Re: [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions

On Fri, Nov 06 2020 at 15:29, ira weiny wrote:

+#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS +/*

• • PKRS is a per-logical-processor MSR which overlays additional protection for
• • pages which have been mapped with a protection key.
• • The register is not maintained with XSAVE so we have to maintain the MSR
• • value in software during context switch and exception handling.
• • Context switches save the MSR in the task struct thus taking that value to
• • other processors if necessary.
• • To protect against exceptions having access to this memory we save the
• • current running value and set the PKRS value for the duration of the
• • exception. Thus preventing exception handlers from having the elevated
• • access of the interrupted task.
• */

+noinstr void irq_save_set_pkrs(irqentry_state_t *irq_state, u32 val) +{

• if (!cpu_feature_enabled(X86_FEATURE_PKS))

• return;
  

• irq_state->thread_pkrs = current->thread.saved_pkrs;
• write_pkrs(INIT_PKRS_VALUE);

Why is this noinstr? Just because it's called from a noinstr function?

Of course the function itself violates the noinstr constraints:

vmlinux.o: warning: objtool: write_pkrs()+0x36: call to do_trace_write_msr() leaves .noinstr.text section

There is absolutely no reason to have this marked noinstr.

Thanks,

tglx