Subject: Re: [PATCH v12 8/8] arm64: expose FAR_EL1 tag bits in siginfo

From: Dave Martin <[email-masked]>
Date: Mon, 2 Nov 2020 17:54:51 +0000

On Fri, Oct 16, 2020 at 05:12:33PM -0700, Peter Collingbourne 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).
> 
> We should not stop clearing these bits in the existing fault address
> fields, because there may be existing userspace applications that are
> expecting the tag bits to be cleared. Instead, create a new pair of
> union fields in siginfo._sigfault, and store the tag bits of FAR_EL1
> there, together with a mask specifying which bits are valid.
> 
> A flag is added to si_xflags to allow userspace to determine whether
> the values in the fields are valid.
> 
> [1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
> 
> Signed-off-by: Peter Collingbourne <[email-masked]>
> Link: https://linux-review.googlesource.com/id/Ia8876bad8c798e0a32df7c2ce1256c4771c81446
> ---
> 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 SIXFLAG_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 | 21 +++++---
>  arch/arm64/include/asm/exception.h      |  2 +-
>  arch/arm64/include/asm/signal.h         | 19 +++++++
>  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                   | 68 ++++++++++++++-----------
>  arch/x86/kernel/signal_compat.c         |  9 +++-
>  include/linux/compat.h                  |  2 +
>  include/linux/signal.h                  | 16 ++++++
>  include/uapi/asm-generic/siginfo.h      | 10 ++++
>  kernel/signal.c                         | 18 ++++++-
>  16 files changed, 148 insertions(+), 73 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..032c09a876f4 100644
> --- a/Documentation/arm64/tagged-pointers.rst
> +++ b/Documentation/arm64/tagged-pointers.rst
> @@ -53,12 +53,21 @@ 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.
> +Non-zero tags are not preserved in the fault address fields
> +siginfo.si_addr or sigcontext.fault_address 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 in these fields. One exception to this rule is for
> +signals raised in response to watchpoint debug exceptions, where the
> +tag information will be preserved.
> +
> +The fault address tag is preserved in the si_addr_tag_bits field
> +of siginfo, which is set for signals raised in response to data aborts
> +and instruction aborts. The si_addr_tag_bits_mask field indicates
> +which bits of the field are valid. The validity of these fields is
> +indicated by the SIXFLAG_ADDR_TAG_BITS flag in siginfo.si_xflags,
> +and the validity of si_xflags in turn is indicated by the kernel
> +indicating support for the sigaction.sa_flags flag SA_XFLAGS.
>  
>  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..46f9b3c61896
> --- /dev/null
> +++ b/arch/arm64/include/asm/signal.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ARM64_ASM_SIGNAL_H
> +#define __ARM64_ASM_SIGNAL_H
> +
> +#include <uapi/asm/signal.h>
> +#include <uapi/asm/siginfo.h>
> +
> +static inline unsigned long arch_addr_tag_bits_mask(unsigned long sig,
> +						    unsigned long si_code)
> +{
> +	if (sig == SIGTRAP && si_code == TRAP_BRKPT)
> +		return 0;
> +	if (sig == SIGSEGV && si_code == SEGV_MTESERR)
> +		return 0xfUL << 56;

Should this be 0xffUL << 56?

I thought MTE ignored bits 63:60, rather than requiring them to be 0?

The whole tag byte may be relevant for debugging purposes, even if only
some of the bits are checked in hardware.

For consistency, I wonder whether it makes sense to expose the bits as
tag bits for watchpoint execeptions, even if they are left un-cleared in
si_addr for historical reasons.

> +	return 0xffUL << 56;
> +}
> +#define arch_addr_tag_bits_mask arch_addr_tag_bits_mask

[...]

Cheers
---Dave

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