From 4bef20bbb9e02568a009efa4d404ce2621289be5 Mon Sep 17 00:00:00 2001 From: Matt Turner Date: Wed, 7 Oct 2026 23:24:05 -0400 Subject: [PATCH] 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 Tested-by: Magnus Lindholm Signed-off-by: Matt Turner Link: https://lore.kernel.org/r/20261007-alpha-hugepages-v2-2-8e108af2d961@gmail.com Signed-off-by: Magnus Lindholm --- arch/alpha/mm/fault.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c index dfe427d93072..24408b53197c 100644 --- a/arch/alpha/mm/fault.c +++ b/arch/alpha/mm/fault.c @@ -8,6 +8,7 @@ #include #include #include +#include #include #define __EXTERN_INLINE inline @@ -113,6 +114,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr, struct mm_struct *mm = current->mm; const struct exception_table_entry *fixup; int si_code = SEGV_MAPERR; + unsigned int lsb; vm_fault_t fault; unsigned int flags = FAULT_FLAG_DEFAULT; @@ -185,6 +187,8 @@ do_page_fault(unsigned long address, unsigned long mmcsr, goto bad_area; else if (fault & VM_FAULT_SIGBUS) goto do_sigbus; + else if (fault & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE)) + goto do_sigbus_mceerr; BUG(); } @@ -248,6 +252,21 @@ do_page_fault(unsigned long address, unsigned long mmcsr, goto no_context; return; + do_sigbus_mceerr: + mmap_read_unlock(mm); + if (!user_mode(regs)) + goto no_context; + /* + * Report the size of the poisoned area, which for a hugetlb fault + * is the size of the huge page that could not be mapped. + */ + lsb = PAGE_SHIFT; + if (fault & VM_FAULT_HWPOISON_LARGE) + lsb = hstate_index_to_shift(VM_FAULT_GET_HINDEX(fault)); + show_signal_msg(regs, address, SIGBUS, "hardware memory error"); + force_sig_mceerr(BUS_MCEERR_AR, (void __user *) address, lsb); + return; + do_sigsegv: show_signal_msg(regs, address, SIGSEGV, si_code == SEGV_MAPERR ? "unmapped access"