linux-kernel - Re: [Bug report] hash_name() may cross page boundary and trigger

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Message-ID: <20251129094448.GL3538@ZenIV>
Date: Sat, 29 Nov 2025 09:44:48 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Xie Yuanbin <xieyuanbin1@...wei.com>
Cc: will@...nel.org, linux@...linux.org.uk, bigeasy@...utronix.de,
	rmk+kernel@...linux.org.uk, akpm@...ux-foundation.org,
	brauner@...nel.org, catalin.marinas@....com, hch@....de,
	jack@...e.com, linux-arm-kernel@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, pangliyuan1@...wei.com,
	wangkefeng.wang@...wei.com, wozizhi@...weicloud.com,
	yangerkun@...wei.com, lilinjie8@...wei.com, liaohua4@...wei.com
Subject: Re: [Bug report] hash_name() may cross page boundary and trigger

On Sat, Nov 29, 2025 at 05:25:45PM +0800, Xie Yuanbin wrote:
> On Sat, 29 Nov 2025 09:08:13 +0000, Al Viro wrote:
> > On Sat, Nov 29, 2025 at 12:08:17PM +0800, Xie Yuanbin wrote:
> >
> >> I think the `user_mode(regs)` check is necessary because the label
> >> no_context actually jumps to __do_kernel_fault(), whereas page fault
> >> from user mode should jump to `__do_user_fault()`.
> >>
> >> Alternatively, we would need to change `goto no_context` to
> >> `goto bad_area`. Or perhaps I misunderstood something, please point it out.
> >
> > FWIW, goto bad_area has an obvious problem: uses of 'fault' value, which
> > contains garbage.
> 
> Yes, I know it, I just omitted it. Thank you for pointing that out.
> 
> > or
> > 	if (unlikely(addr >= TASK_SIZE)) {
> > 		fault = 0;
> > 		code = SEGV_MAPERR;
> > 		goto bad_area;
> > 	}
> 
> In fact, I have already submitted another patch, which is exactly the way
> as you described:
> Link: https://lore.kernel.org/20251127140109.191657-1-xieyuanbin1@huawei.com
> 
> The only difference is that I will move the judgment to before
> local_irq_enable(). The reason for doing this is to fix another bug,
> you can find more details about it here:
> Link: https://lore.kernel.org/20250925025744.6807-1-xieyuanbin1@huawei.com
> Link: https://lore.kernel.org/20251129021815.9679-1-xieyuanbin1@huawei.com

AFAICS, your patch does nothing to the case when we hit kernel address from
kernel mode, which is what triggers that "block in RCU mode for no good reason"
fun...