[RFC PATCH 03/36] arm64: hibernate: mask DAIF before restoring hibernated kernel

[Vladimir Murzin <vladimir.murzin@arm.com>]

From: Ada Couprie Diaz <ada.coupriediaz@arm.com>

The arm64 hibernate code manages the exception masking in an unsound
way, leading to potential crashes and/or warnings during resume.

When a hibernation image is saved in `swsusp_arch_suspend()`, all DAIF
exceptions are masked (by virtue of `local_daif_save()`), and the
suspended image is saved assuming that all DAIF exceptions will remain
masked when the image is restored.

When a hibernation image is resumed by `swsusp_arch_resume()`, only
interrupts are masked (by virtue of `local_irq_save()` in
`resume_target_kernel()`). When pseudo-NMI is enabled the DAIF.IF bits
will be clear, and regardless of pseudo-NMI the DAIF.DA bits will be
clear.

This means that there are two problems:

(1) It is possible to take Debug, SError, or pseudo-NMI exceptions
    during the resume process. This is unsafe, as during the resume
    process both the old ane new kernels will tranisently be in an
    inconsistent state, and swsusp_arch_suspend_exit() won't retain
    an executable mapping of any exception vectors.

    Any exception taken here will be fatal and silent.

(2) When re-entering the resumed kernel, some DAIF bits will be clear
    unexpectedly. This permits Debug, SError, or pseudo-NMI exceptions
    to be taken for a short period while the resumed kernel is not yet
    in a consistent state.

    This is detected by CONFIG_ARM64_DEBUG_PRIORITY_MASKING.

Avoid these issues by masking all DAIF exceptions during resume.

Cc: stable@vger.kernel.org
Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
---
 arch/arm64/kernel/hibernate.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index 9717568518ba..d0d9bd91e639 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c
@@ -405,6 +405,7 @@ int swsusp_arch_suspend(void)
 int __nocfi swsusp_arch_resume(void)
 {
 	int rc;
+	unsigned long flags;
 	void *zero_page;
 	size_t exit_size;
 	pgd_t *tmp_pg_dir;
@@ -465,9 +466,21 @@ int __nocfi swsusp_arch_resume(void)
 	if (el2_reset_needed())
 		__hyp_set_vectors(el2_vectors);
 
+	/*
+	 * It is necessary to mask all DAIF exceptions here as:
+	 *
+	 * - The copy of swsusp_arch_suspend_exit() in the hibernation
+	 *   text cannot handle taking any exceptions.
+	 *
+	 * - The suspended kernel masked all DAIF exceptions in
+	 *   swsusp_arch_resume(), and expects to be re-entered in the
+	 *   same state : with all DAIF exceptions masked.
+	 */
+	flags = local_daif_save();
 	hibernate_exit(virt_to_phys(tmp_pg_dir), resume_hdr.ttbr1_el1,
 		       resume_hdr.reenter_kernel, restore_pblist,
 		       resume_hdr.__hyp_stub_vectors, virt_to_phys(zero_page));
+	local_daif_restore(flags);
 
 	return 0;
 }
-- 
2.34.1