From: Jan Beulich <jbeulich@suse.com>
To: Jason Andryuk <jandryuk@gmail.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
Andrew Cooper <andrew.cooper3@citrix.com>,
George Dunlap <george.dunlap@citrix.com>,
Ian Jackson <iwj@xenproject.org>, Julien Grall <julien@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH] libelf: Handle PVH kernels lacking ENTRY elfnote
Date: Thu, 15 Oct 2020 09:00:09 +0200 [thread overview]
Message-ID: <4229544b-e98d-6f3c-14aa-a884c403ba74@suse.com> (raw)
In-Reply-To: <CAKf6xpv5GNjw0pjOxEqdVj2+C6v+O5PDZG5yYkNfytDjUT_r5w@mail.gmail.com>
On 14.10.2020 18:27, Jason Andryuk wrote:
> On Wed, Oct 14, 2020 at 12:02 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 14.10.2020 17:31, Jason Andryuk wrote:
>>> Linux kernels only have an ENTRY elfnote when built with CONFIG_PV. A
>>> kernel build CONFIG_PVH=y CONFIG_PV=n lacks the note. In this case,
>>> virt_entry will be UNSET_ADDR, overwritten by the ELF header e_entry,
>>> and fail the check against the virt address range.
>
> Oh, these should be CONFIG_XEN_PVH=y and CONFIG_XEN_PV=n
>
>>> Change the code to only check virt_entry against the virtual address
>>> range if it was set upon entry to the function.
>>
>> Not checking at all seems wrong to me. The ELF spec anyway says
>> "virtual address", so an out of bounds value is at least suspicious.
>>
>>> Maybe the overwriting of virt_entry could be removed, but I don't know
>>> if there would be unintended consequences where (old?) kernels don't
>>> have an elfnote, but do have an in-range e_entry? The failing kernel I
>>> just looked at has an e_entry of 0x1000000.
>>
>> And if you dropped the overwriting, what entry point would we use
>> in the absence of an ELF note?
>
> elf_xen_note_check currently has:
>
> /* PVH only requires one ELF note to be set */
> if ( parms->phys_entry != UNSET_ADDR32 )
> {
> elf_msg(elf, "ELF: Found PVH image\n");
> return 0;
> }
>
>> I'd rather put up the option of adjusting the entry (or the check),
>> if it looks like a valid physical address.
>
> The function doesn't know if the image will be booted PV or PVH, so I
> guess we do all the checks, but use 'parms->phys_entry != UNSET_ADDR32
> && parms->virt_entry == UNSET_ADDR' to conditionally skip checking
> virt?
Like Jürgen, the purpose of the patch hadn't become clear to me
from reading the description. As I understand it now, we're currently
refusing to boot such a kernel for no reason. If that's correct,
perhaps you could say so in the description in a more direct way?
As far as actual code adjustments go - how much of
elf_xen_addr_calc_check() is actually applicable when booting PVH?
And why is there no bounds check of ->phys_entry paralleling the
->virt_entry one?
On the whole, as long as we don't know what mode we're planning to
boot in, we can't skip any checks, as the mere presence of
XEN_ELFNOTE_PHYS32_ENTRY doesn't mean that's also what gets used.
Therefore simply bypassing any of the checks is not an option. In
particular what you suggest would lead to failure to check
e_entry-derived ->virt_entry when the PVH-specific note is
present but we're booting in PV mode. For now I don't see how to
address this without making the function aware of the intended
booting mode.
Jan
next prev parent reply other threads:[~2020-10-15 7:00 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-14 15:31 [PATCH] libelf: Handle PVH kernels lacking ENTRY elfnote Jason Andryuk
2020-10-14 15:52 ` Wei Liu
2020-10-14 16:02 ` Jan Beulich
2020-10-14 16:27 ` Jason Andryuk
2020-10-15 7:00 ` Jan Beulich [this message]
2020-10-15 14:50 ` Jason Andryuk
2020-10-15 15:14 ` Jan Beulich
2020-10-15 17:27 ` Andrew Cooper
2020-10-16 16:28 ` Jason Andryuk
2020-10-19 7:38 ` Jan Beulich
2020-10-19 15:26 ` Jason Andryuk
2020-10-19 15:36 ` Jan Beulich
2020-10-15 15:03 ` Roger Pau Monné
2020-10-14 16:12 ` Jürgen Groß
2020-10-14 16:27 ` Jason Andryuk
2020-10-15 4:17 ` Jürgen Groß
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