[PATCH] x86/power: Fix some ordering bugs in __restore_processor_context()

[Andy Lutomirski]

__restore_processor_context() had a couple of ordering bugs.  It
restored GSBASE after calling load_gs_index(), and the latter can
call into tracing code.  It also tried to restore segment registers
before restoring the LDT, which is straight-up wrong.

Reorder the code so that we restore GSBASE, then the descriptor
tables, then the segments.

This fixes two bugs.  First, it fixes a regression that broke resume
under certain configurations due to irqflag tracing in
native_load_gs_index().  Second, it fixes resume when the userspace
process that initiated suspect had funny segments.  The latter can be
reproduced by compiling this:

// SPDX-License-Identifier: GPL-2.0
/*
 * ldt_echo.c - Echo argv[1] while using an LDT segment
 */

int main(int argc, char **argv)
{
	int ret;
	size_t len;
	char *buf;

	const struct user_desc desc = {
                .entry_number    = 0,
                .base_addr       = 0,
                .limit           = 0xfffff,
                .seg_32bit       = 1,
                .contents        = 0, /* Data, grow-up */
                .read_exec_only  = 0,
                .limit_in_pages  = 1,
                .seg_not_present = 1,
                .useable         = 0
        };

	if (argc != 2)
		errx(1, "Usage: %s STRING", argv[0]);

	len = asprintf(&buf, "%s\n", argv[1]);
	if (len < 0)
		errx(1, "Out of memory");

	ret = syscall(SYS_modify_ldt, 1, &desc, sizeof(desc));
	if (ret < -1)
		errno = -ret;
	if (ret)
		err(1, "modify_ldt");

	asm volatile ("movw %0, %%es" :: "rm" ((unsigned short)7));
	write(1, buf, len);
	return 0;
}

and running ldt_echo >/sys/power/mem

Without the fix, the latter causes a triple fault on resume.

Reported-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>
Fixes: ca37e57bbe0c ("x86/entry/64: Add missing irqflags tracing to native_load_gs_index()")
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

Jarkko, can you test this version?

 arch/x86/power/cpu.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index 84fcfde53f8f..5191de14f4df 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -226,8 +226,20 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
 	load_idt((const struct desc_ptr *)&ctxt->idt_limit);
 #endif
 
+#ifdef CONFIG_X86_64
 	/*
-	 * segment registers
+	 * We need GSBASE restored before percpu access can work.
+	 * percpu access can happen in exception handlers or in complicated
+	 * helpers like load_gs_index().
+	 */
+	wrmsrl(MSR_GS_BASE, ctxt->gs_base);
+#endif
+
+	fix_processor_context();
+
+	/*
+	 * Restore segment registers.  This happens after restoring the GDT
+	 * and LDT, which happen in fix_processor_context().
 	 */
 #ifdef CONFIG_X86_32
 	loadsegment(es, ctxt->es);
@@ -248,13 +260,14 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
 	load_gs_index(ctxt->gs);
 	asm volatile ("movw %0, %%ss" :: "r" (ctxt->ss));
 
+	/*
+	 * Restore FSBASE and user GSBASE after reloading the respective
+	 * segment selectors.
+	 */
 	wrmsrl(MSR_FS_BASE, ctxt->fs_base);
-	wrmsrl(MSR_GS_BASE, ctxt->gs_base);
 	wrmsrl(MSR_KERNEL_GS_BASE, ctxt->gs_kernel_base);
 #endif
 
-	fix_processor_context();
-
 	do_fpu_end();
 	tsc_verify_tsc_adjust(true);
 	x86_platform.restore_sched_clock_state();
-- 
2.13.6