[PATCH] x86: Turn the copy_from_user check into an (optional) compile time warning

[Arjan van de Ven]

A previous patch added the buffer size check to copy_from_user().

One of the things learned from analyzing the result of the previous patch
is that in general, gcc is really good at proving that the code contains
sufficient security checks to not need to do a runtime check. But that
for those cases where gcc could not prove this, there was a relatively
high percentage of real security issues.

This patch turns the case of "gcc cannot prove" into a compile time
warning, as long as a sufficiently new gcc is in use that supports this.
The objective is that these warnings will trigger developers checking
new cases out before a security hole enters a linux kernel release.

Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/uaccess_32.h |   12 +++++++++---
 arch/x86/lib/usercopy_32.c        |    6 ++++++
 include/linux/compiler-gcc4.h     |    3 +++
 include/linux/compiler.h          |    4 ++++
 4 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/uaccess_32.h b/arch/x86/include/asm/uaccess_32.h
index 582d6ae..7826639 100644
--- a/arch/x86/include/asm/uaccess_32.h
+++ b/arch/x86/include/asm/uaccess_32.h
@@ -191,6 +191,13 @@ unsigned long __must_check _copy_from_user(void *to,
 					  const void __user *from,
 					  unsigned long n);
 
+
+extern void copy_from_user_overflow(void)
+#ifdef CONFIG_DEBUG_STACKOVERFLOW
+	__compiletime_warning("copy_from_user buffer size is not provably correct")
+#endif
+;
+
 static inline unsigned long __must_check copy_from_user(void *to,
 					  const void __user *from,
 					  unsigned long n)
@@ -200,10 +207,9 @@ static inline unsigned long __must_check copy_from_user(void *to,
 
 	if (likely(sz == -1 || sz >= n))
 		ret = _copy_from_user(to, from, n);
-#ifdef CONFIG_DEBUG_VM
 	else
-		WARN(1, "Buffer overflow detected!\n");
-#endif
+		copy_from_user_overflow();
+
 	return ret;
 }
 
diff --git a/arch/x86/lib/usercopy_32.c b/arch/x86/lib/usercopy_32.c
index 8498684..e218d5d 100644
--- a/arch/x86/lib/usercopy_32.c
+++ b/arch/x86/lib/usercopy_32.c
@@ -883,3 +883,9 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
 	return n;
 }
 EXPORT_SYMBOL(_copy_from_user);
+
+void copy_from_user_overflow(void)
+{
+	WARN(1, "Buffer overflow detected!\n");
+}
+EXPORT_SYMBOL(copy_from_user_overflow);
diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index a3aef5d..f1709c1 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -39,3 +39,6 @@
 #endif
 
 #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
+#if __GNUC_MINOR__ >= 4
+#define __compiletime_warning(message) __attribute__((warning(message)))
+#endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 9c42853..241dfd8 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -189,6 +189,10 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 #ifndef __compiletime_object_size
 # define __compiletime_object_size(obj) -1
 #endif
+#ifndef __compiletime_warning
+# define __compiletime_warning(message)
+#endif
+
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler
  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
-- 
1.6.2.5


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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