Describe CVE-2021-46997.
What is CVE-2021-46997?
In the Linux kernel, the following vulnerability has been resolved:

arm64: entry: always set GIC_PRIO_PSR_I_SET during entry

Zenghui reports that booting a kernel with "irqchip.gicv3_pseudo_nmi=1"
on the command line hits a warning during kernel entry, due to the way
we manipulate the PMR.

Early in the entry sequence, we call lockdep_hardirqs_off() to inform
lockdep that interrupts have been masked (as the HW sets DAIF wqhen
entering an exception). Architecturally PMR_EL1 is not affected by
exception entry, and we don't set GIC_PRIO_PSR_I_SET in the PMR early in
the exception entry sequence, so early in exception entry the PMR can
indicate that interrupts are unmasked even though they are masked by
DAIF.

If DEBUG_LOCKDEP is selected, lockdep_hardirqs_off() will check that
interrupts are masked, before we set GIC_PRIO_PSR_I_SET in any of the
exception entry paths, and hence lockdep_hardirqs_off() will WARN() that
something is amiss.

We can avoid this by consistently setting GIC_PRIO_PSR_I_SET during
exception entry so that kernel code sees a consistent environment. We
must also update local_daif_inherit() to undo this, as currently only
touches DAIF. For other paths, local_daif_restore() will update both
DAIF and the PMR. With this done, we can remove the existing special
cases which set this later in the entry code.

We always use (GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET) for consistency with
local_daif_save(), as this will warn if it ever encounters
(GIC_PRIO_IRQOFF | GIC_PRIO_PSR_I_SET), and never sets this itself. This
matches the gic_prio_kentry_setup that we have to retain for
ret_to_user.

The original splat from Zenghui's report was:

| DEBUG_LOCKS_WARN_ON(!irqs_disabled())
| WARNING: CPU: 3 PID: 125 at kernel/locking/lockdep.c:4258 lockdep_hardirqs_off+0xd4/0xe8
| Modules linked in:
| CPU: 3 PID: 125 Comm: modprobe Tainted: G        W         5.12.0-rc8+ #463
| Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
| pstate: 604003c5 (nZCv DAIF +PAN -UAO -TCO BTYPE=--)
| pc : lockdep_hardirqs_off+0xd4/0xe8
| lr : lockdep_hardirqs_off+0xd4/0xe8
| sp : ffff80002a39bad0
| pmr_save: 000000e0
| x29: ffff80002a39bad0 x28: ffff0000de214bc0
| x27: ffff0000de1c0400 x26: 000000000049b328
| x25: 0000000000406f30 x24: ffff0000de1c00a0
| x23: 0000000020400005 x22: ffff8000105f747c
| x21: 0000000096000044 x20: 0000000000498ef9
| x19: ffff80002a39bc88 x18: ffffffffffffffff
| x17: 0000000000000000 x16: ffff800011c61eb0
| x15: ffff800011700a88 x14: 0720072007200720
| x13: 0720072007200720 x12: 0720072007200720
| x11: 0720072007200720 x10: 0720072007200720
| x9 : ffff80002a39bad0 x8 : ffff80002a39bad0
| x7 : ffff8000119f0800 x6 : c0000000ffff7fff
| x5 : ffff8000119f07a8 x4 : 0000000000000001
| x3 : 9bcdab23f2432800 x2 : ffff800011730538
| x1 : 9bcdab23f2432800 x0 : 0000000000000000
| Call trace:
|  lockdep_hardirqs_off+0xd4/0xe8
|  enter_from_kernel_mode.isra.5+0x7c/0xa8
|  el1_abort+0x24/0x100
|  el1_sync_handler+0x80/0xd0
|  el1_sync+0x6c/0x100
|  __arch_clear_user+0xc/0x90
|  load_elf_binary+0x9fc/0x1450
|  bprm_execve+0x404/0x880
|  kernel_execve+0x180/0x188
|  call_usermodehelper_exec_async+0xdc/0x158
|  ret_from_fork+0x10/0x18.