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lib: harden strncpy_from_user

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The strncpy_from_user() accessor is effectively a copy_from_user()
specialised to copy strings, terminating early at a NUL byte if possible.
In other respects it is identical, and can be used to copy an arbitrarily
large buffer from userspace into the kernel.  Conceptually, it exposes a
similar attack surface.

As with copy_from_user(), we check the destination range when the kernel
is built with KASAN, but unlike copy_from_user() we do not check the
destination buffer when using HARDENED_USERCOPY.  As strncpy_from_user()
calls get_user() in a loop, we must call check_object_size() explicitly.

This patch adds this instrumentation to strncpy_from_user(), per the same
rationale as with the regular copy_from_user().  In the absence of
hardened usercopy this will have no impact as the instrumentation expands
to an empty static inline function.

Link: http://lkml.kernel.org/r/1472221903-31181-1-git-send-email-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mark Rutland 2016-10-11 13:51:27 -07:00 committed by Linus Torvalds
parent e0176a2f1e
commit bf90e56e46
1 changed files with 2 additions and 0 deletions
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2
lib/strncpy_from_user.c
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@ -1,6 +1,7 @@
#include <linux/compiler.h> #include <linux/compiler.h>
#include <linux/export.h> #include <linux/export.h>
#include <linux/kasan-checks.h> #include <linux/kasan-checks.h>
#include <linux/thread_info.h>
#include <linux/uaccess.h> #include <linux/uaccess.h>
#include <linux/kernel.h> #include <linux/kernel.h>
#include <linux/errno.h> #include <linux/errno.h>
@ -111,6 +112,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
long retval; long retval;
kasan_check_write(dst, count); kasan_check_write(dst, count);
check_object_size(dst, count, false);
user_access_begin(); user_access_begin();
retval = do_strncpy_from_user(dst, src, count, max); retval = do_strncpy_from_user(dst, src, count, max);
user_access_end(); user_access_end();