<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Jan 16, 2013, at 4:58 PM, JF Bastien <<a href="mailto:jfb@google.com">jfb@google.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr"><div class="gmail_default" style="">The sneaky part is indeed our original intent: we want to mitigate mode-switching attacks so we sketched out a mitigation that's kind of like constant blinding, it makes the final code somewhat unpredictable to the attacker. I do think that it's possible to bias instruction selection without incurring an undue performance hit while still messing with known-code creation.</div></div></blockquote><div><br></div><div>With clever help from register allocation, that's quite possible. The low bits are often enough mostly register and immediate values that they can be twiddled creatively. I'm curious what you come up with there. That could be quite interesting!</div><br><blockquote type="cite"><div dir="ltr"><div class="gmail_default" style=""> The one issue we had was that we needed to freeze our ARM ABI, and that implied freezing all the trap/halt/debug instructions that the NaCl "OS" recognizes, hence this change.</div>
<div class="gmail_default" style=""><br></div><div class="gmail_default" style="">The mitigation is something we'll implement later, once PNaCl reaches its performance goals. We've already branched NaCl ARM with a frozen ABI for Chrome M25, and although the current backend is GCC we'll keep the same ABI once we migrate to PNaCl.</div>
<div class="gmail_default" style=""><br></div><div class="gmail_default" style="">In the meantime we'd like to reduce PNaCl's localmods, so while this change on its own is quite silly, we think it's critical as part of a whole system.</div>
<div class="gmail_default" style=""><br></div><div class="gmail_default" style="">Given this, would it make sense to commit this change, once I specialize it for the NaCl ARM target?</div></div><div class="gmail_extra"><br></div></blockquote><div><br></div><div>Yeah, that's fine. I'll probably grumble a bit about the added complexity, but it's the right thing to do. :)</div><div><br></div><div>-Jim</div><br><blockquote type="cite"><div class="gmail_extra"><br>
<div class="gmail_quote">On Wed, Jan 16, 2013 at 4:33 PM, Jim Grosbach <span dir="ltr"><<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word">Hi JF,<div><br></div><div>Assuming there's real security benefits to be had, yes. However, I'm reticent to add complexity to the code on a purely theoretical benefit. Can you elaborate a bit more on why this is worth it?<div>
<br></div><div>In particular, I'm skeptical of benefits to an overlapping ARM/Thumb TRAP instruction. Now, I can definitely see benefit if you could find a way to get ARM ISel to more frequently have Thumb2 undefined bitpatterns in the bitstream (as the low-order bits of normal ARM instructions, that is). That would be quite clever and downright sneaky. Also likely pretty hard to do it w/o completely crushing performance…</div>
<span class="HOEnZb"><font color="#888888"><div><br></div><div>-Jim</div></font></span><div><div class="h5"><br><div><div>On Jan 16, 2013, at 4:27 PM, JF Bastien <<a href="mailto:jfb@google.com" target="_blank">jfb@google.com</a>> wrote:</div>
<br><blockquote type="cite"><div dir="ltr"><div class="gmail_default">As discussed over IRC: it then makes sense to only change the encoding for the NaCl triple (which is effectively its own OS).</div></div><div class="gmail_extra">
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On Wed, Jan 16, 2013 at 4:17 PM, Jim Grosbach <span dir="ltr"><<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word"><br><div><div><div>On Jan 16, 2013, at 3:45 PM, Renato Golin Linaro <<a href="mailto:renato.golin@linaro.org" target="_blank">renato.golin@linaro.org</a>> wrote:</div><br>
<blockquote type="cite"><div dir="ltr">On 16 January 2013 22:39, Jim Grosbach <span dir="ltr"><<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote">
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<div style="word-wrap:break-word">The entire encoding is semantically significant on Darwin. I suspect that's true on other platforms, too, but I don't know for sure.</div></blockquote><div><br></div><div>I'm not sure either. I agree with Bastien that it *should* trap on both ARM and Thumb, but it also depends on what catch routine is installed and other hard-to-know problems.</div>
<div><br></div></div></div></div></blockquote><div><br></div></div><div>Both instructions will trap; however, how they trap is also important. In this case, it's the difference between the user program terminating with SIGILL vs. SIGTRAP.</div>
<span><font color="#888888"><div><br></div><div>-Jim</div></font></span><div><div><br></div><br><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>Bastien, Have you tested in which platforms?<br>
</div><div><br></div><div>cheers,</div><div>--renato</div><div><br></div><div><br></div></div></div></div>
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