src: keep the first snapshot blob alive for later isolates - #65779
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`NewIsolate()` creates every isolate from the snapshot blob the first isolate in the process used, because V8 shares the read-only heap between isolates, and did so by keeping a pointer to the first `CreateParams`. When that blob came from an `EmbedderSnapshotData` the embedder had since released, e.g. a second `CommonEnvironmentSetup::CreateFromSnapshot()` after the first setup and its snapshot were destroyed, V8 deserialized freed memory. Record the first blob and external references under a mutex instead of copying the caller's `CreateParams`, and make `~SnapshotData()` leave that one blob allocated, since its owner can go away before the last isolate is created. Nothing is copied and `node` itself is unaffected. embedtest grows an `--embedder-run-twice` switch so the sequence can be tested. Refs: nodejs#45885 Signed-off-by: Shelley Vohr <[email protected]>
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #65779 +/- ##
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+ Coverage 89.99% 90.06% +0.06%
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Files 757 769 +12
Lines 257739 261413 +3674
Branches 48881 49632 +751
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+ Hits 231961 235431 +3470
- Misses 16861 17022 +161
- Partials 8917 8960 +43
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V8 shares the read-only heap between isolates, so
NewIsolate()creates every isolate from the snapshot blob that the first isolate in the process used. It did that by stashing a pointer to the firstCreateParamsin a function static (the comment next to it already said "this isn't really memory-safe"). When the first blob came from anEmbedderSnapshotDatathe embedder has since released, which is the natural thing to do after tearing down aCommonEnvironmentSetupcreated withCreateFromSnapshot(), the nextNewIsolate()hands V8 freed memory andSnapshot::Initialize()reads from it. It happens to work on glibc because the pages are still intact; ASAN reports the use-after-free.NewIsolate()now records the first blob and external references under a mutex instead of keeping the caller'sCreateParams, and~SnapshotData()leaves that one blob allocated, since its owner can go away before the last isolate is created. Nothing is copied andnodeitself is unaffected (its snapshot already lives untilTearDownOncePerProcess()). node.h now says thatsnapshot_datahas to outlive the setup and that every setup in a process has to use the same snapshot.embedtestgets an--embedder-run-twiceswitch to exercise two instances in one process.Tests:
test/embedding/test-embedding-snapshot-twice.jsruns a snapshot twice in one embedtest process, freeing theEmbedderSnapshotDatain between; heap-use-after-free inv8::internal::Snapshot::Initializeunder ASAN before, clean after. The rest oftest/embeddingpasses.Refs: #45885
Disclosure: the code, test and this description were written by Claude Code, directed and reviewed by @codebytere.