[PATCH v4 0/2] arm64: Clamp ID overrides and gate the GMID_EL1 read

[Fuad Tabba <fuad.tabba@linux.dev>]

Hi folks,

Changes since v3 [1]:
  - Gate on __read_sysreg_by_encoding() and drop the helper's argument,
    so the command-line override is not open-coded in the header.
    (Catalin)
  - Pick up Suzuki's override clamp as patch 1, rather than adding a
    raw-presence check to the gate. (Will)

Patch 2 fixes a GMID_EL1 read that traps to EL2 when the kernel has
disabled MTE, panicking the host under pKVM on a CPU that is offlined
and brought back online. It gates the read on
__read_sysreg_by_encoding().

That needs patch 1, Suzuki's override clamp [2]. Without it
__read_sysreg_by_encoding() applies the command-line override raw, so
id_aa64pfr1.mte=2 on a CPU without FEAT_MTE2 makes the gate true and the
kernel reads a GMID_EL1 that is not there. On the boot CPU that read
comes before init_cpu_ftr_reg() strips the unsafe override, so the gate
cannot rely on it.

Tested on QEMU under pKVM. With -machine virt,mte=on, offlining and
onlining CPU1 with arm64.nomte panics an unpatched v7.2 in
__cpuinfo_store_cpu() and does not with the series. Without tag memory,
where QEMU implements no GMID_EL1, id_aa64pfr1.mte=2 covers the
force-up case.

Based on Linux 7.2 (8d3ae59288f1e).

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260824184155.2644646-1-fuad.tabba@linux.dev/
[2] https://lore.kernel.org/all/afc5bd00-28ca-413b-b047-ee53589c285d@arm.com/

Fuad Tabba (1):
  arm64: Don't read GMID_EL1 when MTE is disabled

Suzuki K Poulose (1):
  arm64: Apply overrides to CPU local capabilities

 arch/arm64/include/asm/cpufeature.h |  9 ++++++
 arch/arm64/kernel/cpufeature.c      | 48 +++++++++++++++++++++--------
 arch/arm64/kernel/cpuinfo.c         |  2 +-
 3 files changed, 46 insertions(+), 13 deletions(-)

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2.39.5