linux-kernel - [PATCH rdma-next v4 1/3] overflow.h: Add arithmetic shift helper

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20180801212541.22889-2-keescook@chromium.org>
Date:   Wed,  1 Aug 2018 14:25:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Leon Romanovsky <leon@...nel.org>,
        Bart Van Assche <Bart.VanAssche@....com>,
        Doug Ledford <dledford@...hat.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH rdma-next v4 1/3] overflow.h: Add arithmetic shift helper

From: Jason Gunthorpe <jgg@...lanox.com>

Add shift_overflow() helper to assist driver authors in ensuring that
shift operations don't cause overflows or other odd conditions.

Signed-off-by: Jason Gunthorpe <jgg@...lanox.com>
Signed-off-by: Leon Romanovsky <leonro@...lanox.com>
[kees: tweaked comments and commit log, dropped unneeded assignment]
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/linux/overflow.h | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 8712ff70995f..40b48e2133cb 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -202,6 +202,37 @@
 
 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 
+/** check_shl_overflow() - Calculate a left-shifted value and check overflow
+ *
+ * @a: Value to be shifted
+ * @s: How many bits left to shift
+ * @d: Pointer to where to store the result
+ *
+ * Computes *@d = (@a << @s)
+ *
+ * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
+ * make sense. Example conditions:
+ * - 'a << s' causes bits to be lost when stored in *d.
+ * - 's' is garbage (e.g. negative) or so large that the result of
+ *   'a << s' is guaranteed to be 0.
+ * - 'a' is negative.
+ * - 'a << s' sets the sign bit, if any, in '*d'.
+ *
+ * '*d' will hold the results of the attempted shift, but is not
+ * considered "safe for use" if false is returned.
+ */
+#define check_shl_overflow(a, s, d) ({					\
+	typeof(a) _a = a;						\
+	typeof(s) _s = s;						\
+	typeof(d) _d = d;						\
+	u64 _a_full = _a;						\
+	unsigned int _to_shift =					\
+		_s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;		\
+	*_d = (_a_full << _to_shift);					\
+	(_to_shift != _s || *_d < 0 || _a < 0 ||			\
+		(*_d >> _to_shift) != _a);				\
+})
+
 /**
  * array_size() - Calculate size of 2-dimensional array.
  *
-- 
2.17.1