[tip:x86/asm] x86: Unify copy_from_user() size checking

From: tip-bot for Jan Beulich
Date: Sat Oct 26 2013 - 09:51:46 EST

Commit-ID: 3df7b41aa5e7797f391d0a41f8b0dce1fe366a09
Gitweb: http://git.kernel.org/tip/3df7b41aa5e7797f391d0a41f8b0dce1fe366a09
Author: Jan Beulich <JBeulich@xxxxxxxx>
AuthorDate: Mon, 21 Oct 2013 09:43:57 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 26 Oct 2013 12:27:36 +0200

x86: Unify copy_from_user() size checking

Commits 4a3127693001c61a21d1ce680db6340623f52e93 ("x86: Turn the
copy_from_user check into an (optional) compile time warning")
and 63312b6a6faae3f2e5577f2b001e3b504f10a2aa ("x86: Add a
Kconfig option to turn the copy_from_user warnings into errors")
touched only the 32-bit variant of copy_from_user(), whereas the
original commit 9f0cf4adb6aa0bfccf675c938124e68f7f06349d ("x86:
Use __builtin_object_size() to validate the buffer size for
copy_from_user()") also added the same code to the 64-bit one.

Further the earlier conversion from an inline WARN() to the call
to copy_from_user_overflow() went a little too far: When the
number of bytes to be copied is not a constant (e.g. [looking at
3.11] in drivers/net/tun.c:__tun_chr_ioctl() or
drivers/pci/pcie/aer/aer_inject.c:aer_inject_write()), the
compiler will always have to keep the funtion call, and hence
there will always be a warning. By using __builtin_constant_p()
we can avoid this.

And then this slightly extends the effect of
converting warnings to errors in the constant size case, it
retains the (possibly wrong) warnings in the non-constant size
case, such that if someone is prepared to get a few false
positives, (s)he'll be able to recover the current behavior
(except that these diagnostics now will never be converted to

Since the 32-bit variant (intentionally) didn't call
might_fault(), the unification results in this being called
twice now. Adding a suitable #ifdef would be the alternative if
that's a problem.

I'd like to point out though that with
__compiletime_object_size() being restricted to gcc before 4.6,
the whole construct is going to become more and more pointless
going forward. I would question however that commit
2fb0815c9ee6b9ac50e15dd8360ec76d9fa46a2 ("gcc4: disable
__compiletime_object_size for GCC 4.6+") was really necessary,
and instead this should have been dealt with as is done here
from the beginning.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/5265056D02000078000FC4F3@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/include/asm/uaccess.h | 68 +++++++++++++++++++++++++++++++++++++++
arch/x86/include/asm/uaccess_32.h | 26 ---------------
arch/x86/include/asm/uaccess_64.h | 18 -----------
arch/x86/lib/usercopy_32.c | 3 +-
4 files changed, 69 insertions(+), 46 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 5838fa9..c9799ed 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -542,5 +542,73 @@ extern struct movsl_mask {
# include <asm/uaccess_64.h>

+unsigned long __must_check _copy_from_user(void *to, const void __user *from,
+ unsigned n);
+# define copy_user_diag __compiletime_error
+# define copy_user_diag __compiletime_warning
+extern void copy_user_diag("copy_from_user() buffer size is too small")
+#undef copy_user_diag
+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()
+static inline void
+__copy_from_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, unsigned long n)
+ int sz = __compiletime_object_size(to);
+ might_fault();
+ /*
+ * 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))
+ n = _copy_from_user(to, from, n);
+ else if(__builtin_constant_p(n))
+ copy_from_user_overflow();
+ else
+ __copy_from_user_overflow(sz, n);
+ return n;
+#undef __copy_from_user_overflow
#endif /* _ASM_X86_UACCESS_H */

diff --git a/arch/x86/include/asm/uaccess_32.h b/arch/x86/include/asm/uaccess_32.h
index 7f760a9..c348c87 100644
--- a/arch/x86/include/asm/uaccess_32.h
+++ b/arch/x86/include/asm/uaccess_32.h
@@ -186,31 +186,5 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,

unsigned long __must_check copy_to_user(void __user *to,
const void *from, unsigned long n);
-unsigned long __must_check _copy_from_user(void *to,
- const void __user *from,
- unsigned long n);
-extern void copy_from_user_overflow(void)
- __compiletime_error("copy_from_user() buffer size is not provably correct")
- __compiletime_warning("copy_from_user() buffer size is not provably correct")
-static inline unsigned long __must_check copy_from_user(void *to,
- const void __user *from,
- unsigned long n)
- int sz = __compiletime_object_size(to);
- if (likely(sz == -1 || sz >= n))
- n = _copy_from_user(to, from, n);
- else
- copy_from_user_overflow();
- return n;

#endif /* _ASM_X86_UACCESS_32_H */
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 4f7923d..4df93c4 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -48,26 +48,8 @@ copy_user_generic(void *to, const void *from, unsigned len)
__must_check unsigned long
_copy_to_user(void __user *to, const void *from, unsigned len);
__must_check unsigned long
-_copy_from_user(void *to, const void __user *from, unsigned len);
-__must_check unsigned long
copy_in_user(void __user *to, const void __user *from, unsigned len);

-static inline unsigned long __must_check copy_from_user(void *to,
- const void __user *from,
- unsigned long n)
- int sz = __compiletime_object_size(to);
- might_fault();
- if (likely(sz == -1 || sz >= n))
- n = _copy_from_user(to, from, n);
- else
- WARN(1, "Buffer overflow detected!\n");
- return n;
static __always_inline __must_check
int copy_to_user(void __user *dst, const void *src, unsigned size)
diff --git a/arch/x86/lib/usercopy_32.c b/arch/x86/lib/usercopy_32.c
index 3eb18ac..c408d02 100644
--- a/arch/x86/lib/usercopy_32.c
+++ b/arch/x86/lib/usercopy_32.c
@@ -679,8 +679,7 @@ EXPORT_SYMBOL(copy_to_user);
* If some data could not be copied, this function will pad the copied
* data to the requested size using zero bytes.
-unsigned long
-_copy_from_user(void *to, const void __user *from, unsigned long n)
+unsigned long _copy_from_user(void *to, const void __user *from, unsigned n)
if (access_ok(VERIFY_READ, from, n))
n = __copy_from_user(to, from, n);
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