linux-kernel - Re: [PATCH 1/2] arm64/pageattr: Propagate return value from __change_memory

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Message-ID: <aSStn55E-omGunwA@willie-the-truck>
Date: Mon, 24 Nov 2025 19:10:23 +0000
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Dev Jain <dev.jain@....com>, catalin.marinas@....com, rppt@...nel.org,
	shijie@...amperecomputing.com, yang@...amperecomputing.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64/pageattr: Propagate return value from
 __change_memory_common

On Mon, Nov 24, 2025 at 06:09:31PM +0000, Ryan Roberts wrote:
> On 24/11/2025 15:11, Will Deacon wrote:
> > On Thu, Nov 13, 2025 at 11:55:48AM +0000, Ryan Roberts wrote:
> >> On 12/11/2025 06:27, Dev Jain wrote:
> >>> The rodata=on security measure requires that any code path which does
> >>> vmalloc -> set_memory_ro/set_memory_rox must protect the linear map alias
> >>> too. Therefore, if such a call fails, we must abort set_memory_* and caller
> >>> must take appropriate action; currently we are suppressing the error, and
> >>> there is a real chance of such an error arising post commit a166563e7ec3
> >>> ("arm64: mm: support large block mapping when rodata=full"). Therefore,
> >>> propagate any error to the caller.
> >>>
> >>> Fixes: a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
> >>> Signed-off-by: Dev Jain <dev.jain@....com>
> >>
> >> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> >>
> >> It would be good to get this into v6.18 I guess?
> > 
> > I'm not sure I see the urgency. When the commit message says:
> > 
> >   "there is a real chance of such an error arising post commit a166563e7ec3"
> > 
> > afaict that's either due to -ENOMEM 
> 
> Yes this is the main risk; failing to allocate an intermediate page table during
> split_kernel_leaf_mapping(). I have no idea how likely that is in production.
> The only data point I have is that for the theoretical memory hotplug -ENOMEM
> that Chaitanya and Linu fixed recently, we tried to provoke it by hotplugging a
> lot of memory on a system under high memory pressure; no matter what we did, we
> couldn't get that 4K pgtable allocation to fail. So on that basis, I think we
> are unlikely to see it.
> 
> > or some hideous issue with the
> > page-tables (e.g. the -EINVALs in pageattr_pXd_entry() seem completely
> > unnecessary to me).
> 
> I thought the -EINVALs were trying to catch the case where someone tries to set
> permissions on a sub-portion of a vmalloc_huge() area on a system that doesn't
> support BBML2_NOABORT. But looking again, we already refuse vmalloc_huge in
> change_memory_common.
> 
> > 
> > Do you think failure is actually likely and recoverable?
> 
> As above, I think failure is unlikely, but not impossible. I guess the result
> would be memory that remains RW when it should have been set RO or RX. But I
> think it will all work itself out at vfree. I think.
> 
> We can defer this to next cycle if you prefer.

Yeah, I think we have bigger problems if we can't find memory to allocate
page tables tbh.

Will