CVE-2024-42271: net/iucv: fix use after free in iucv_sock_close()

In the Linux kernel, the following vulnerability has been resolved:

net/iucv: fix use after free in iucvsockclose()

iucvseverpath() is called from process context and from bh context. iucv->path is used as indicator whether somebody else is taking care of severing the path (or it is already removed / never existed). This needs to be done with atomic compare and swap, otherwise there is a small window where iucvsockclose() will try to work with a path that has already been severed and freed by iucvcallbackconnrej() called by iucvtaskletfn().

Example: [452744.123844] Call Trace: [452744.123845] ([<0000001e87f03880>] 0x1e87f03880) [452744.123966] [<00000000d593001e>] iucvpathsever+0x96/0x138 [452744.124330] [<000003ff801ddbca>] iucvseverpath+0xc2/0xd0 [afiucv] [452744.124336] [<000003ff801e01b6>] iucvsockclose+0xa6/0x310 [afiucv] [452744.124341] [<000003ff801e08cc>] iucvsockrelease+0x3c/0xd0 [afiucv] [452744.124345] [<00000000d574794e>] sockrelease+0x5e/0xe8 [452744.124815] [<00000000d5747a0c>] sockclose+0x34/0x48 [452744.124820] [<00000000d5421642>] fput+0xba/0x268 [452744.124826] [<00000000d51b382c>] taskworkrun+0xbc/0xf0 [452744.124832] [<00000000d5145710>] donotifyresume+0x88/0x90 [452744.124841] [<00000000d5978096>] systemcall+0xe2/0x2c8 [452744.125319] Last Breaking-Event-Address: [452744.125321] [<00000000d5930018>] iucvpathsever+0x90/0x138 [452744.125324] [452744.125325] Kernel panic - not syncing: Fatal exception in interrupt

Note that bhlocksock() is not serializing the tasklet context against process context, because the check for sockownedbyuser() and corresponding handling is missing.

Ideas for a future clean-up patch: A) Correct usage of bhlocksock() in tasklet context, as described in Re-enqueue, if needed. This may require adding return values to the tasklet functions and thus changes to all users of iucv.

B) Change iucv tasklet into worker and use only locksock() in afiucv.