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Subject[PATCH] zsmalloc: calling zs_map_object() from irq is a bug
Date
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.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
---
mm/zsmalloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 7c38e850a8fc..685049a9048d 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1349,7 +1349,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
* 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);
--
2.14.2
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