[3.9] gh-95778: CVE-2020-10735: Prevent DoS by very large int() #96502
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Co-authored-by: Christian Heimes <christian@python.org>
Ned pointed this out on the 3.7 review, it matches other patch changes and stands out better.
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I've been using the opening message text for the PR as the commit message when merging. The automatic one is gross. |
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wait for #96537 to be integrated into this PR before merging, i'll remove the do-not-merge label then. |
Per mdickinson@'s comment on the main branch PR.
…#96537) Converting a large enough `int` to a decimal string raises `ValueError` as expected. However, the raise comes _after_ the quadratic-time base-conversion algorithm has run to completion. For effective DOS prevention, we need some kind of check before entering the quadratic-time loop. Oops! =) The quick fix: essentially we catch _most_ values that exceed the threshold up front. Those that slip through will still be on the small side (read: sufficiently fast), and will get caught by the existing check so that the limit remains exact. The justification for the current check. The C code check is: ```c max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10 ``` In GitHub markdown math-speak, writing $M$ for `max_str_digits`, $L$ for `PyLong_SHIFT` and $s$ for `size_a`, that check is: $$\left\lfloor\frac{M}{3L}\right\rfloor \le \left\lfloor\frac{s - 11}{10}\right\rfloor$$ From this it follows that $$\frac{M}{3L} < \frac{s-1}{10}$$ hence that $$\frac{L(s-1)}{M} > \frac{10}{3} > \log_2(10).$$ So $$2^{L(s-1)} > 10^M.$$ But our input integer $a$ satisfies $|a| \ge 2^{L(s-1)}$, so $|a|$ is larger than $10^M$. This shows that we don't accidentally capture anything _below_ the intended limit in the check. <!-- gh-issue-number: pythongh-95778 --> * Issue: pythongh-95778 <!-- /gh-issue-number --> Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org>
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@tiran can wasm32 failures be ignored here? This is a 3.9 backport and that version is not supported on WASM? Those fail to build with errors like |
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The FeeBSD Shared PR failure is unrelated. It's an openssl test failure due to trying to use a protocol that is not supported on the machine: In GH-95312 we did backport test changes that were supposed to fix this but apparently not. This is to be followed up separately. |
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Integer to and from text conversions via CPython's bignum
inttype is not safe against denial of service attacks due to malicious input. Very large input strings with hundred thousands of digits can consume several CPU seconds.This PR comes fresh from a pile of work done in our private PSRT security response team repo.
This backports #96499 aka 511ca94
Signed-off-by: Christian Heimes [Red Hat] christian@python.org
Tons-of-polishing-up-by: Gregory P. Smith [Google] greg@krypto.org
Reviews via the private PSRT repo via many others (see the NEWS entry in the PR).
I wrote up a one pager for the release managers.