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@@ -164,11 +164,11 @@ place where the thread is safe to be frozen (no kernel semaphores
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should be held at that point and it must be safe to sleep there), and
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add:
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- if (current->flags & PF_FREEZE)
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- refrigerator(PF_FREEZE);
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+ try_to_freeze(PF_FREEZE);
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If the thread is needed for writing the image to storage, you should
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-instead set the PF_NOFREEZE process flag when creating the thread.
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+instead set the PF_NOFREEZE process flag when creating the thread (and
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+be very carefull).
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Q: What is the difference between between "platform", "shutdown" and
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@@ -233,3 +233,81 @@ A: Try running
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cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null
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after resume. swapoff -a; swapon -a may also be usefull.
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+
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+Q: What happens to devices during swsusp? They seem to be resumed
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+during system suspend?
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+
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+A: That's correct. We need to resume them if we want to write image to
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+disk. Whole sequence goes like
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+
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+ Suspend part
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+ ~~~~~~~~~~~~
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+ running system, user asks for suspend-to-disk
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+
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+ user processes are stopped
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+
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+ suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
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+ with state snapshot
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+
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+ state snapshot: copy of whole used memory is taken with interrupts disabled
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+
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+ resume(): devices are woken up so that we can write image to swap
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+
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+ write image to swap
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+
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+ suspend(PMSG_SUSPEND): suspend devices so that we can power off
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+
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+ turn the power off
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+
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+ Resume part
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+ ~~~~~~~~~~~
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+ (is actually pretty similar)
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+
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+ running system, user asks for suspend-to-disk
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+
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+ user processes are stopped (in common case there are none, but with resume-from-initrd, noone knows)
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+
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+ read image from disk
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+
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+ suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
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+ with image restoration
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+
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+ image restoration: rewrite memory with image
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+
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+ resume(): devices are woken up so that system can continue
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+
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+ thaw all user processes
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+
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+Q: What is this 'Encrypt suspend image' for?
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+
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+A: First of all: it is not a replacement for dm-crypt encrypted swap.
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+It cannot protect your computer while it is suspended. Instead it does
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+protect from leaking sensitive data after resume from suspend.
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+
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+Think of the following: you suspend while an application is running
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+that keeps sensitive data in memory. The application itself prevents
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+the data from being swapped out. Suspend, however, must write these
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+data to swap to be able to resume later on. Without suspend encryption
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+your sensitive data are then stored in plaintext on disk. This means
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+that after resume your sensitive data are accessible to all
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+applications having direct access to the swap device which was used
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+for suspend. If you don't need swap after resume these data can remain
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+on disk virtually forever. Thus it can happen that your system gets
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+broken in weeks later and sensitive data which you thought were
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+encrypted and protected are retrieved and stolen from the swap device.
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+To prevent this situation you should use 'Encrypt suspend image'.
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+
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+During suspend a temporary key is created and this key is used to
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+encrypt the data written to disk. When, during resume, the data was
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+read back into memory the temporary key is destroyed which simply
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+means that all data written to disk during suspend are then
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+inaccessible so they can't be stolen later on. The only thing that
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+you must then take care of is that you call 'mkswap' for the swap
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+partition used for suspend as early as possible during regular
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+boot. This asserts that any temporary key from an oopsed suspend or
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+from a failed or aborted resume is erased from the swap device.
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+
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+As a rule of thumb use encrypted swap to protect your data while your
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+system is shut down or suspended. Additionally use the encrypted
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+suspend image to prevent sensitive data from being stolen after
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+resume.
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Pavel Machek