Re: [PATCH] ses: Fix racy cleanup of /sys in remove_dev()

From: Calvin Owens
Date: Thu Jun 02 2016 - 18:50:42 EST

On 05/13/2016 01:28 PM, Calvin Owens wrote:
Currently we free the resources backing the enclosure device before we
call device_unregister(). This is racy: during rmmod of low-level SCSI
drivers that hook into enclosure, we end up with a small window of time
during which writing to /sys can OOPS. Example trace with mpt3sas:


general protection fault: 0000 [#1] SMP KASAN
Modules linked in: mpt3sas(-) <...>
RIP: [<ffffffffa0388a98>] ses_get_page2_descriptor.isra.6+0x38/0x220 [ses]
Call Trace:
[<ffffffffa0389d14>] ses_set_fault+0xf4/0x400 [ses]
[<ffffffffa0361069>] set_component_fault+0xa9/0xf0 [enclosure]
[<ffffffff8205bffc>] dev_attr_store+0x3c/0x70
[<ffffffff81677df5>] sysfs_kf_write+0x115/0x180
[<ffffffff81675725>] kernfs_fop_write+0x275/0x3a0
[<ffffffff8151f810>] __vfs_write+0xe0/0x3e0
[<ffffffff8152281f>] vfs_write+0x13f/0x4a0
[<ffffffff81526731>] SyS_write+0x111/0x230
[<ffffffff828b401b>] entry_SYSCALL_64_fastpath+0x13/0x94

Fortunately the solution is extremely simple: call device_unregister()
before we free the resources, and the race no longer exists. The driver
core holds a reference over ->remove_dev(), so AFAICT this is safe.

Signed-off-by: Calvin Owens <calvinowens@xxxxxx>
drivers/scsi/ses.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/ses.c b/drivers/scsi/ses.c
index 53ef1cb..0e8601a 100644
--- a/drivers/scsi/ses.c
+++ b/drivers/scsi/ses.c
@@ -778,6 +778,8 @@ static void ses_intf_remove_enclosure(struct scsi_device *sdev)
if (!edev)

+ enclosure_unregister(edev);
ses_dev = edev->scratch;
edev->scratch = NULL;

@@ -789,7 +791,6 @@ static void ses_intf_remove_enclosure(struct scsi_device *sdev)

- enclosure_unregister(edev);

static void ses_intf_remove(struct device *cdev,