Re: [PATCH] selinux: fix off-by-one in setprocattr

[Paul Moore <paul@paul-moore.com>]

On Tue, Jan 31, 2017 at 11:54 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> SELinux tries to support setting/clearing of /proc/pid/attr attributes
> from the shell by ignoring terminating newlines and treating an
> attribute value that begins with a NUL or newline as an attempt to
> clear the attribute.  However, the test for clearing attributes has
> always been wrong; it has an off-by-one error, and this could further
> lead to reading past the end of the allocated buffer since commit
> bb646cdb12e75d82258c2f2e7746d5952d3e321a ("proc_pid_attr_write():
> switch to memdup_user()").  Fix the off-by-one error.
>
> Even with this fix, setting and clearing /proc/pid/attr attributes
> from the shell is not straightforward since the interface does not
> support multiple write() calls (so shells that write the value and
> newline separately will set and then immediately clear the attribute,
> requiring use of echo -n to set the attribute), whereas trying to use
> echo -n "" to clear the attribute causes the shell to skip the
> write() call altogether since POSIX says that a zero-length write
> causes no side effects. Thus, one must use echo -n to set and echo
> without -n to clear, as in the following example:
> $ echo -n unconfined_u:object_r:user_home_t:s0 > /proc/$$/attr/fscreate
> $ cat /proc/$$/attr/fscreate
> unconfined_u:object_r:user_home_t:s0
> $ echo "" > /proc/$$/attr/fscreate
> $ cat /proc/$$/attr/fscreate
>
> Note the use of /proc/$$ rather than /proc/self, as otherwise
> the cat command will read its own attribute value, not that of the shell.
>
> There are no users of this facility to my knowledge; possibly we
> should just get rid of it.
>
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> ---
>  security/selinux/hooks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Thanks, merged into selinux/stable-4.10 with the following added to
the end of the commit description.  I'll be sending this to James as
soon as my test kernel finishes building.

"UPDATE: Upon further investigation it appears that a local process
with the process:setfscreate permission can cause a kernel panic as a
result of this bug.  This patch fixes CVE-2017-2618."

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index a5398fe..6a047bf 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -5882,7 +5882,7 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
>                 return error;
>
>         /* Obtain a SID for the context, if one was specified. */
> -       if (size && str[1] && str[1] != '\n') {
> +       if (size && str[0] && str[0] != '\n') {
>                 if (str[size-1] == '\n') {
>                         str[size-1] = 0;
>                         size--;
> --
> 2.7.4
>

-- 
paul moore
www.paul-moore.com