Re: [PATCH v2] arm64: Work around missing `bti c` in modules

[D Scott Phillips <scott@os.amperecomputing.com>]

D Scott Phillips <scott@os.amperecomputing.com> writes:

> GCC does not insert a `bti c` instruction at the beginning of a function
> when all callers reach the function through a direct branch[1]. In the case
> of cross-section calls (like __init to non __init), a thunk may be inserted
> which uses an indirect branch. If that happens, the first instruction in
> the callee function will result in a Branch Target Exception due to the
> missing `bti c`.
>
> Handle Branch Target Exceptions which happen in the kernel due to module
> calls from __init to non-__init by clearing PSTATE.BTYPE and resuming.
>
> [1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
>
> Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
> ---
> Changes since v1:
>  - Add the gcc bug id into the traps.c comment
>  - Cover the try_module_get with the preempt_disable
>  - Add a CC_HAS_ config for the compiler bug that we'll eventually refine
>
>  arch/arm64/Kconfig               |  3 +++
>  arch/arm64/kernel/entry-common.c | 12 +++++++++
>  arch/arm64/kernel/traps.c        | 43 ++++++++++++++++++++++++++++++--
>  3 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index cd93c9041679..d5d4d2891657 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1860,6 +1860,9 @@ config ARM64_BTI_KERNEL
>  	  is enabled and the system supports BTI all kernel code including
>  	  modular code must have BTI enabled.
>  
> +config CC_HAS_CROSS_SECTION_BTI_MISSING
> +	def_bool CC_IS_GCC # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
> +
>  config CC_HAS_BRANCH_PROT_PAC_RET_BTI
>  	# GCC 9 or later, clang 8 or later
>  	def_bool $(cc-option,-mbranch-protection=pac-ret+leaf+bti)
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 56cefd33eb8e..696e3f3c90ea 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -388,6 +388,15 @@ static void noinstr el1_undef(struct pt_regs *regs)
>  	exit_to_kernel_mode(regs);
>  }
>  
> +static void noinstr el1_bti(struct pt_regs *regs)
> +{
> +	enter_from_kernel_mode(regs);
> +	local_daif_inherit(regs);
> +	do_bti(regs);
> +	local_daif_mask();
> +	exit_to_kernel_mode(regs);
> +}
> +
>  static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
>  {
>  	unsigned long far = read_sysreg(far_el1);
> @@ -427,6 +436,9 @@ asmlinkage void noinstr el1h_64_sync_handler(struct pt_regs *regs)
>  	case ESR_ELx_EC_UNKNOWN:
>  		el1_undef(regs);
>  		break;
> +	case ESR_ELx_EC_BTI:
> +		el1_bti(regs);
> +		break;
>  	case ESR_ELx_EC_BREAKPT_CUR:
>  	case ESR_ELx_EC_SOFTSTP_CUR:
>  	case ESR_ELx_EC_WATCHPT_CUR:

There's a change in behavior here that I don't want to go by
unnoticed. Previously BTI exceptions would fall through to the default
case and cause a panic. With this change they'll go into do_bti, and
then kill the task if not handled by the gcc workaround case. I think
that change is a good one, but I don't want to sneak it in.

Would it be better if I split that out into a separate patch so that it
gets noticed on its own?

Scott

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