[PATCH AUTOSEL 4.9 62/98] x86/power: Fix some ordering bugs in __restore_processor_context()

Sasha Levin
2018-10-25T14:13:47Z

From: Andy Lutomirski <luto@kernel.org>

[ Upstream commit 5b06bbcfc2c621da3009da8decb7511500c293ed ]

__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 = 0,
.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.

Fixes: ca37e57bbe0c ("x86/entry/64: Add missing irqflags tracing to native_load_gs_index()")
Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/6b31721ea92f51ea839e79bd97ade4a75b1eeea2.1512057304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 53cace2ec0e2..73063dfed476 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -222,8 +222,20 @@ 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);
@@ -244,13 +256,14 @@ 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();
x86_platform.restore_sched_clock_state();
mtrr_bp_restore();
--
2.17.1