[PATCH 4.4 32/63] x86/mm/64: Fix reboot interaction with CR4.PCIDE

Greg Kroah-Hartman
2018-01-01T14:28:53Z

From: Andy Lutomirski <luto@kernel.org>

commit 924c6b900cfdf376b07bccfd80e62b21914f8a5a upstream.

Trying to reboot via real mode fails with PCID on: long mode cannot
be exited while CR4.PCIDE is set. (No, I have no idea why, but the
SDM and actual CPUs are in agreement here.) The result is a GPF and
a hang instead of a reboot.

I didn't catch this in testing because neither my computer nor my VM
reboots this way. I can trigger it with reboot=bios, though.

Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")
Reported-and-tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.luto@kernel.org
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kernel/reboot.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -93,6 +93,10 @@ void __noreturn machine_real_restart(uns
load_cr3(initial_page_table);
#else
write_cr3(real_mode_header->trampoline_pgd);
+
+ /* Exiting long mode will fail if CR4.PCIDE is set. */
+ if (static_cpu_has(X86_FEATURE_PCID))
+ cr4_clear_bits(X86_CR4_PCIDE);
#endif

/* Jump to the identity-mapped low memory code */