linux-kernel - Re: [RFC PATCH] vfs: Fix might sleep in load_unaligned

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Message-ID: <20251126192640.GD3538@ZenIV>
Date: Wed, 26 Nov 2025 19:26:40 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Xie Yuanbin <xieyuanbin1@...wei.com>, brauner@...nel.org, jack@...e.cz,
	will@...nel.org, nico@...xnic.net, akpm@...ux-foundation.org,
	hch@....de, jack@...e.com, wozizhi@...weicloud.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	lilinjie8@...wei.com, liaohua4@...wei.com,
	wangkefeng.wang@...wei.com, pangliyuan1@...wei.com
Subject: Re: [RFC PATCH] vfs: Fix might sleep in load_unaligned_zeropad()
 with rcu read lock held

On Wed, Nov 26, 2025 at 07:05:05PM +0000, Russell King (Oracle) wrote:
> On Wed, Nov 26, 2025 at 06:48:20PM +0000, Al Viro wrote:
> > It's been years since I looked at 32bit arm exception handling, so I'd need
> > quite a bit of (re)RTF{S,M} before I'm comfortable with poking in
> > arch/arm/mm/fault.c; better let ARM folks deal with that.  But arch/* is
> > where it should be dealt with; as for papering over that in fs/*:
> 
> Don't expect that to happen. I've not looked at it for over a decade,
> I do very little 32-bit ARM stuff anymore. Others have modified the
> fault handling, the VM has changed, I basically no longer have the
> knowledge. Effectively, 32-bit ARM is unmaintained now, although it
> still has many users.

Joy...  For quick and dirty variant (on current tree), how about
adding
	if (unlikely(addr > TASK_SIZE) && !user_mode(regs))
		goto no_context;

right after

	if (!ttbr0_usermode_access_allowed(regs))
		goto no_context;

in do_page_fault() there?

NOTE: that might or might not break vdso; I don't think it would, but...