CVE-2024-56693

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

brd: defer automatic disk creation until module initialization succeeds

My colleague Wupeng found the following problems during fault injection:

BUG: unable to handle page fault for address: fffffbfff809d073 PGD 6e648067 P4D 123ec8067 PUD 123ec4067 PMD 100e38067 PTE 0 Oops: Oops: 0000 [#1] PREEMPT SMP KASAN NOPTI CPU: 5 UID: 0 PID: 755 Comm: modprobe Not tainted 6.12.0-rc3+ #17 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 RIP: 0010:__asanload8+0x4c/0xa0 ... Call Trace: <TASK> blkdevput_whole+0x41/0x70 bdevrelease+0x1a3/0x250 blkdevrelease+0x11/0x20 __fput+0x1d7/0x4a0 taskworkrun+0xfc/0x180 syscallexittousermode+0x1de/0x1f0 dosyscall64+0x6b/0x170 entrySYSCALL64afterhwframe+0x76/0x7e

loopinit() is calling loopadd() after _registerblkdev() succeeds and is ignoring diskadd() failure from loopadd(), for loopadd() failure is not fatal and successfully created disks are already visible to bdevopen().

brdinit() is currently calling brdalloc() before __registerblkdev() succeeds and is releasing successfully created disks when brdinit() returns an error. This can cause UAF for the latter two case:

case 1: T1: modprobe brd brdinit brdalloc(0) // success adddisk diskscanpartitions bdevfileopenbydev // alloc file fput // won't free until back to userspace brdalloc(1) // failed since mem alloc error inject // error path for modprobe will release code segment // back to userspace _fput blkdevrelease bdevrelease blkdevputwhole bdev->bddisk->fops->release // fops is freed now, UAF!

case 2: T1: T2: modprobe brd brdinit brdalloc(0) // success open(/dev/ram0) brd_alloc(1) // fail // error path for modprobe

                               close(/dev/ram0)
                               ...
                               /* UAF! */
                               bdev->bd_disk->fops->release

Fix this problem by following what loopinit() does. Besides, reintroduce brddevicesmutex to help serialize modifications to brdlist.