ACK/cmnt: [CVE-2017-2618][PATCH T/Y] selinux: fix off-by-one in setprocattr

[Stefan Bader]

On 30.06.2017 09:21, Po-Hsu Lin wrote:
> From: Stephen Smalley <sds at tycho.nsa.gov>
> 
> CVE-2017-2618
> 
> 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.
> 
> 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.
> 
> Signed-off-by: Stephen Smalley <sds at tycho.nsa.gov>
> [PM: added the update about CVE-2017-2618 to the commit description]
> Cc: stable at vger.kernel.org # 3.5: d6ea83ec6864e
> Signed-off-by: Paul Moore <paul at paul-moore.com>
> 
> Signed-off-by: James Morris <james.l.morris at oracle.com>
> (cherry picked from commit 0c461cb727d146c9ef2d3e86214f498b78b7d125)
> 
> Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>

Acked-by: Stefan Bader <stefan.bader at canonical.com>

> ---

While the SHA1 mentioned in the commit message would be 4.5 and for that be
after Trusty, the cc for stable refers to a commit from 3.8 (possibly backported
up to 3.5). So it may or may not have the same impact. But regardless the change
looks sensible and for that I would say it should go into both Trusty and Yakkety.

-Stefan

>  security/selinux/hooks.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 357762a..d2246a8 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -5548,7 +5548,7 @@ static int selinux_setprocattr(struct task_struct *p,
>  		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--;
> 


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