Re: A possible pointer_overflow in xen-4.13

[=?gb18030?B?UnJvYWNo?= <2284696125@qq.com>]

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After patching it, this works fine and UBSAN dose not have any error report about it.


------------------ Original ------------------
From: &nbsp;"Andrew Cooper";<andrew.cooper3@citrix.com&gt;;
Send time:&nbsp;Saturday, Jun 26, 2021 9:50 PM
To:&nbsp;"Rroach"<2284696125@qq.com&gt;; "xen-devel"<xen-devel@lists.xenproject.org&gt;; 

Subject: &nbsp;Re: A possible pointer_overflow in xen-4.13



           On 26/06/2021 14:29, Rroach wrote:
     
                              Hi, I compile Xen-4.13 with CONFIG_UBSAN, and try test             it. However, during testing, xl dmesg got the output as             shown below.
           
           
           It seems that there is a potential pointer overflow             within arch/x86/pv/emul-priv-op.c:131 where xen try to             execute instruction ''' APPEND_CALL(save_guest_gprs)             '''£¬where APPEND_CALL try to add an offset on *p without             proper checking.
           
           
           I compiled xen-4.13 by clang-9, with following             instructions: ''' export CONFIG_UBSAN=y ''' &amp;&amp; '''             make clang=y debug=y ''' . Do you have any idea what going             on here?
         
          
     You say Xen 4.13, but APPEND_CALL() doesn't exist       there.&nbsp; I added it in 4.14 when I rewrote this mess to be       compatible with CET by not using a ROP gadget.&nbsp; Your backtrace       says 4.15 unstable which means its an old staging build (not that       that is going to have any effect on this specific issue).
       
       The fact that it continued executing correctly means the       calculation did the right thing, whether or not UBSAN was happy.        The displacement will end up negative as the stub we're writing is       numerically higher than {load,save}_guest_gprs(), which I guess       means that f - stub_va will underflow.
       
       I'm very confused as to why UBSAN reports against       save_guest_gprs() considering that load_guest_gprs() when through       the exact same logic a few instructions earlier.
       
       Either way, does this make the problem go away?
       
       diff --git a/xen/arch/x86/pv/emul-priv-op.c       b/xen/arch/x86/pv/emul-priv-op.c
       index 11467a1e3a..be41bced76 100644
       --- a/xen/arch/x86/pv/emul-priv-op.c
       +++ b/xen/arch/x86/pv/emul-priv-op.c
       @@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct       priv_op_ctxt *ctxt, u8 opcode,
       &nbsp;#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p +=       sizeof(b); })
       &nbsp;#define       APPEND_CALL(f)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
       &nbsp;&nbsp;&nbsp;        ({&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;        \
       -&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; long disp = (long)(f) - (stub_va + p -       ctxt-&gt;io_emul_stub + 5); \
       +&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; long disp = (long)(f) - (long)(stub_va + p -       ctxt-&gt;io_emul_stub + 5); \
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; BUG_ON((int32_t)disp !=       disp);&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; *p++ =       0xe8;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; *(int32_t *)p = disp; p +=       4;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
       
       ~Andrew

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