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Nikos Chantziaras <realnc@×××××.com> wrote: |
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> |
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> For example, if you don't trust Firefox, don't install Firefox. But you |
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> *do* trust Firefox. What you don't trust is the JS code Firefox is |
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> executing. |
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That's an artificial distinction, because it is actually firefox |
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which is executing the code during the interpretation of JS. |
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|
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> So Firefox needs protection. |
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|
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The only "protection" firefox could be giving is that interpreted |
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JS is not able to do anything (not access any file or any program |
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outside of a well-defined sandbox). For various reasons, this is |
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not the case and will not be the case in any foreseeable future. |
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|
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What does need protection is all the programs which can be accessed |
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(almost no matter how indirectly) by JS code - which is almost everything |
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running on a typical desktop. |
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|
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Admittedly, firefox can help by e.g. making it harder to use timers |
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for the precise time measurement needed for "simple" spectre exploits. |
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This is already happening, but this is independent of the mitigations |
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provided by the compiler. |
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|
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> I was under the impression that it's the function that performs the call |
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> that needs protection. The called function doesn't need protection, |
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> because if it ends up being actually called, then it's too late already. |
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The opposite is true. There should be no problem for any code (even |
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untrusted) to query gnupg to verify the validity of the signature. |
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But with the spectre exploits, this query might be used to get the |
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secret key which gnupg must access. So it is gnupg which needs the |
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protection (unless you want to make it much harder to use gnupg). |
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|
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> For example, if sandboxed, untrusted code wants to speculatively execute |
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> a memcpy(), then the sandbox would need to call it on behalf of the |
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> untrusted code. |
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|
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In my above example, it is gnupg which speculatively executes some code |
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if queried e.g. to verify a signature. The "attacking" program then might |
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use this fact (under some conditions) to understand which secret key was |
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used. |
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Everything which untrusted code can influence (no matter how indirectly) |
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is possible subject to be exploited if it uses speculative execution. |
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Therefore, the only way to be on the safe side is to avoid speculative |
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execution altogether. |
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|
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> But if the sandbox is protected, the memcpy() call would |
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> never be made speculatively, since retpoline will trap it. |
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|
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This means that the sandbox-code itself is protected from being |
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"read" by other code. But already if e.g. a webpage uses libjpeg to |
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render a picture (even if it would do it inside of an otherwise |
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protected sandbox), and libjpeg uses speculative execution (which |
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it does if you do not have compiled it with the new compiler switches), |
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javascript might (theoretically) be used to get image data it normally |
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has no access to. |
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Admittedly, this is all very theoretical and whether it is |
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actually possible to write working exploits is a different story |
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(that's why I wrote "(under some conditions)" and "(theoretically)"): |
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Not every speculative execution actually _can_ be exploited, |
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but there is always a certain probability (the more direct the |
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victim program can be accessed by the thief code, the higher the |
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probability is). |
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