Re: [PATCH v21 2/2] arm64: expose FAR_EL1 tag bits in siginfo

[Andrey Konovalov <andreyknvl@google.com>]

On Fri, Nov 20, 2020 at 9:34 PM Peter Collingbourne <pcc@google.com> wrote:
>
> The kernel currently clears the tag bits (i.e. bits 56-63) in the fault
> address exposed via siginfo.si_addr and sigcontext.fault_address. However,
> the tag bits may be needed by tools in order to accurately diagnose
> memory errors, such as HWASan [1] or future tools based on the Memory
> Tagging Extension (MTE).
>
> Expose these bits via the arch_untagged_si_addr mechanism, so that
> they are only exposed to signal handlers with the SA_EXPOSE_TAGBITS
> flag set.
>
> [1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Link: https://linux-review.googlesource.com/id/Ia8876bad8c798e0a32df7c2ce1256c4771c81446
> ---
> v21:
> - split out the arch-generic part
> - add some comments
>
> v20:
> - use the sigaction copy
>
> v19:
> - move the code for hiding the tag bits outside the lock
>
> v18:
> - move the SA_EXPOSE_TAGBITS handling to get_signal
>
> v16:
> - add missing file
>
> v15:
> - switched to the SA_EXPOSE_TAGBITS approach
>   proposed by Eric
> - rebased to 5.10-rc3
>
> v14:
> - fix an inaccuracy in the commit message
> - add some comments to arch_addr_tag_bits_mask
>
> v13:
> - renamed si_xflags to si_faultflags
> - rebased to 5.10-rc2
>
> v12:
> - add new fields to signal_compat.c test cases
> - rebased to 5.10-rc1
> - mask out bits 63:60 for tag check faults
>
> v11:
> - add a comment explaining what the arch hook should do
> - rename ignored bits to tag bits
>
> v10:
> - rename the flag to SIFAULTFLAG_ADDR_IGNORED_BITS
> - use an arch hook to specify which bits are ignored, instead
>   of passing them explicitly
> - while refactoring for the arch hook, noticed that my previous
>   patches missed a case involving cache maintenance instructions,
>   so expose the tag bits for that signal as well
>
> v9:
> - make the ignored bits fields generic
> - add some new dependent patches that prepare us to store the
>   field in such a way that userspace can detect their presence
>
> v8:
> - rebase onto 5.8rc2
>
> v7:
> - switch to a new siginfo field instead of using sigcontext
> - merge the patch back into one since the other patches are now
>   unnecessary
>
> v6:
> - move fault address and fault code into the kernel_siginfo data structure
> - split the patch in three since it was getting large and now has
>   generic and arch-specific parts
>
> v5:
> - add padding to fault_addr_top_byte_context in order to ensure the correct
>   size and preserve sp alignment
>
> v4:
> - expose only the tag bits in the context instead of the entire FAR_EL1
> - remove mention of the new context from the sigcontext.__reserved[] note
>
> v3:
> - add documentation to tagged-pointers.rst
> - update comments in sigcontext.h
>
> v2:
> - revert changes to hw_breakpoint.c
> - rename set_thread_esr to set_thread_far_esr
>
>  Documentation/arm64/tagged-pointers.rst | 25 ++++++--
>  arch/arm64/include/asm/exception.h      |  2 +-
>  arch/arm64/include/asm/signal.h         | 25 ++++++++
>  arch/arm64/include/asm/system_misc.h    |  2 +-
>  arch/arm64/include/asm/traps.h          |  6 +-
>  arch/arm64/kernel/debug-monitors.c      |  5 +-
>  arch/arm64/kernel/entry-common.c        |  2 -
>  arch/arm64/kernel/ptrace.c              |  7 +--
>  arch/arm64/kernel/sys_compat.c          |  5 +-
>  arch/arm64/kernel/traps.c               | 29 ++++-----
>  arch/arm64/mm/fault.c                   | 83 +++++++++++++++----------
>  11 files changed, 120 insertions(+), 71 deletions(-)
>  create mode 100644 arch/arm64/include/asm/signal.h
>
> diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
> index eab4323609b9..19d284b70384 100644
> --- a/Documentation/arm64/tagged-pointers.rst
> +++ b/Documentation/arm64/tagged-pointers.rst
> @@ -53,12 +53,25 @@ visibility.
>  Preserving tags
>  ---------------
>
> -Non-zero tags are not preserved when delivering signals. This means that
> -signal handlers in applications making use of tags cannot rely on the
> -tag information for user virtual addresses being maintained for fields
> -inside siginfo_t. One exception to this rule is for signals raised in
> -response to watchpoint debug exceptions, where the tag information will
> -be preserved.
> +When delivering signals, non-zero tags are not preserved in
> +siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in
> +sigaction.sa_flags when the signal handler was installed. This means
> +that signal handlers in applications making use of tags cannot rely
> +on the tag information for user virtual addresses being maintained
> +in these fields unless the flag was set.
> +
> +Due to architecture limitations, bits 63:60 of the fault address
> +are not preserved in response to synchronous tag check faults
> +(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should
> +treat the values of these bits as undefined in order to accommodate
> +future architecture revisions which may preserve the bits.
> +
> +For signals raised in response to watchpoint debug exceptions, the
> +tag information will be preserved regardless of the SA_EXPOSE_TAGBITS
> +flag setting.
> +
> +Non-zero tags are never preserved in sigcontext.fault_address
> +regardless of the SA_EXPOSE_TAGBITS flag setting.
>
>  The architecture prevents the use of a tagged PC, so the upper byte will
>  be set to a sign-extension of bit 55 on exception return.
> diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
> index 99b9383cd036..2a8aa1884d8a 100644
> --- a/arch/arm64/include/asm/exception.h
> +++ b/arch/arm64/include/asm/exception.h
> @@ -32,7 +32,7 @@ static inline u32 disr_to_esr(u64 disr)
>  }
>
>  asmlinkage void enter_from_user_mode(void);
> -void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs);
> +void do_mem_abort(unsigned long far, unsigned int esr, struct pt_regs *regs);
>  void do_undefinstr(struct pt_regs *regs);
>  void do_bti(struct pt_regs *regs);
>  asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr);
> diff --git a/arch/arm64/include/asm/signal.h b/arch/arm64/include/asm/signal.h
> new file mode 100644
> index 000000000000..ef449f5f4ba8
> --- /dev/null
> +++ b/arch/arm64/include/asm/signal.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ARM64_ASM_SIGNAL_H
> +#define __ARM64_ASM_SIGNAL_H
> +
> +#include <asm/memory.h>
> +#include <uapi/asm/signal.h>
> +#include <uapi/asm/siginfo.h>
> +
> +static inline void __user *arch_untagged_si_addr(void __user *addr,
> +                                                unsigned long sig,
> +                                                unsigned long si_code)
> +{
> +       /*
> +        * For historical reasons, all bits of the fault address are exposed as
> +        * address bits for watchpoint exceptions. New architectures should
> +        * handle the tag bits consistently.
> +        */
> +       if (sig == SIGTRAP && si_code == TRAP_BRKPT)
> +               return addr;
> +
> +       return untagged_addr(addr);
> +}
> +#define arch_untagged_si_addr arch_untagged_si_addr
> +
> +#endif
> diff --git a/arch/arm64/include/asm/system_misc.h b/arch/arm64/include/asm/system_misc.h
> index 1ab63cfbbaf1..673be2d1263c 100644
> --- a/arch/arm64/include/asm/system_misc.h
> +++ b/arch/arm64/include/asm/system_misc.h
> @@ -22,7 +22,7 @@ void die(const char *msg, struct pt_regs *regs, int err);
>
>  struct siginfo;
>  void arm64_notify_die(const char *str, struct pt_regs *regs,
> -                     int signo, int sicode, void __user *addr,
> +                     int signo, int sicode, unsigned long far,
>                       int err);
>
>  void hook_debug_fault_code(int nr, int (*fn)(unsigned long, unsigned int,
> diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
> index d96dc2c7c09d..54f32a0675df 100644
> --- a/arch/arm64/include/asm/traps.h
> +++ b/arch/arm64/include/asm/traps.h
> @@ -26,9 +26,9 @@ void register_undef_hook(struct undef_hook *hook);
>  void unregister_undef_hook(struct undef_hook *hook);
>  void force_signal_inject(int signal, int code, unsigned long address, unsigned int err);
>  void arm64_notify_segfault(unsigned long addr);
> -void arm64_force_sig_fault(int signo, int code, void __user *addr, const char *str);
> -void arm64_force_sig_mceerr(int code, void __user *addr, short lsb, const char *str);
> -void arm64_force_sig_ptrace_errno_trap(int errno, void __user *addr, const char *str);
> +void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);
> +void arm64_force_sig_mceerr(int code, unsigned long far, short lsb, const char *str);
> +void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far, const char *str);
>
>  /*
>   * Move regs->pc to next instruction and do necessary setup before it
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index fa76151de6ff..4f3661eeb7ec 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -234,9 +234,8 @@ static void send_user_sigtrap(int si_code)
>         if (interrupts_enabled(regs))
>                 local_irq_enable();
>
> -       arm64_force_sig_fault(SIGTRAP, si_code,
> -                            (void __user *)instruction_pointer(regs),
> -                            "User debug trap");
> +       arm64_force_sig_fault(SIGTRAP, si_code, instruction_pointer(regs),
> +                             "User debug trap");
>  }
>
>  static int single_step_handler(unsigned long unused, unsigned int esr,
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 43d4c329775f..dbbddfbf4a72 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -22,7 +22,6 @@ static void notrace el1_abort(struct pt_regs *regs, unsigned long esr)
>         unsigned long far = read_sysreg(far_el1);
>
>         local_daif_inherit(regs);
> -       far = untagged_addr(far);
>         do_mem_abort(far, esr, regs);
>  }
>  NOKPROBE_SYMBOL(el1_abort);
> @@ -114,7 +113,6 @@ static void notrace el0_da(struct pt_regs *regs, unsigned long esr)
>
>         user_exit_irqoff();
>         local_daif_restore(DAIF_PROCCTX);
> -       far = untagged_addr(far);
>         do_mem_abort(far, esr, regs);
>  }
>  NOKPROBE_SYMBOL(el0_da);
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index f49b349e16a3..8ac487c84e37 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -192,14 +192,11 @@ static void ptrace_hbptriggered(struct perf_event *bp,
>                                 break;
>                         }
>                 }
> -               arm64_force_sig_ptrace_errno_trap(si_errno,
> -                                                 (void __user *)bkpt->trigger,
> +               arm64_force_sig_ptrace_errno_trap(si_errno, bkpt->trigger,
>                                                   desc);
>         }
>  #endif
> -       arm64_force_sig_fault(SIGTRAP, TRAP_HWBKPT,
> -                             (void __user *)(bkpt->trigger),
> -                             desc);
> +       arm64_force_sig_fault(SIGTRAP, TRAP_HWBKPT, bkpt->trigger, desc);
>  }
>
>  /*
> diff --git a/arch/arm64/kernel/sys_compat.c b/arch/arm64/kernel/sys_compat.c
> index 3c18c2454089..265fe3eb1069 100644
> --- a/arch/arm64/kernel/sys_compat.c
> +++ b/arch/arm64/kernel/sys_compat.c
> @@ -68,7 +68,7 @@ do_compat_cache_op(unsigned long start, unsigned long end, int flags)
>   */
>  long compat_arm_syscall(struct pt_regs *regs, int scno)
>  {
> -       void __user *addr;
> +       unsigned long addr;
>
>         switch (scno) {
>         /*
> @@ -111,8 +111,7 @@ long compat_arm_syscall(struct pt_regs *regs, int scno)
>                 break;
>         }
>
> -       addr  = (void __user *)instruction_pointer(regs) -
> -               (compat_thumb_mode(regs) ? 2 : 4);
> +       addr = instruction_pointer(regs) - (compat_thumb_mode(regs) ? 2 : 4);
>
>         arm64_notify_die("Oops - bad compat syscall(2)", regs,
>                          SIGILL, ILL_ILLTRP, addr, scno);
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 8af4e0e85736..f4ddbe9ed3f1 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -170,32 +170,32 @@ static void arm64_show_signal(int signo, const char *str)
>         __show_regs(regs);
>  }
>
> -void arm64_force_sig_fault(int signo, int code, void __user *addr,
> +void arm64_force_sig_fault(int signo, int code, unsigned long far,
>                            const char *str)
>  {
>         arm64_show_signal(signo, str);
>         if (signo == SIGKILL)
>                 force_sig(SIGKILL);
>         else
> -               force_sig_fault(signo, code, addr);
> +               force_sig_fault(signo, code, (void __user *)far);
>  }
>
> -void arm64_force_sig_mceerr(int code, void __user *addr, short lsb,
> +void arm64_force_sig_mceerr(int code, unsigned long far, short lsb,
>                             const char *str)
>  {
>         arm64_show_signal(SIGBUS, str);
> -       force_sig_mceerr(code, addr, lsb);
> +       force_sig_mceerr(code, (void __user *)far, lsb);
>  }
>
> -void arm64_force_sig_ptrace_errno_trap(int errno, void __user *addr,
> +void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far,
>                                        const char *str)
>  {
>         arm64_show_signal(SIGTRAP, str);
> -       force_sig_ptrace_errno_trap(errno, addr);
> +       force_sig_ptrace_errno_trap(errno, (void __user *)far);
>  }
>
>  void arm64_notify_die(const char *str, struct pt_regs *regs,
> -                     int signo, int sicode, void __user *addr,
> +                     int signo, int sicode, unsigned long far,
>                       int err)
>  {
>         if (user_mode(regs)) {
> @@ -203,7 +203,7 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
>                 current->thread.fault_address = 0;
>                 current->thread.fault_code = err;
>
> -               arm64_force_sig_fault(signo, sicode, addr, str);
> +               arm64_force_sig_fault(signo, sicode, far, str);
>         } else {
>                 die(str, regs, err);
>         }
> @@ -374,7 +374,7 @@ void force_signal_inject(int signal, int code, unsigned long address, unsigned i
>                 signal = SIGKILL;
>         }
>
> -       arm64_notify_die(desc, regs, signal, code, (void __user *)address, err);
> +       arm64_notify_die(desc, regs, signal, code, address, err);
>  }
>
>  /*
> @@ -385,7 +385,7 @@ void arm64_notify_segfault(unsigned long addr)
>         int code;
>
>         mmap_read_lock(current->mm);
> -       if (find_vma(current->mm, addr) == NULL)
> +       if (find_vma(current->mm, untagged_addr(addr)) == NULL)
>                 code = SEGV_MAPERR;
>         else
>                 code = SEGV_ACCERR;
> @@ -448,12 +448,13 @@ NOKPROBE_SYMBOL(do_ptrauth_fault);
>
>  static void user_cache_maint_handler(unsigned int esr, struct pt_regs *regs)
>  {
> -       unsigned long address;
> +       unsigned long tagged_address, address;
>         int rt = ESR_ELx_SYS64_ISS_RT(esr);
>         int crm = (esr & ESR_ELx_SYS64_ISS_CRM_MASK) >> ESR_ELx_SYS64_ISS_CRM_SHIFT;
>         int ret = 0;
>
> -       address = untagged_addr(pt_regs_read_reg(regs, rt));
> +       tagged_address = pt_regs_read_reg(regs, rt);
> +       address = untagged_addr(tagged_address);
>
>         switch (crm) {
>         case ESR_ELx_SYS64_ISS_CRM_DC_CVAU:     /* DC CVAU, gets promoted */
> @@ -480,7 +481,7 @@ static void user_cache_maint_handler(unsigned int esr, struct pt_regs *regs)
>         }
>
>         if (ret)
> -               arm64_notify_segfault(address);
> +               arm64_notify_segfault(tagged_address);
>         else
>                 arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
>  }
> @@ -772,7 +773,7 @@ asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr)
>   */
>  void bad_el0_sync(struct pt_regs *regs, int reason, unsigned int esr)
>  {
> -       void __user *pc = (void __user *)instruction_pointer(regs);
> +       unsigned long pc = instruction_pointer(regs);
>
>         current->thread.fault_address = 0;
>         current->thread.fault_code = esr;
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 1ee94002801f..29a6b8c9e830 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -40,7 +40,7 @@
>  #include <asm/traps.h>
>
>  struct fault_info {
> -       int     (*fn)(unsigned long addr, unsigned int esr,
> +       int     (*fn)(unsigned long far, unsigned int esr,
>                       struct pt_regs *regs);
>         int     sig;
>         int     code;
> @@ -385,8 +385,11 @@ static void set_thread_esr(unsigned long address, unsigned int esr)
>         current->thread.fault_code = esr;
>  }
>
> -static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> +static void do_bad_area(unsigned long far, unsigned int esr,
> +                       struct pt_regs *regs)
>  {
> +       unsigned long addr = untagged_addr(far);
> +
>         /*
>          * If we are in kernel mode at this point, we have no context to
>          * handle this fault with.
> @@ -395,8 +398,7 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
>                 const struct fault_info *inf = esr_to_fault_info(esr);
>
>                 set_thread_esr(addr, esr);
> -               arm64_force_sig_fault(inf->sig, inf->code, (void __user *)addr,
> -                                     inf->name);
> +               arm64_force_sig_fault(inf->sig, inf->code, far, inf->name);
>         } else {
>                 __do_kernel_fault(addr, esr, regs);
>         }
> @@ -448,7 +450,7 @@ static bool is_write_abort(unsigned int esr)
>         return (esr & ESR_ELx_WNR) && !(esr & ESR_ELx_CM);
>  }
>
> -static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
> +static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
>                                    struct pt_regs *regs)
>  {
>         const struct fault_info *inf;
> @@ -456,6 +458,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>         vm_fault_t fault;
>         unsigned long vm_flags = VM_ACCESS_FLAGS;
>         unsigned int mm_flags = FAULT_FLAG_DEFAULT;
> +       unsigned long addr = untagged_addr(far);
>
>         if (kprobe_page_fault(regs, esr))
>                 return 0;
> @@ -567,8 +570,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>                  * We had some memory, but were unable to successfully fix up
>                  * this page fault.
>                  */
> -               arm64_force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)addr,
> -                                     inf->name);
> +               arm64_force_sig_fault(SIGBUS, BUS_ADRERR, far, inf->name);
>         } else if (fault & (VM_FAULT_HWPOISON_LARGE | VM_FAULT_HWPOISON)) {
>                 unsigned int lsb;
>
> @@ -576,8 +578,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>                 if (fault & VM_FAULT_HWPOISON_LARGE)
>                         lsb = hstate_index_to_shift(VM_FAULT_GET_HINDEX(fault));
>
> -               arm64_force_sig_mceerr(BUS_MCEERR_AR, (void __user *)addr, lsb,
> -                                      inf->name);
> +               arm64_force_sig_mceerr(BUS_MCEERR_AR, far, lsb, inf->name);
>         } else {
>                 /*
>                  * Something tried to access memory that isn't in our memory
> @@ -585,8 +586,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>                  */
>                 arm64_force_sig_fault(SIGSEGV,
>                                       fault == VM_FAULT_BADACCESS ? SEGV_ACCERR : SEGV_MAPERR,
> -                                     (void __user *)addr,
> -                                     inf->name);
> +                                     far, inf->name);
>         }
>
>         return 0;
> @@ -596,33 +596,35 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>         return 0;
>  }
>
> -static int __kprobes do_translation_fault(unsigned long addr,
> +static int __kprobes do_translation_fault(unsigned long far,
>                                           unsigned int esr,
>                                           struct pt_regs *regs)
>  {
> +       unsigned long addr = untagged_addr(far);
> +
>         if (is_ttbr0_addr(addr))
> -               return do_page_fault(addr, esr, regs);
> +               return do_page_fault(far, esr, regs);
>
> -       do_bad_area(addr, esr, regs);
> +       do_bad_area(far, esr, regs);
>         return 0;
>  }
>
> -static int do_alignment_fault(unsigned long addr, unsigned int esr,
> +static int do_alignment_fault(unsigned long far, unsigned int esr,
>                               struct pt_regs *regs)
>  {
> -       do_bad_area(addr, esr, regs);
> +       do_bad_area(far, esr, regs);
>         return 0;
>  }
>
> -static int do_bad(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> +static int do_bad(unsigned long far, unsigned int esr, struct pt_regs *regs)
>  {
>         return 1; /* "fault" */
>  }
>
> -static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> +static int do_sea(unsigned long far, unsigned int esr, struct pt_regs *regs)
>  {
>         const struct fault_info *inf;
> -       void __user *siaddr;
> +       unsigned long siaddr;
>
>         inf = esr_to_fault_info(esr);
>
> @@ -634,19 +636,30 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>                 return 0;
>         }
>
> -       if (esr & ESR_ELx_FnV)
> -               siaddr = NULL;
> -       else
> -               siaddr  = (void __user *)addr;
> +       if (esr & ESR_ELx_FnV) {
> +               siaddr = 0;
> +       } else {
> +               /*
> +                * The architecture specifies that the tag bits of FAR_EL1 are
> +                * UNKNOWN for synchronous external aborts. Mask them out now
> +                * so that userspace doesn't see them.
> +                */
> +               siaddr  = untagged_addr(far);
> +       }
>         arm64_notify_die(inf->name, regs, inf->sig, inf->code, siaddr, esr);
>
>         return 0;
>  }
>
> -static int do_tag_check_fault(unsigned long addr, unsigned int esr,
> +static int do_tag_check_fault(unsigned long far, unsigned int esr,
>                               struct pt_regs *regs)
>  {
> -       do_bad_area(addr, esr, regs);
> +       /*
> +        * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN for tag
> +        * check faults. Mask them out now so that userspace doesn't see them.
> +        */
> +       far &= (1UL << 60) - 1;
> +       do_bad_area(far, esr, regs);
>         return 0;
>  }
>
> @@ -717,11 +730,12 @@ static const struct fault_info fault_info[] = {
>         { do_bad,               SIGKILL, SI_KERNEL,     "unknown 63"                    },
>  };
>
> -void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> +void do_mem_abort(unsigned long far, unsigned int esr, struct pt_regs *regs)
>  {
>         const struct fault_info *inf = esr_to_fault_info(esr);
> +       unsigned long addr = untagged_addr(far);
>
> -       if (!inf->fn(addr, esr, regs))
> +       if (!inf->fn(far, esr, regs))
>                 return;
>
>         if (!user_mode(regs)) {
> @@ -730,8 +744,12 @@ void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>                 show_pte(addr);
>         }
>
> -       arm64_notify_die(inf->name, regs,
> -                        inf->sig, inf->code, (void __user *)addr, esr);
> +       /*
> +        * At this point we have an unrecognized fault type whose tag bits may
> +        * have been defined as UNKNOWN. Therefore we only expose the untagged
> +        * address to the signal handler.
> +        */
> +       arm64_notify_die(inf->name, regs, inf->sig, inf->code, addr, esr);
>  }
>  NOKPROBE_SYMBOL(do_mem_abort);
>
> @@ -744,8 +762,8 @@ NOKPROBE_SYMBOL(do_el0_irq_bp_hardening);
>
>  void do_sp_pc_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>  {
> -       arm64_notify_die("SP/PC alignment exception", regs,
> -                        SIGBUS, BUS_ADRALN, (void __user *)addr, esr);
> +       arm64_notify_die("SP/PC alignment exception", regs, SIGBUS, BUS_ADRALN,
> +                        addr, esr);
>  }
>  NOKPROBE_SYMBOL(do_sp_pc_abort);
>
> @@ -871,8 +889,7 @@ void do_debug_exception(unsigned long addr_if_watchpoint, unsigned int esr,
>                 arm64_apply_bp_hardening();
>
>         if (inf->fn(addr_if_watchpoint, esr, regs)) {
> -               arm64_notify_die(inf->name, regs,
> -                                inf->sig, inf->code, (void __user *)pc, esr);
> +               arm64_notify_die(inf->name, regs, inf->sig, inf->code, pc, esr);
>         }
>
>         debug_exception_exit(regs);
> --
> 2.29.2.454.gaff20da3a2-goog
>

Not sure if it's expected, but this patch changes the way in-kernel
MTE faults are reported. The address of any in-kernel fault now gets
its top byte zeroed out, even though it was set to 0xf. However I
guess it's related to the fact that MTE always uses pointer tags in
0x0_ format.