[PATCH] sysctl: enable strict writes

[Kees Cook <keescook@chromium.org>]

SYSCTL_WRITES_WARN was added in f4aacea2f5d1a ("sysctl: allow for strict
write position handling"), and released in v3.16 in August of 2014. Since
then I can find only 1 instance of non-zero offset writing[1], and it
was fixed immediately in CRIU[2]. As such, it appears safe to flip this
to the strict state now.

[1] https://www.google.com/search?q="when%20file%20position%20was%20not%200"
[2] http://lists.openvz.org/pipermail/criu/2015-April/019819.html

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 Documentation/sysctl/kernel.txt | 15 +++++++--------
 kernel/sysctl.c                 |  2 +-
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index af70d1541d3a..be61d53e997f 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -810,14 +810,13 @@ via the /proc/sys interface:
        Each write syscall must fully contain the sysctl value to be
        written, and multiple writes on the same sysctl file descriptor
        will rewrite the sysctl value, regardless of file position.
-   0 - (default) Same behavior as above, but warn about processes that
-       perform writes to a sysctl file descriptor when the file position
-       is not 0.
-   1 - Respect file position when writing sysctl strings. Multiple writes
-       will append to the sysctl value buffer. Anything past the max length
-       of the sysctl value buffer will be ignored. Writes to numeric sysctl
-       entries must always be at file position 0 and the value must be
-       fully contained in the buffer sent in the write syscall.
+   0 - Same behavior as above, but warn about processes that perform writes
+       to a sysctl file descriptor when the file position is not 0.
+   1 - (default) Respect file position when writing sysctl strings. Multiple
+       writes will append to the sysctl value buffer. Anything past the max
+       length of the sysctl value buffer will be ignored. Writes to numeric
+       sysctl entries must always be at file position 0 and the value must
+       be fully contained in the buffer sent in the write syscall.
 
 ==============================================================
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index dc6858d6639e..081873afba6b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -173,7 +173,7 @@ extern int no_unaligned_warning;
 #define SYSCTL_WRITES_WARN	 0
 #define SYSCTL_WRITES_STRICT	 1
 
-static int sysctl_writes_strict = SYSCTL_WRITES_WARN;
+static int sysctl_writes_strict = SYSCTL_WRITES_STRICT;
 
 static int proc_do_cad_pid(struct ctl_table *table, int write,
 		  void __user *buffer, size_t *lenp, loff_t *ppos);
-- 
1.9.1


-- 
Kees Cook
Chrome OS & Brillo Security