1 files changed, 81 insertions, 2 deletions

diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt
index 4e1627cc5b51..7a6b78966459 100644
--- a/Documentation/power/swsusp.txt
+++ b/Documentation/power/swsusp.txt
@@ -164,10 +164,11 @@ place where the thread is safe to be frozen (no kernel semaphores
 should be held at that point and it must be safe to sleep there), and
 add:
 
-            try_to_freeze();
+       try_to_freeze();
 
 If the thread is needed for writing the image to storage, you should
-instead set the PF_NOFREEZE process flag when creating the thread.
+instead set the PF_NOFREEZE process flag when creating the thread (and
+be very carefull).
 
 
 Q: What is the difference between between "platform", "shutdown" and
@@ -232,3 +233,81 @@ A: Try running
 cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null
 
 after resume. swapoff -a; swapon -a may also be usefull.
+
+Q: What happens to devices during swsusp? They seem to be resumed
+during system suspend?
+
+A: That's correct. We need to resume them if we want to write image to
+disk. Whole sequence goes like
+
+      Suspend part
+      ~~~~~~~~~~~~
+      running system, user asks for suspend-to-disk
+
+      user processes are stopped
+
+      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
+                      with state snapshot
+
+      state snapshot: copy of whole used memory is taken with interrupts disabled
+
+      resume(): devices are woken up so that we can write image to swap
+
+      write image to swap
+
+      suspend(PMSG_SUSPEND): suspend devices so that we can power off
+
+      turn the power off
+
+      Resume part
+      ~~~~~~~~~~~
+      (is actually pretty similar)
+
+      running system, user asks for suspend-to-disk
+
+      user processes are stopped (in common case there are none, but with resume-from-initrd, noone knows)
+
+      read image from disk
+
+      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
+                      with image restoration
+
+      image restoration: rewrite memory with image
+
+      resume(): devices are woken up so that system can continue
+
+      thaw all user processes
+
+Q: What is this 'Encrypt suspend image' for?
+
+A: First of all: it is not a replacement for dm-crypt encrypted swap.
+It cannot protect your computer while it is suspended. Instead it does
+protect from leaking sensitive data after resume from suspend.
+
+Think of the following: you suspend while an application is running
+that keeps sensitive data in memory. The application itself prevents
+the data from being swapped out. Suspend, however, must write these
+data to swap to be able to resume later on. Without suspend encryption
+your sensitive data are then stored in plaintext on disk.  This means
+that after resume your sensitive data are accessible to all
+applications having direct access to the swap device which was used
+for suspend. If you don't need swap after resume these data can remain
+on disk virtually forever. Thus it can happen that your system gets
+broken in weeks later and sensitive data which you thought were
+encrypted and protected are retrieved and stolen from the swap device.
+To prevent this situation you should use 'Encrypt suspend image'.
+
+During suspend a temporary key is created and this key is used to
+encrypt the data written to disk. When, during resume, the data was
+read back into memory the temporary key is destroyed which simply
+means that all data written to disk during suspend are then
+inaccessible so they can't be stolen later on.  The only thing that
+you must then take care of is that you call 'mkswap' for the swap
+partition used for suspend as early as possible during regular
+boot. This asserts that any temporary key from an oopsed suspend or
+from a failed or aborted resume is erased from the swap device.
+
+As a rule of thumb use encrypted swap to protect your data while your
+system is shut down or suspended. Additionally use the encrypted
+suspend image to prevent sensitive data from being stolen after
+resume.