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To: David Howells <dhowells@redhat.com>
From: Hillf Danton <hdanton@sina.com>
Subject: Re: Oops (request_key_auth_describe) while running cve-2016-7042
 from LTP
Date: Fri, 30 Aug 2019 22:54:37 +0800
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Cc: "dhowells@redhat.com" <dhowells@redhat.com>,
 Sachin Sant <sachinp@linux.vnet.ibm.com>,
 "keyrings@vger.kernel.org" <keyrings@vger.kernel.org>,
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David Howells <dhowells@redhat.com> wrote:
>
> Hillf Danton <hdanton@sina.com> wrote:
>
> > -	struct request_key_auth *rka =3D dereference_key_rcu(key);
> > +	struct request_key_auth *rka;
> > +
> > +	rcu_read_lock();
> > +	rka =3D dereference_key_rcu(key);
>
> This shouldn't help as the caller, proc_keys_show(), is holding the RCU r=
ead
> lock across the call.  The end of the function reads:
> and the documentation says "This method will be called with the RCU read =
lock
> held".
>
1, callee has no pre defined duty to help caller in general; they should
not try to do anything, however, to help their callers in principle due to
limited info on their hands IMO.
2, uses of rcu can be nested.
3, no comment can be found in security/keys/request_key_auth.c about
the rcu already documented.
4, the newly added rcu can avoid incidental messup anywhere else.

Hillf

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--></style></head><body lang=3DZH-CN link=3Dblue vlink=3D"#954F72"><div cla=
ss=3DWordSection1><p class=3DMsoNormal><span lang=3DEN-US style=3D'color:bl=
ack'>David Howells &lt;dhowells@redhat.com&gt; wrote:</span><span lang=3DEN=
-US><o:p></o:p></span></p><p class=3DMsoNormal><span lang=3DEN-US>&gt;<o:p>=
&nbsp;</o:p></span></p><p class=3DMsoNormal><span lang=3DEN-US>&gt; Hillf D=
anton &lt;hdanton@sina.com&gt; wrote:</span></p><p class=3DMsoNormal><span =
lang=3DEN-US>&gt;<o:p>&nbsp;</o:p></span></p><p class=3DMsoNormal><span lan=
g=3DEN-US>&gt; &gt; -=C2=A0=C2=A0=C2=A0=C2=A0 struct request_key_auth *rka =
=3D dereference_key_rcu(key);</span></p><p class=3DMsoNormal><span lang=3DE=
N-US>&gt; &gt; +=C2=A0=C2=A0=C2=A0=C2=A0 struct request_key_auth *rka;</spa=
n></p><p class=3DMsoNormal><span lang=3DEN-US>&gt; &gt; +</span></p><p clas=
s=3DMsoNormal><span lang=3DEN-US>&gt; &gt; +=C2=A0=C2=A0=C2=A0=C2=A0 rcu_re=
ad_lock();</span></p><p class=3DMsoNormal><span lang=3DEN-US>&gt; &gt; +=C2=
=A0=C2=A0=C2=A0=C2=A0 rka =3D dereference_key_rcu(key);</span></p><p class=
=3DMsoNormal><span lang=3DEN-US>&gt;<o:p>&nbsp;</o:p></span></p><p class=3D=
MsoNormal><span lang=3DEN-US>&gt; This shouldn't help as the caller, proc_k=
eys_show(), is holding the RCU read</span></p><p class=3DMsoNormal><span la=
ng=3DEN-US>&gt; lock across the call.=C2=A0 The end of the function reads:<=
/span></p><p class=3DMsoNormal><span lang=3DEN-US>&gt; and the documentatio=
n says &quot;This method will be called with the RCU read lock</span></p><p=
 class=3DMsoNormal><span lang=3DEN-US>&gt; held&quot;.</span></p><p class=
=3DMsoNormal><span lang=3DEN-US>&gt;<o:p>&nbsp;</o:p></span></p><p class=3D=
MsoNormal><span lang=3DEN-US>1, callee has no pre defined duty to help call=
er in general; they should</span></p><p class=3DMsoNormal><span lang=3DEN-U=
S>not try to do anything, however, to help their callers in principle due t=
o</span></p><p class=3DMsoNormal><span lang=3DEN-US>limited info on their h=
ands IMO.</span></p><p class=3DMsoNormal><span lang=3DEN-US>2, uses of rcu =
can be nested.</span></p><p class=3DMsoNormal><span lang=3DEN-US>3, no comm=
ent can be found in security/keys/request_key_auth.c about</span></p><p cla=
ss=3DMsoNormal><span lang=3DEN-US>the rcu already documented.</span></p><p =
class=3DMsoNormal><span lang=3DEN-US>4, the newly added rcu can avoid incid=
ental messup anywhere else.</span></p><p class=3DMsoNormal><span lang=3DEN-=
US><o:p>&nbsp;</o:p></span></p><p class=3DMsoNormal><span lang=3DEN-US>Hill=
f<span class=3DDefaultFontHxMailStyle><o:p></o:p></span></span></p></div></=
body></html>=

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