Re: [RFC PATCH v2 03/45] arm64: hibernate: mask DAIF before restoring hibernated kernel

From: Ada Couprie Diaz <redacted>

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_disable()` 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.

Fixes: 82869ac57b5d ("arm64: kernel: Add support for hibernate/suspend-to-disk")
Signed-off-by: Ada Couprie Diaz <redacted>
Signed-off-by: Vladimir Murzin <redacted>
---
 arch/arm64/kernel/hibernate.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index 9717568518ba..1eb1c1074c5b 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c

@@ -465,9 +465,21 @@ 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.
+	 */
+	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));
+	unreachable();