Re: [PATCH v2 0/3] sparc64: fix a window fill fixup lockup and two fault bugs

[Andreas Larsson <andreas@gaisler.com>]

On 2026-10-07 10:53, Stian Halseth wrote:
> A register window fill that faults is mishandled in three ways on
> sparc64.  The first wedges the CPU in kernel mode and takes the machine
> with it; the other two corrupt what the kernel reports about the fault,
> and were found while chasing the first.
> 
> 1/3 is the lockup.  user_rtt_fill_fixup_common() re-enters the kernel
> without passing through etrap, so %asi is left as the ASI_AIUP that
> rtrap set for the fill while the primary context has been restored to
> the kernel's.  Every subsequent %asi-based user access then translates
> in the kernel context, and a user address below the VA hole can never be
> resolved: the fault repeats forever, the task survives SIGKILL, and RCU
> eventually reports the CPU stalled.
> 
> The reproducer in the v1 cover letter moves %sp onto a page, flushes the
> register windows, mprotects that page PROT_NONE and takes a signal, so
> the fill on the signal return path faults.  It needs no unusual
> configuration and wedges a stock kernel in a few seconds; mind where you
> run it, as the machine needs a hard reset afterwards.  On a fixed kernel
> it prints PASS and exits.
> 
> 2/3 and 3/3 came out of that investigation.  do_fault_siginfo() decodes
> the instruction at regs->tpc to compute si_addr, which is not the
> faulting access when the fault came from a spill or fill; in one capture
> it decoded a branch and reported the contents of %g5.  And the huge-page
> path in the TSB miss handler tested TL after switching the global
> register bank, so it consumed a fault code and a PTE that no longer
> existed and killed the task with SIGSEGV at an address that had never
> been mapped.  That one needs CONFIG_TRANSPARENT_HUGEPAGE and only
> appears under load.
> 
> 1/3 is reproduced and fixed on an UltraSPARC T4-1 (sun4v, Niagara4) and
> on a Sun Fire V240 (sun4u, UltraSPARC-IIIi), which between them cover
> both forms of the global register bank switch, SET_GL and the
> PSTATE_AG|PSTATE_MG write.  2/3 and 3/3 are tested on the T4-1, where
> with all three applied a Go toolchain build now completes five times
> running with THP enabled, having previously died within a minute.
> 
> Link: https://github.com/sparclinux/issues/issues/87
> 
> v2:
>   - 1/3: reword the comment above the %asi write as suggested by
>     Andreas: state the restored invariant first, phrase it as why the
>     write is needed, and leave the failure-mode details to the commit
>     message.  No code change.
>   - 2/3, 3/3: unchanged.
> 
> v1: https://lore.kernel.org/all/20260828123707.1852437-1-stian@itx.no/
> 
> Stian Halseth (3):
>   sparc64: restore %asi in user_rtt_fill_fixup_common
>   sparc64: use the fault address for si_addr on window fixup faults
>   sparc64: decide the TSB huge-page window fixup before the bank switch
> 
>  arch/sparc/kernel/tsb.S       | 24 ++++++++++++++++++++----
>  arch/sparc/kernel/urtt_fill.S | 10 ++++++++++
>  arch/sparc/mm/fault_64.c      |  9 +++++++++
>  3 files changed, 39 insertions(+), 4 deletions(-)
> 

The Tested-by from v1 was unfortunately not incorporated in this
version, so I add it here:

Tested-by: Imre Kaloz <kaloz@kernel.org>


Reviewed-by: Andreas Larsson <andreas@gaisler.com>

Picking this up to my for-next.

Thanks,
Andreas