Re: [PATCH RFC v4] jfs: validate BT_ROOT flag in xt_getpage and DT_GETPAGE

On 29-07-2026 04:15 pm, syzbot wrote:
> A corrupted JFS image can trick the kernel into calling release_metapage()
> on a pseudo-metapage embedded within the inode, leading to an out-of-bounds
> memory access and a spinlock crash.
> 
> When operating on the root page of an xtree or dtree (which is stored
> inline within the inode), JFS doesn't use a real struct metapage. Instead,
> BT_GETPAGE() sets the metapage pointer to the address of
> JFS_IP(ip)->bxflag. Because this is a pseudo-metapage, it does not have a
> valid wait_queue_head_t wait field. Accessing mp->wait reads out-of-bounds
> memory in struct jfs_inode_info.
> 
> In xt_getpage(), the code validates nextindex and maxentry, but it fails to
> verify that the BT_ROOT flag is set in p->header.flag for the root page (bn
> == 0). If the inline xtree root page has a corrupted header.flag where
> BT_ROOT is missing, xtTruncate() will mistakenly treat the root page as a
> regular leaf page and call discard_metapage(mp).
> 
> Unlike XT_PUTPAGE() which safely checks !BT_IS_ROOT(mp) before releasing,
> discard_metapage() calls release_metapage(mp) directly. This assumes mp is
> a real metapage and calls unlock_metapage(mp), which executes
> wake_up(&mp->wait), accessing garbage memory and resulting in a spinlock
> bad magic BUG or UBSAN out-of-bounds array access.
> 
> Fix this by enforcing the invariant that the BT_ROOT flag is set if and
> only if bn == 0 in both xt_getpage() and DT_GETPAGE().
> 
> BUG: spinlock bad magic on CPU#1, comm/6074
>   lock: 0xffff8881f71d1b48, .magic: ffffffff, .owner: /-1902340672,
>   .owner_cpu: 768
> CPU: 1 UID: 0 PID: 6074 Comm: comm Not tainted #1 PREEMPT(full)
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>   spin_bug kernel/locking/spinlock_debug.c:78 [inline]
>   debug_spin_lock_before kernel/locking/spinlock_debug.c:86 [inline]
>   do_raw_spin_lock+0x1e5/0x2f0 kernel/locking/spinlock_debug.c:115
>   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:133 [inline]
>   _raw_spin_lock_irqsave+0x4c/0x60 kernel/locking/spinlock.c:166
>   __wake_up_common_lock+0x30/0x1f0 kernel/sched/wait.c:124
>   unlock_metapage fs/jfs/jfs_metapage.c:40 [inline]
>   release_metapage+0x131/0xa60 fs/jfs/jfs_metapage.c:872
>   xtTruncate+0xeaa/0x2eb0 fs/jfs/jfs_xtree.c:-1
>   jfs_free_zero_link+0x35b/0x4c0 fs/jfs/namei.c:760
>   jfs_evict_inode+0x356/0x430 fs/jfs/inode.c:159
>   evict+0x624/0xb50 fs/inode.c:841
>   ...
>   </TASK>
> 
> UBSAN: array-index-out-of-bounds in kernel/locking/qspinlock.h:68:9
> index 8945 is out of range for type 'unsigned long[8]'
> CPU: 1 UID: 0 PID: 6074 Comm: comm Not tainted #1 PREEMPT(full)
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>   ubsan_epilogue+0xa/0x30 lib/ubsan.c:233
>   __ubsan_handle_out_of_bounds+0xe8/0xf0 lib/ubsan.c:455
>   decode_tail kernel/locking/qspinlock.h:68 [inline]
>   __pv_queued_spin_lock_slowpath+0xaf3/0xbc0 kernel/locking/qspinlock.c:285
>   pv_queued_spin_lock_slowpath arch/x86/include/asm/paravirt-spinlock.h:35
>   [inline]
>   queued_spin_lock_slowpath arch/x86/include/asm/paravirt-spinlock.h:66
>   [inline]
>   queued_spin_lock include/asm-generic/qspinlock.h:114 [inline]
>   do_raw_spin_lock+0x26c/0x2f0 kernel/locking/spinlock_debug.c:116
>   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:133 [inline]
>   _raw_spin_lock_irqsave+0x4c/0x60 kernel/locking/spinlock.c:166
>   __wake_up_common_lock+0x30/0x1f0 kernel/sched/wait.c:124
>   unlock_metapage fs/jfs/jfs_metapage.c:40 [inline]
>   release_metapage+0x131/0xa60 fs/jfs/jfs_metapage.c:872
>   xtTruncate+0xeaa/0x2eb0 fs/jfs/jfs_xtree.c:-1
>   ...
>   </TASK>
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=e380443eaa59bfb75a84
> Link: https://syzkaller.appspot.com/ai_job?id=a0b45181-6c6e-42a4-85c9-89f4273612e3
> To: <[email protected]>
> To: "Dave Kleikamp" <[email protected]>
> Cc: "Arnd Bergmann" <[email protected]>
> Cc: <[email protected]>
> Cc: "Yun Zhou" <[email protected]>
> Cc: "Zheng Yu" <[email protected]>
> 
> ---
> v4:
> - Avoided unnecessary whitespace and formatting changes in DT_GETPAGE and xt_getpage.
> - Kept the existing DT_GETPAGE error message for both corruption checks.
> 
> v3:
> - Validated root/non-root invariant failures and non-root dtpage integrity failures.
> - Removed '------------[ cut here ]------------' from the commit message.
> https://lore.kernel.org/all/[email protected]/T/
> 
> v2:
> - Combined the BT_ROOT flag validation with the existing page integrity check in DT_GETPAGE.
> - Unified the error message in DT_GETPAGE for both root and leaf page corruption.
> https://lore.kernel.org/all/[email protected]/T/
> 
> v1:
> https://lore.kernel.org/all/[email protected]/T/
> ---
> diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
> index ac0f79faf..0d372a237 100644
> --- a/fs/jfs/jfs_dtree.c
> +++ b/fs/jfs/jfs_dtree.c
> @@ -115,7 +115,8 @@ struct dtsplit {
>   do {									\
>   	BT_GETPAGE(IP, BN, MP, dtpage_t, SIZE, P, RC, i_dtroot);	\
>   	if (!(RC)) {							\
> -		if ((BN) && !check_dtpage(P)) {				\
> +		if ((((BN) == 0) != !!((P)->header.flag & BT_ROOT)) ||	\
> +		    ((BN) && !check_dtpage(P))) {			\
>   			BT_PUTPAGE(MP);					\
>   			jfs_error((IP)->i_sb,				\
>   				  "DT_GETPAGE: dtree page corrupt\n");	\
> diff --git a/fs/jfs/jfs_xtree.c b/fs/jfs/jfs_xtree.c
> index 28c3cf960..3a5f9d035 100644
> --- a/fs/jfs/jfs_xtree.c
> +++ b/fs/jfs/jfs_xtree.c
> @@ -121,7 +121,8 @@ static inline xtpage_t *xt_getpage(struct inode *ip, s64 bn, struct metapage **m
>   		(le16_to_cpu(p->header.nextindex) >
>   			le16_to_cpu(p->header.maxentry)) ||
>   		(le16_to_cpu(p->header.maxentry) >
> -			((bn == 0) ? XTROOTMAXSLOT : PSIZE >> L2XTSLOTSIZE))) {
> +			((bn == 0) ? XTROOTMAXSLOT : PSIZE >> L2XTSLOTSIZE)) ||
> +		((bn == 0) != !!(p->header.flag & BT_ROOT))) {
>   		jfs_error(ip->i_sb, "xt_getpage: xtree page corrupt\n");
>   		BT_PUTPAGE(*mp);
>   		*mp = NULL;
> 
> 
> base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6

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