[PATCH v3 1/2] arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

[Will Deacon]

On Tue, Jun 06, 2017 at 05:00:21PM +0000, Ard Biesheuvel wrote:
> When turning branch instructions into NOPs, we attempt to validate the
> action by comparing the old value at the call site with the opcode of
> a direct relative branch instruction pointing at the old target.
> 
> However, these call sites are statically initialized to call _mcount(),
> and may be redirected via a PLT entry if the module is loaded far away
> from the kernel text, leading to false negatives and spurious errors.
> 
> So skip the validation if CONFIG_ARM64_MODULE_PLTS is configured.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/kernel/ftrace.c | 46 ++++++++++++++++++--
>  1 file changed, 43 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 40ad08ac569a..4cb576374b82 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -84,12 +84,52 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  		    unsigned long addr)
>  {
>  	unsigned long pc = rec->ip;
> -	u32 old, new;
> +	long offset = (long)pc - (long)addr;
> +	bool validate = true;
> +	u32 old = 0, new;
> +
> +	if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
> +	    (offset < -SZ_128M || offset >= SZ_128M)) {
> +		u32 replaced;
> +
> +		/*
> +		 * 'mod' is only set at module load time, but if we end up
> +		 * dealing with an out-of-range condition, we can assume it
> +		 * is due to a module being loaded far away from the kernel.
> +		 */
> +		if (!mod) {
> +			preempt_disable();
> +			mod = __module_text_address(pc);
> +			preempt_enable();

The comment in __module_text_address says that preempt must be disabled so
that the module doesn't get freed under its feet, but if that's a
possibility here then it feels really dangerous to re-enable preemption
before we've done the patching. Shouldn't we take module_mutex or something?

Other than that, this looks fine. Thanks.

Will