AZL-92531

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

block: Avoid mounting the bdev pseudo-filesystem in userspace

The bdev pseudo-filesystem is an internal kernel filesystem with which userspace should not interfere. Unregister it so that userspace cannot even attempt to mount it.

This fixes a bug [1] that occurs when attempting to access files, because the system call movemount() uses pointers declared in the inodeoperations structure, which for the bdev pseudo-filesystem are always equal to 0. inode->i_op = &empty_iops;

[1]

BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD 23380067 P4D 23380067 PUD 23381067 PMD 0 Oops: 0010 [#1] PREEMPT SMP KASAN NOPTI CPU: 2 PID: 17125 Comm: syz-executor.0 Not tainted 6.1.155-syzkaller-00350-g84221fde2681 #0 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 RIP: 0010:0x0

Call Trace: <TASK> lookupopen.isra.0+0x700/0x1180 fs/namei.c:3460 openlastlookups fs/namei.c:3550 [inline] pathopenat+0x953/0x2700 fs/namei.c:3780 dofilpopen+0x1c5/0x410 fs/namei.c:3810 dosysopenat2+0x171/0x4d0 fs/open.c:1318 dosysopen fs/open.c:1334 [inline] __dosysopenat fs/open.c:1350 [inline] __sesysopenat fs/open.c:1345 [inline] __x64sysopenat+0x13c/0x1f0 fs/open.c:1345 dosyscallx64 arch/x86/entry/common.c:51 [inline] dosyscall64+0x35/0x80 arch/x86/entry/common.c:81 entrySYSCALL64afterhwframe+0x6e/0xd8

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.