Patch "zsmalloc: calling zs_map_object() from irq is a bug" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

zsmalloc: calling zs_map_object() from irq is a bug

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
zsmalloc-calling-zs_map_object-from-irq-is-a-bug.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sergey Senozhatsky <[email protected]>
Date: Wed, 15 Nov 2017 17:34:03 -0800
Subject: zsmalloc: calling zs_map_object() from irq is a bug

From: Sergey Senozhatsky <[email protected]>

[ Upstream commit 1aedcafbf32b3f232c159b14cd0d423fcfe2b861 ]

Use BUG_ON(in_interrupt()) in zs_map_object(). This is not a new BUG_ON(), it's always been there, but was recently changed to VM_BUG_ON(). There are several problems there. First, we use use per-CPU mappings both in zsmalloc and in zram, and interrupt may easily corrupt those buffers. Second, and more importantly, we believe it's possible to start leaking sensitive information. Consider the following case:

-> process P
swap out
zram
per-cpu mapping CPU1
compress page A
-> IRQ

swap out
zram
per-cpu mapping CPU1
compress page B
write page from per-cpu mapping CPU1 to zsmalloc pool
iret

-> process P
write page from per-cpu mapping CPU1 to zsmalloc pool [*]
return

* so we store overwritten data that actually belongs to another page (task) and potentially contains sensitive data. And when process P will page fault it's going to read (swap in) that other task's data.

---
 mm/zsmalloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1407,7 +1407,7 @@ void *zs_map_object(struct zs_pool *pool
 	 * pools/users, we can't allow mapping in interrupt context
 	 * because it can corrupt another users mappings.
 	 */
-	WARN_ON_ONCE(in_interrupt());
+	BUG_ON(in_interrupt());
 
 	/* From now on, migration cannot move the object */
 	pin_tag(handle);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/x86-hpet-prevent-might-sleep-splat-on-resume.patch queue-4.9/zsmalloc-calling-zs_map_object-from-irq-is-a-bug.patch