alpha: handle VM_FAULT_HWPOISON in do_page_fault()

do_page_fault() handles VM_FAULT_OOM, VM_FAULT_SIGSEGV and
VM_FAULT_SIGBUS and then falls into BUG() for anything else in
VM_FAULT_ERROR. VM_FAULT_HWPOISON and VM_FAULT_HWPOISON_LARGE are in
that set, so a fault on a poisoned page takes down the kernel instead of
delivering a signal.

Alpha does not select ARCH_SUPPORTS_MEMORY_FAILURE, so the machine check
paths cannot produce these. UFFDIO_POISON can: it installs a poison PTE
marker without any memory failure support, and a subsequent access
returns VM_FAULT_HWPOISON. Registering an anonymous range with
userfaultfd, poisoning it and reading it back reliably hits the BUG():

  Kernel bug at arch/alpha/mm/fault.c:188
  poison(50809): Kernel Bug 1
  pc is at do_page_fault+0x4e8/0x550
  ra is at do_page_fault+0xfc/0x550

The BUG() fires with mmap_read_lock() still held, so the faulting task
is left unkillable in D state holding the lock, and shutdown stalls
behind it. A UFFD_USER_MODE_ONLY userfaultfd is always allowed, so any
local user can do this.

Deliver SIGBUS with BUS_MCEERR_AR instead, reporting the size of the
poisoned area, as the other architectures do. That is the huge page size
for VM_FAULT_HWPOISON_LARGE, which becomes reachable once alpha
implements huge pages.

Tested on an UP1500 (EV68AL): the reproducer above now takes a SIGBUS
and the kernel logs

  poison[373]: hardware memory error at 0000020000030000 pc 00000200010007ec

with no oops, no wedged task and no taint.

A backport needs the show_signal_msg() call dropped, since alpha only
has it since commit 9ca6af1aac40 ("alpha: select
SYSCTL_EXCEPTION_TRACE").

Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
Cc: stable@vger.kernel.org # 6.6+
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Link: https://lore.kernel.org/r/20261007-alpha-hugepages-v2-2-8e108af2d961@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

0 files changed, 0 insertions, 0 deletions