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

[Harshit Mogalapalli]

Hi all,

On 01/12/25 23:49, Borislav Petkov wrote:
> On Mon, Dec 01, 2025 at 09:20:20AM -0800, Andrew Morton wrote:
> > On Wed, 12 Nov 2025 11:30:02 -0800 Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx> 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.
>
> Then why isn't there a ima_validate_range() function there which everyone
> calls instead of the same check being replicated everywhere?

Thanks for the reviews.

Sure, have tried this, will send a V2 with a generic helper.

> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Fixes: b69a2afd5afc ("x86/kexec: Carry forward IMA measurement log on kexec")
> >
> > That was via the x86 tree so I assume the x86 team (Boris?) will be
> > processing this patch.
>
> Yeah, it is on my to-deal-with-after-the-merge-window pile.
>
> But since you've forced my hand... :-P
>
> > I'll put it into mm.git's mm-hotfixes branch for now, to get a bit of
> > testing and to generally track its progress.
> >
> > > --- 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.
> >
> > "within" ;)

Thanks for spotting.

> > > +	 */
> > > +	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",
>
> This error message needs to be made a lot more user-friendly.
>
> And pls do a generic helper as suggested above which ima code calls.

Will do, thanks for the suggestion.

> And by looking at the diff, there are two ima_get_kexec_buffer() functions in
> the tree which could use some unification too ontop.

In drivers/of/kexec.c we have:

int __init ima_get_kexec_buffer(void **addr, size_t *size)
{
        int ret, len;
        unsigned long tmp_addr;
        unsigned long start_pfn, end_pfn;
        size_t tmp_size;
        const void *prop;

        prop = of_get_property(of_chosen, "linux,ima-kexec-buffer", &len);
        if (!prop)
                return -ENOENT;

        ret = do_get_kexec_buffer(prop, len, &tmp_addr, &tmp_size);
        if (ret)
                return ret;

        /* Do some sanity on the returned size for the ima-kexec buffer */
        if (!tmp_size)
                return -ENOENT;

        /*
         * Calculate the PFNs for the buffer and ensure
         * they are with in addressable memory.
         */
        start_pfn = PHYS_PFN(tmp_addr);
        end_pfn = PHYS_PFN(tmp_addr + tmp_size - 1);
        if (!page_is_ram(start_pfn) || !page_is_ram(end_pfn)) {
                pr_warn("IMA buffer at 0x%lx, size = 0x%zx beyond 
memory\n",
                        tmp_addr, tmp_size);
                return -EINVAL;
        }

        *addr = __va(tmp_addr);
        *size = tmp_size;

        return 0;
}

In arch/x86/kernel/setup.c we have something like:

int __init ima_get_kexec_buffer(void **addr, size_t *size)
{
        if (!ima_kexec_buffer_size)
                return -ENOENT;

        *addr = __va(ima_kexec_buffer_phys);
        *size = ima_kexec_buffer_size;

        return 0;
}

I will try to generalize common parts in another patch.

Will send a V2 adding ima_validate_range() helper.

Thanks,
Harshit.

> Right?
>
> Thx.