linux-kernel - Re: [syzbot] [efi?] [fs?] possible deadlock in efivarfs

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Message-ID: <20250310232530.GK2023217@ZenIV>
Date: Mon, 10 Mar 2025 23:25:30 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: James Bottomley <James.Bottomley@...senpartnership.com>,
	syzbot <syzbot+019072ad24ab1d948228@...kaller.appspotmail.com>,
	jk@...abs.org, linux-efi@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [efi?] [fs?] possible deadlock in efivarfs_actor

On Mon, Mar 10, 2025 at 07:24:43PM +0100, Ard Biesheuvel wrote:

> And one of the other logs has
> 
> [   47.650966][ T6617] syz.2.9/6617 is trying to acquire lock:
> [   47.652339][ T6617] ffff0000d69f6558
> (&sb->s_type->i_mutex_key#25){++++}-{4:4}, at:
> efivarfs_actor+0x1b8/0x2b8
> [   47.654943][ T6617]
> [   47.654943][ T6617] but task is already holding lock:
> [   47.656931][ T6617] ffff0000f5b84558
> (&sb->s_type->i_mutex_key#25){++++}-{4:4}, at: iterate_dir+0x3b4/0x5f4
> 
> where the locks have the same name but the address is different.
> 
> So there is something dodgy going on here, and I'm inclined to just ignore it.

That one is a false positive - iterate_dir() locks parent, then
callback locks child, but without bothering to tell lockdep about
that.  IOW, in actor you should use inode_lock_nested(inode, INODE_CHILD);
instead of inode_lock(inode).