FAILED: patch "[PATCH] block: Avoid mounting the bdev pseudo-filesystem in userspace" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x f73aa66dffcb8e61e78f01b56163ec16a15d06d2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026070222-ducking-flanked-b10f@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f73aa66dffcb8e61e78f01b56163ec16a15d06d2 Mon Sep 17 00:00:00 2001
From: Denis Arefev <[email protected]>
Date: Thu, 21 May 2026 10:28:56 +0300
Subject: [PATCH] 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 move_mount() uses pointers declared in the inode_operations 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>
 lookup_open.isra.0+0x700/0x1180 fs/namei.c:3460
 open_last_lookups fs/namei.c:3550 [inline]
 path_openat+0x953/0x2700 fs/namei.c:3780
 do_filp_open+0x1c5/0x410 fs/namei.c:3810
 do_sys_openat2+0x171/0x4d0 fs/open.c:1318
 do_sys_open fs/open.c:1334 [inline]
 __do_sys_openat fs/open.c:1350 [inline]
 __se_sys_openat fs/open.c:1345 [inline]
 __x64_sys_openat+0x13c/0x1f0 fs/open.c:1345
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x6e/0xd8

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

index e44a73903201..85ce57bd2ae4 100644
@@ -446,15 +446,10 @@ EXPORT_SYMBOL_GPL(blockdev_superblock);
 
 void __init bdev_cache_init(void)
 {
-	int err;
-
 	bdev_cachep = kmem_cache_create("bdev_cache", sizeof(struct bdev_inode),
 			0, (SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|
 				SLAB_ACCOUNT|SLAB_PANIC),
 			init_once);
-	err = register_filesystem(&bd_type);
-	if (err)
-		panic("Cannot register bdev pseudo-fs");
 	blockdev_mnt = kern_mount(&bd_type);
 	if (IS_ERR(blockdev_mnt))
 		panic("Cannot create bdev pseudo-fs");

From: Denis Arefev <[email protected]>

[ Upstream commit f73aa66dffcb8e61e78f01b56163ec16a15d06d2 ]

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 move_mount() uses pointers declared in the inode_operations 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>
 lookup_open.isra.0+0x700/0x1180 fs/namei.c:3460
 open_last_lookups fs/namei.c:3550 [inline]
 path_openat+0x953/0x2700 fs/namei.c:3780
 do_filp_open+0x1c5/0x410 fs/namei.c:3810
 do_sys_openat2+0x171/0x4d0 fs/open.c:1318
 do_sys_open fs/open.c:1334 [inline]
 __do_sys_openat fs/open.c:1350 [inline]
 __se_sys_openat fs/open.c:1345 [inline]
 __x64_sys_openat+0x13c/0x1f0 fs/open.c:1345
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x6e/0xd8

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

---
 block/bdev.c | 4 ----
 1 file changed, 4 deletions(-)

index ce7c20c2661793..e015fea1521e2b 100644
@@ -459,16 +459,12 @@ EXPORT_SYMBOL_GPL(blockdev_superblock);
 
 void __init bdev_cache_init(void)
 {
-	int err;
 	static struct vfsmount *bd_mnt;
 
 	bdev_cachep = kmem_cache_create("bdev_cache", sizeof(struct bdev_inode),
 			0, (SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|
 				SLAB_MEM_SPREAD|SLAB_ACCOUNT|SLAB_PANIC),
 			init_once);
-	err = register_filesystem(&bd_type);
-	if (err)
-		panic("Cannot register bdev pseudo-fs");
 	bd_mnt = kern_mount(&bd_type);
 	if (IS_ERR(bd_mnt))
 		panic("Cannot create bdev pseudo-fs");

2.53.0