[PATCH 3.18 22/36] sysctl: enable strict writes

Greg Kroah-Hartman
2017-07-03T13:35:12Z

From: Kees Cook <keescook@chromium.org>

commit 41662f5cc55335807d39404371cfcbb1909304c4 upstream.

SYSCTL_WRITES_WARN was added in commit f4aacea2f5d1 ("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>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
Documentation/sysctl/kernel.txt | 15 +++++++--------
kernel/sysctl.c | 2 +-
2 files changed, 8 insertions(+), 9 deletions(-)

--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -775,14 +775,13 @@ 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.

==============================================================

--- 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);