n))
+        if (likely(sz == -1 || sz >= n))
                 ret = __copy_from_user(to, from, n);
-        else
-                copy_from_user_overflow();
+        else (!__builtin_constant_p(n))
+		copy_user_overflow(sz, n);
+	else
+                __bad_copy_user();
 
         return ret;
 }
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index e751fe2..c109f07 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -68,7 +68,6 @@ config DEBUG_RODATA
 config S390
 	def_bool y
 	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_GCOV_PROFILE_ALL
diff --git a/arch/s390/configs/default_defconfig b/arch/s390/configs/default_defconfig
index 26e0c7f..412b1bd 100644
--- a/arch/s390/configs/default_defconfig
+++ b/arch/s390/configs/default_defconfig
@@ -602,7 +602,6 @@ CONFIG_FAIL_FUTEX=y
 CONFIG_FAULT_INJECTION_DEBUG_FS=y
 CONFIG_FAULT_INJECTION_STACKTRACE_FILTER=y
 CONFIG_LATENCYTOP=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_IRQSOFF_TRACER=y
 CONFIG_PREEMPT_TRACER=y
 CONFIG_SCHED_TRACER=y
diff --git a/arch/s390/configs/gcov_defconfig b/arch/s390/configs/gcov_defconfig
index 24879da..bec279e 100644
--- a/arch/s390/configs/gcov_defconfig
+++ b/arch/s390/configs/gcov_defconfig
@@ -552,7 +552,6 @@ CONFIG_NOTIFIER_ERROR_INJECTION=m
 CONFIG_CPU_NOTIFIER_ERROR_INJECT=m
 CONFIG_PM_NOTIFIER_ERROR_INJECT=m
 CONFIG_LATENCYTOP=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_BLK_DEV_IO_TRACE=y
 # CONFIG_KPROBE_EVENT is not set
 CONFIG_TRACE_ENUM_MAP_FILE=y
diff --git a/arch/s390/configs/performance_defconfig b/arch/s390/configs/performance_defconfig
index a5c1e5f..1751446 100644
--- a/arch/s390/configs/performance_defconfig
+++ b/arch/s390/configs/performance_defconfig
@@ -549,7 +549,6 @@ CONFIG_TIMER_STATS=y
 CONFIG_RCU_TORTURE_TEST=m
 CONFIG_RCU_CPU_STALL_TIMEOUT=60
 CONFIG_LATENCYTOP=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_SCHED_TRACER=y
 CONFIG_FTRACE_SYSCALLS=y
 CONFIG_STACK_TRACER=y
diff --git a/arch/s390/defconfig b/arch/s390/defconfig
index 73610f2..2d40ef0 100644
--- a/arch/s390/defconfig
+++ b/arch/s390/defconfig
@@ -172,7 +172,6 @@ CONFIG_DEBUG_NOTIFIERS=y
 CONFIG_RCU_CPU_STALL_TIMEOUT=60
 CONFIG_RCU_TRACE=y
 CONFIG_LATENCYTOP=y
-CONFIG_DEBUG_STRICT_USER_COPY_CHECKS=y
 CONFIG_SCHED_TRACER=y
 CONFIG_FTRACE_SYSCALLS=y
 CONFIG_TRACER_SNAPSHOT_PER_CPU_SWAP=y
diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
index 9b49cf1..95aefdb 100644
--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -311,6 +311,14 @@ int __get_user_bad(void) __attribute__((noreturn));
 #define __put_user_unaligned __put_user
 #define __get_user_unaligned __get_user
 
+extern void __compiletime_error("usercopy buffer size is too small")
+__bad_copy_user(void);
+
+static inline void copy_user_overflow(int size, unsigned long count)
+{
+	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
+}
+
 /**
  * copy_to_user: - Copy a block of data into user space.
  * @to:   Destination address, in user space.
@@ -332,12 +340,6 @@ copy_to_user(void __user *to, const void *from, unsigned long n)
 	return __copy_to_user(to, from, n);
 }
 
-void copy_from_user_overflow(void)
-#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
-__compiletime_warning("copy_from_user() buffer size is not provably correct")
-#endif
-;
-
 /**
  * copy_from_user: - Copy a block of data from user space.
  * @to:   Destination address, in kernel space.
@@ -362,7 +364,10 @@ copy_from_user(void *to, const void __user *from, unsigned long n)
 
 	might_fault();
 	if (unlikely(sz != -1 && sz < n)) {
-		copy_from_user_overflow();
+		if (!__builtin_constant_p(n))
+			copy_user_overflow(sz, n);
+		else
+			__bad_copy_user();
 		return n;
 	}
 	return __copy_from_user(to, from, n);
diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
index 4820a02..78da75b 100644
--- a/arch/tile/Kconfig
+++ b/arch/tile/Kconfig
@@ -4,7 +4,6 @@
 config TILE
 	def_bool y
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
-	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select ARCH_WANT_FRAME_POINTERS
diff --git a/arch/tile/include/asm/uaccess.h b/arch/tile/include/asm/uaccess.h
index 0a9c4265..c664300 100644
--- a/arch/tile/include/asm/uaccess.h
+++ b/arch/tile/include/asm/uaccess.h
@@ -416,14 +416,13 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
 	return n;
 }
 
-#ifdef CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
-/*
- * There are still unprovable places in the generic code as of 2.6.34, so this
- * option is not really compatible with -Werror, which is more useful in
- * general.
- */
-extern void copy_from_user_overflow(void)
-	__compiletime_warning("copy_from_user() size is not provably correct");
+extern void __compiletime_error("usercopy buffer size is too small")
+__bad_copy_user(void);
+
+static inline void copy_user_overflow(int size, unsigned long count)
+{
+	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
+}
 
 static inline unsigned long __must_check copy_from_user(void *to,
 					  const void __user *from,
@@ -433,8 +432,10 @@ static inline unsigned long __must_check copy_from_user(void *to,
 
 	if (likely(sz == -1 || sz >= n))
 		n = _copy_from_user(to, from, n);
+	else if (!__builtin_constant_p(n))
+		copy_user_overflow();
 	else
-		copy_from_user_overflow();
+		__bad_copy_user();
 
 	return n;
 }
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c580d8c..2a1f0ce 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 a0ae610..c3f2911 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.
diff --git a/include/asm-generic/uaccess.h b/include/asm-generic/uaccess.h
index 1bfa602..5dea1fb 100644
--- a/include/asm-generic/uaccess.h
+++ b/include/asm-generic/uaccess.h
@@ -72,6 +72,7 @@ struct exception_table_entry
 /* Returns 0 if exception not found and fixup otherwise.  */
 extern unsigned long search_exception_table(unsigned long);
 
+
 /*
  * architectures with an MMU should override these two
  */
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 8dbc892..573c5a1 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -158,7 +158,7 @@
 #define __compiler_offsetof(a, b)					\
 	__builtin_offsetof(a, b)
 
-#if GCC_VERSION >= 40100 && GCC_VERSION < 40600
+#if GCC_VERSION >= 40100
 # define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
 #endif
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2307d7c..2e2cca5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1686,24 +1686,6 @@ config LATENCYTOP
 	  Enable this option if you want to use the LatencyTOP tool
 	  to find out which userspace is blocking on what kernel operations.
 
-config ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
-	bool
-
-config DEBUG_STRICT_USER_COPY_CHECKS
-	bool "Strict user copy size checks"
-	depends on ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
-	depends on DEBUG_KERNEL && !TRACE_BRANCH_PROFILING
-	help
-	  Enabling this option turns a certain set of sanity checks for user
-	  copy operations into compile time failures.
-
-	  The copy_from_user() etc checks are there to help test if there
-	  are sufficient security checks on the length argument of
-	  the copy operation, by having gcc prove that the argument is
-	  within bounds.
-
-	  If unsure, say N.
-
 source kernel/trace/Kconfig
 
 menu "Runtime Testing"
diff --git a/lib/Makefile b/lib/Makefile
index cfa68eb..5dc77a8 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -24,7 +24,6 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
 	 earlycpio.o seq_buf.o nmi_backtrace.o nodemask.o
 
-obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o
 lib-$(CONFIG_HAS_DMA) += dma-noop.o
diff --git a/lib/usercopy.c b/lib/usercopy.c
deleted file mode 100644
index 4f5b1dd..0000000
--- a/lib/usercopy.c
+++ /dev/null
@@ -1,9 +0,0 @@
-#include <linux/export.h>
-#include <linux/bug.h>
-#include <linux/uaccess.h>
-
-void copy_from_user_overflow(void)
-{
-	WARN(1, "Buffer overflow detected!\n");
-}
-EXPORT_SYMBOL(copy_from_user_overflow);
-- 
2.7.4