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GeezerVison is showing github.com — [3.9] bpo-40791: Make compare_digest more constant-time. (GH-20444) by miss-islington · Pull Request #23436 · python/cpython

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@miss-islington

@miss-islington miss-islington commented Nov 21, 2020 •
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  • bpo-40791: Make compare_digest more constant-time.

The existing volatile left/right pointers guarantee that the reads will all occur, but does not guarantee that they will be used. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between result and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.

(This is change GH-1 from https://bugs.python.org/issue40791 .)
(cherry picked from commit 3172936)

Co-authored-by: Devin Jeanpierre [email protected]

https://bugs.python.org/issue40791

Automerge-Triggered-By: GH:gpshead

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@ssbr @miss-islington
bpo-40791: Make compare_digest more constant-time. (pythonGH-20444)
0e4ccb3 
* bpo-40791: Make compare_digest more constant-time.

The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.

(This is change GH-1 from https://bugs.python.org/issue40791 .)
(cherry picked from commit 3172936)

Co-authored-by: Devin Jeanpierre <[email protected]>
@the-knights-who-say-ni the-knights-who-say-ni added the CLA signed label Nov 21, 2020
@bedevere-bot bedevere-bot added type-bug An unexpected behavior, bug, or error type-security A security issue labels Nov 21, 2020
@bedevere-bot bedevere-bot mentioned this pull request Nov 21, 2020
bpo-40791: Make compare_digest more constant-time. #20444
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@bedevere-bot bedevere-bot added the awaiting review label Nov 21, 2020
gpshead
gpshead approved these changes Nov 21, 2020
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@bedevere-bot bedevere-bot added awaiting merge and removed awaiting review labels Nov 21, 2020
@gpshead gpshead added the 🤖 automerge label Nov 21, 2020
@miss-islington

miss-islington commented Nov 21, 2020

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@ssbr and @gpshead: Status check is done, and it's a success ✅ .
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@miss-islington
miss-islington merged commit c1bbca5 into python:3.9 Nov 21, 2020
@bedevere-bot bedevere-bot removed the awaiting merge label Nov 21, 2020
@miss-islington
miss-islington deleted the backport-3172936-3.9 branch November 21, 2020 09:18
@miss-islington

miss-islington commented Nov 21, 2020

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@ssbr and @gpshead: Status check is done, and it's a success ✅ .
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@ssbr ssbr mannequin mentioned this pull request Aug 24, 2023
hmac.compare_digest could try harder to be constant-time. #84968
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@miss-islington @gpshead @the-knights-who-say-ni @bedevere-bot @ssbr
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