On 22.04.2024 13:34, Roger Pau Monné wrote:
> On Mon, Apr 22, 2024 at 12:57:55PM +0200, Jan Beulich wrote:
>> On 22.04.2024 12:49, Roger Pau Monné wrote:
>>> On Mon, Apr 22, 2024 at 10:25:40AM +0200, Jan Beulich wrote:
>>>> On 22.04.2024 09:54, Roger Pau Monné wrote:
>>>>> On Fri, Apr 19, 2024 at 04:34:41PM +0200, Jan Beulich wrote:
>>>>>> On 19.04.2024 12:50, Roger Pau Monné wrote:
>>>>>>> On Fri, Apr 19, 2024 at 12:15:19PM +0200, Jan Beulich wrote:
>>>>>>>> On 19.04.2024 12:02, Roger Pau Monne wrote:
>>>>>>>>> Livepatch payloads containing symbols that belong to init sections
>>>>>>>>> can only
>>>>>>>>> lead to page faults later on, as by the time the livepatch is loaded
>>>>>>>>> init
>>>>>>>>> sections have already been freed.
>>>>>>>>>
>>>>>>>>> Refuse to resolve such symbols and return an error instead.
>>>>>>>>>
>>>>>>>>> Note such resolutions are only relevant for symbols that point to
>>>>>>>>> undefined
>>>>>>>>> sections (SHN_UNDEF), as that implies the symbol is not in the
>>>>>>>>> current payload
>>>>>>>>> and hence must either be a Xen or a different livepatch payload
>>>>>>>>> symbol.
>>>>>>>>>
>>>>>>>>> Do not allow to resolve symbols that point to __init_begin, as that
>>>>>>>>> address is
>>>>>>>>> also unmapped. On the other hand, __init_end is not unmapped, and
>>>>>>>>> hence allow
>>>>>>>>> resolutions against it.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Roger Pau Monné <roger....@citrix.com>
>>>>>>>>> ---
>>>>>>>>> Changes since v1:
>>>>>>>>> - Fix off-by-one in range checking.
>>>>>>>>
>>>>>>>> Which means you addressed Andrew's comment while at the same time
>>>>>>>> extending
>>>>>>>> the scope of ...
>>>>>>>>
>>>>>>>>> @@ -310,6 +311,21 @@ int livepatch_elf_resolve_symbols(struct
>>>>>>>>> livepatch_elf *elf)
>>>>>>>>> break;
>>>>>>>>> }
>>>>>>>>> }
>>>>>>>>> +
>>>>>>>>> + /*
>>>>>>>>> + * Ensure not an init symbol. Only applicable to Xen
>>>>>>>>> symbols, as
>>>>>>>>> + * livepatch payloads don't have init sections or
>>>>>>>>> equivalent.
>>>>>>>>> + */
>>>>>>>>> + else if ( st_value >= (uintptr_t)&__init_begin &&
>>>>>>>>> + st_value < (uintptr_t)&__init_end )
>>>>>>>>> + {
>>>>>>>>> + printk(XENLOG_ERR LIVEPATCH
>>>>>>>>> + "%s: symbol %s is in init section, not
>>>>>>>>> resolving\n",
>>>>>>>>> + elf->name, elf->sym[i].name);
>>>>>>>>
>>>>>>>> ... what I raised concern about (and I had not seen any verbal reply
>>>>>>>> to that,
>>>>>>>> I don't think).
>>>>>>>
>>>>>>> I've extended the commit message to explicitly mention the handling of
>>>>>>> bounds for __init_{begin,end} checks. Let me know if you are not fine
>>>>>>> with it (or maybe you expected something else?).
>>>>>>
>>>>>> Well, you mention the two symbols you care about, but you don't mention
>>>>>> at all that these two may alias other symbols which might be legal to
>>>>>> reference from a livepatch.
>>>>>
>>>>> I'm having a hard time understanding why a livepatch would want to
>>>>> reference those, or any symbol in the .init sections for that matter.
>>>>> __init_{begin,end} are exclusively used to unmap the init region after
>>>>> boot. What's the point in livepatch referencing data that's no
>>>>> longer there? The only application I would think of is to calculate
>>>>> some kind of offsets, but that would better be done using other
>>>>> symbols instead.
>>>>>
>>>>> Please bear with me, it would be easier for me to understand if you
>>>>> could provide a concrete example.
>>>>
>>>> The issue is that you do comparison by address, not by name. Let's assume
>>>> that .rodata ends exactly where .init.* starts. Then &__init_begin ==
>>>> &_erodata == &__2M_rodata_end. Access to the latter two symbols wants to
>>>> be permitted; only __init_begin and whatever ends up being the very first
>>>> symbol in .init.* ought to not be referenced.
>>>
>>> Hm, I guess I could add comparison by name additionally, but it looks
>>> fragile to me.
>>
>> It looks fragile, yes. Thing is that you're trying to deal with this when
>> crucial information was lost already. Or wait - isn't the section number
>> still available in ->st_shndx?
>
> But that's the section number of the to be resolved symbol? In the
> livepatch payload it would for example appear as:
>
> 0000000000000000 F *UND* 0000000000000000 .hidden
> setup_boot_APIC_clock
>
> With undefined section.
>
> It's possible I'm not following, is there a way to get the section
> number of the current Xen symbols, and then correlate it with the
> .init section?
Hmm, yes, looks like I was forgetting that livepatch resolves symbols
using kallsyms data, not Xen's ELF symbol table.
Jan
