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<p class="MsoNormal">I would like to write a checker to make sure that pointers from untrusted sources are not dereferenced. So I am playing around with the alpha.security.taint checker to try to understand how taint propagation works. I put together some
simple test cases:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img width="518" height="596" id="Picture_x0020_1" src="cid:image001.png@01D28604.F242C920"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The indicated calls to printf should be flagged because an untrusted value is passed as the format string (the first argument). When I run the alpha.security.taint checker on this, it only flags the last two. Looking into this, I found
that the static analyzer often assigns an unknown value to the result of casting from an integer to a pointer. Taint can’t be attached to an unknown value, so I need a known value for the pointer. Although I’m not sure if this is really what we want to do,
for now I added a post-check call-back on cast expressions to my checker that casts the integer to unsigned and then assigns the result as the pointer value.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img width="760" height="226" id="Picture_x0020_2" src="cid:image002.png@01D28605.94CB8510"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">When I run alpha.security.taint together with this on the test file, it flags the calls to printf where (char *)l and (char *)u are passed, but not the calls where sv1, sp1, (char *)NULL+l, sv2, or sp2 are passed, even though these are
just different expressions with the same value. I tried looking into this using the debugger, and found that when the analyzer calls ExprEngine::evalStore for the assignment statement (say “sv1 = (char *)ll”), it gives the pointer the appropriate value, but
when the analyzer calls GenericTaintChecker::checkUncontrolledFormatString from the pre-check call-back for the call to printf on the following line of the test file, the pointer has an UnknownVal. (See excepts from the debugging session below.)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Can anyone help me understand what is going on here?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Ray<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img width="911" height="316" id="Picture_x0020_4" src="cid:image003.png@01D2861A.EB785810"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img width="886" height="438" id="Picture_x0020_5" src="cid:image004.png@01D2861B.4E6FBB70"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><img width="924" height="730" id="Picture_x0020_6" src="cid:image005.png@01D2861C.95CDE270"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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