CVE-2026-46316: KVM: arm64: vgic-its: Drop the translation cache reference only for the erased entry

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: vgic-its: Drop the translation cache reference only for the erased entry

vgicitsinvalidatecache() walks the per-ITS translation cache with xaforeach() and drops the cache's reference on each entry with vgicputirq(). It puts the iterated pointer, though, rather than the value returned by xaerase().

The function is called from contexts that do not exclude one another: the ITS command handlers hold itslock, the GITSCTLR write path holds cmdlock, and the path that clears EnableLPIs in a redistributor's GICRCTLR holds neither. Two or more of them can drain the same cache concurrently, and if each one observes the same entry, erases it and then puts it, the single reference the cache holds on that entry is dropped more than once. The entry can then be freed while an ITE still maps it.

xaerase() is atomic and returns the previous entry, so put only the entry that this context actually removed. The cache reference is then dropped exactly once per entry even when the invalidations run concurrently, and the behavior is unchanged when only one context runs.