linux-kernel - Re: [PATCH] x86/kexec: Add a sanity check on previous kernel's ima kexec buffer

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Message-ID: <20251201112221.GA137726@noodles-fedora.dhcp.thefacebook.com>
Date: Mon, 1 Dec 2025 11:22:22 +0000
From: Jonathan McDowell <noodles@...a.com>
To: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
CC: "henry.willard@...cle.com" <henry.willard@...cle.com>,
        Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov
	<bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org"
	<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton
	<akpm@...ux-foundation.org>,
        "Mike Rapoport (Microsoft)" <rppt@...nel.org>,
        Jiri Bohac <jbohac@...e.cz>, Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Guo
 Weikang <guoweikang.kernel@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Joel
 Granados <joel.granados@...nel.org>,
        Alexander Graf <graf@...zon.com>, Sohil
 Mehta <sohil.mehta@...el.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Jonathan
 McDowell <noodles@...a.com>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        "yifei.l.liu@...cle.com"
	<yifei.l.liu@...cle.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Paul Webb <paul.x.webb@...cle.com>
Subject: Re: [PATCH] x86/kexec: Add a sanity check on previous kernel's ima
 kexec buffer

On Wed, Nov 12, 2025 at 11:30:02AM -0800, Harshit Mogalapalli wrote:
> > 
> When the second-stage kernel is booted via kexec with a limiting command
> line such as "mem=<size>", the physical range that contains the carried
> over IMA measurement list may fall outside the truncated RAM leading to
> a kernel panic.
> 
>     BUG: unable to handle page fault for address: ffff97793ff47000
>     RIP: ima_restore_measurement_list+0xdc/0x45a
>     #PF: error_code(0x0000) – not-present page
> 
> Other architectures already validate the range with page_is_ram(), as
> done in commit: cbf9c4b9617b ("of: check previous kernel's
> ima-kexec-buffer against memory bounds") do a similar check on x86.
> 
> Cc: stable@...r.kernel.org
> Fixes: b69a2afd5afc ("x86/kexec: Carry forward IMA measurement log on kexec")
> Reported-by: Paul Webb <paul.x.webb@...cle.com>
> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>

Seems legit; this applies on the loaded kernel side, so we do end up
losing the measurements but that matches what OF does.

Reviewed-by: Jonathan McDowell <noodles@...a.com>

> ---
> Have tested the kexec for x86 kernel with IMA_KEXEC enabled and the
> above patch works good. Paul initially reported this on 6.12 kernel but
> I was able to reproduce this on 6.18, so I tried replicating how this
> was fixed in drivers/of/kexec.c
> ---
>  arch/x86/kernel/setup.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 1b2edd07a3e1..fcef197d180e 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -439,9 +439,23 @@ int __init ima_free_kexec_buffer(void)
>  
>  int __init ima_get_kexec_buffer(void **addr, size_t *size)
>  {
> +	unsigned long start_pfn, end_pfn;
> +
>  	if (!ima_kexec_buffer_size)
>  		return -ENOENT;
>  
> +	/*
> +	 * Calculate the PFNs for the buffer and ensure
> +	 * they are with in addressable memory.
> +	 */
> +	start_pfn = PFN_DOWN(ima_kexec_buffer_phys);
> +	end_pfn = PFN_DOWN(ima_kexec_buffer_phys + ima_kexec_buffer_size - 1);
> +	if (!pfn_range_is_mapped(start_pfn, end_pfn)) {
> +		pr_warn("IMA buffer at 0x%llx, size = 0x%zx beyond memory\n",
> +			ima_kexec_buffer_phys, ima_kexec_buffer_size);
> +		return -EINVAL;
> +	}
> +
>  	*addr = __va(ima_kexec_buffer_phys);
>  	*size = ima_kexec_buffer_size;
>  
> -- 
> 2.50.1
> 

-- 
Jonathan McDowell (he/him/his)
Production Engineer | PE Host Integrity
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