[Backport] CVE-2024-12693: Out of bounds memory access in V8

Cherry-pick of patch originally reviewed on
https://chromium-review.googlesource.com/c/v8/v8/+/6084686:
Merged: [maglev] Avoid retagging loop phi backedges too early

When we decide that a loop phi should remain tagged, we call
EnsurePhiInputsTagged to ensures that it only has tagged inputs, which
calls EnsurePhiTagged, which might cause retagging of any untagged
phi it has as input.

In order to avoid retagging multiple times the same Phi, we have a
SnaphotTable (`phi_taggings_`), which records existing tagging in the
predecessors, and in which EnsurePhiTagged looks to avoid creating
new retagging nodes. For loop phis, the backedge predecessor won't
have an entry yet in this SnapshotTable (since we only visit loops
once, this has to be the first time we visit the header and thus
we can't have already visited the backedge block), and we should
thus not call EnsurePhiTagged on the backedge.

Note that the backedge input will anyways be properly tagged when
FixLoopPhisBackedge is later called from the JumpLoop backedge.

Fixed: chromium:382190919
(cherry picked from commit e4ecfc909687511aeb20b88ce6ae2a7a1a80afe5)

Change-Id: Ib24f311cb443eabe278f537c00bbc3274bf82415
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6084686
Auto-Submit: Olivier Flückiger <[email protected]>
Commit-Queue: Olivier Flückiger <[email protected]>
Commit-Queue: Camillo Bruni <[email protected]>
Reviewed-by: Camillo Bruni <[email protected]>
Cr-Commit-Position: refs/branch-heads/13.0@{#41}
Cr-Branched-From: 4be854bd71ea878a25b236a27afcecffa2e29360-refs/heads/13.0.245@{#1}
Cr-Branched-From: 1f5183f7ad6cca21029fd60653d075730c644432-refs/heads/main@{#96103}
Reviewed-on: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/615712
Reviewed-by: Anu Aliyas <[email protected]>

Author: DadaIsCrazy

chromium/v8/src/maglev/maglev-phi-representation-selector.cc

@@ -327,7 +327,8 @@ void MaglevPhiRepresentationSelector::ConvertTaggedPhiTo(
   // registers to floating registers.
   phi->InitializeRegisterData();
 
-  for (int i = 0; i < phi->input_count(); i++) {
+  const int skip_backedge = phi->is_loop_phi() ? 1 : 0;
+  for (int i = 0; i < phi->input_count() - skip_backedge; i++) {
     ValueNode* input = phi->input(i).node();
 #define TRACE_INPUT_LABEL \
   "    @ Input " << i << " (" << PrintNodeLabel(graph_labeller(), input) << ")"