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

commit 13031fb6b8357fbbcded2a7f4cba73e4781ee594 upstream.

vgic_its_invalidate_cache() walks the per-ITS translation cache with
xa_for_each() and drops the cache's reference on each entry with
vgic_put_irq(). It puts the iterated pointer, though, rather than the
value returned by xa_erase().

The function is called from contexts that do not exclude one another: the
ITS command handlers hold its_lock, the GITS_CTLR write path holds
cmd_lock, and the path that clears EnableLPIs in a redistributor's
GICR_CTLR 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.

xa_erase() 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.

BUG=b/522218744
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-46316 in the Linux kernel.

cos-patch: security-moderate
Fixes: 8201d1028caa ("KVM: arm64: vgic-its: Maintain a translation cache per ITS")
Change-Id: I7b2b0a6b6e858529c92e0665cb9ac09026a14670
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/ah2c5lu4JbUg7dj-@v4bel
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kevin Berry <kpberry@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/159164
Reviewed-by: Daniel Velasquez <rdvelasquez@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>