CVE-2022-48726

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

RDMA/ucma: Protect mc during concurrent multicast leaves

Partially revert the commit mentioned in the Fixes line to make sure that allocation and erasing multicast struct are locked.

BUG: KASAN: use-after-free in ucmacleanupmulticast drivers/infiniband/core/ucma.c:491 [inline] BUG: KASAN: use-after-free in ucmadestroyprivate_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579 Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529 CPU: 0 PID: 25529 Comm: syz-executor.1 Not tainted 5.16.0-rc7-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dumpstack lib/dumpstack.c:88 [inline] dumpstacklvl+0xcd/0x134 lib/dumpstack.c:106 printaddress_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247 __kasanreport mm/kasan/report.c:433 [inline] kasanreport.cold+0x83/0xdf mm/kasan/report.c:450 ucmacleanupmulticast drivers/infiniband/core/ucma.c:491 [inline] ucmadestroyprivatectx+0x914/0xb70 drivers/infiniband/core/ucma.c:579 ucmadestroyid+0x1e6/0x280 drivers/infiniband/core/ucma.c:614 ucmawrite+0x25c/0x350 drivers/infiniband/core/ucma.c:1732 vfswrite+0x28e/0xae0 fs/readwrite.c:588 ksyswrite+0x1ee/0x250 fs/readwrite.c:643 dosyscallx64 arch/x86/entry/common.c:50 [inline] dosyscall64+0x35/0xb0 arch/x86/entry/common.c:80 entrySYSCALL64afterhwframe+0x44/0xae

Currently the xarray search can touch a concurrently freeing mc as the xaforeach() is not surrounded by any lock. Rather than hold the lock for a full scan hold it only for the effected items, which is usually an empty list.