Re: [PATCH v6] Avoid crash calling PrintErrMesg from efi_multiboot2

[Jan Beulich <jbeulich@suse.com>]

On 26.02.2025 19:54, Marek Marczykowski-Górecki wrote:
> On Mon, Feb 24, 2025 at 02:31:00PM +0000, Frediano Ziglio wrote:
>> On Mon, Feb 24, 2025 at 1:16 PM Marek Marczykowski-Górecki
>> <marmarek@invisiblethingslab.com> wrote:
>>>
>>> On Mon, Feb 24, 2025 at 12:57:13PM +0000, Frediano Ziglio wrote:
>>>> On Fri, Feb 21, 2025 at 8:20 PM Marek Marczykowski-Górecki
>>>> <marmarek@invisiblethingslab.com> wrote:
>>>>>
>>>>> On Mon, Feb 17, 2025 at 04:26:59PM +0000, Frediano Ziglio wrote:
>>>>>> Although code is compiled with -fpic option data is not position
>>>>>> independent. This causes data pointer to become invalid if
>>>>>> code is not relocated properly which is what happens for
>>>>>> efi_multiboot2 which is called by multiboot entry code.
>>>>>>
>>>>>> Code tested adding
>>>>>>    PrintErrMesg(L"Test message", EFI_BUFFER_TOO_SMALL);
>>>>>> in efi_multiboot2 before calling efi_arch_edd (this function
>>>>>> can potentially call PrintErrMesg).
>>>>>>
>>>>>> Before the patch (XenServer installation on Qemu, xen replaced
>>>>>> with vanilla xen.gz):
>>>>>>   Booting `XenServer (Serial)'Booting `XenServer (Serial)'
>>>>>>   Test message: !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU Apic ID - 00000000 !!!!
>>>>>>   ExceptionData - 0000000000000000  I:0 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0
>>>>>>   RIP  - 000000007EE21E9A, CS  - 0000000000000038, RFLAGS - 0000000000210246
>>>>>>   RAX  - 000000007FF0C1B5, RCX - 0000000000000050, RDX - 0000000000000010
>>>>>>   RBX  - 0000000000000000, RSP - 000000007FF0C180, RBP - 000000007FF0C210
>>>>>>   RSI  - FFFF82D040467CE8, RDI - 0000000000000000
>>>>>>   R8   - 000000007FF0C1C8, R9  - 000000007FF0C1C0, R10 - 0000000000000000
>>>>>>   R11  - 0000000000001020, R12 - FFFF82D040467CE8, R13 - 000000007FF0C1B8
>>>>>>   R14  - 000000007EA33328, R15 - 000000007EA332D8
>>>>>>   DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
>>>>>>   GS   - 0000000000000030, SS  - 0000000000000030
>>>>>>   CR0  - 0000000080010033, CR2 - FFFF82D040467CE8, CR3 - 000000007FC01000
>>>>>>   CR4  - 0000000000000668, CR8 - 0000000000000000
>>>>>>   DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
>>>>>>   DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
>>>>>>   GDTR - 000000007F9DB000 0000000000000047, LDTR - 0000000000000000
>>>>>>   IDTR - 000000007F48E018 0000000000000FFF,   TR - 0000000000000000
>>>>>>   FXSAVE_STATE - 000000007FF0BDE0
>>>>>>   !!!! Find image based on IP(0x7EE21E9A) (No PDB)  (ImageBase=000000007EE20000, EntryPoint=000000007EE23935) !!!!
>>>>>>
>>>>>> After the patch:
>>>>>>   Booting `XenServer (Serial)'Booting `XenServer (Serial)'
>>>>>>   Test message: Buffer too small
>>>>>>   BdsDxe: loading Boot0000 "UiApp" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
>>>>>>   BdsDxe: starting Boot0000 "UiApp" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
>>>>>>
>>>>>> This partially rollback commit 00d5d5ce23e6.
>>>>>>
>>>>>> Fixes: 9180f5365524 ("x86: add multiboot2 protocol support for EFI platforms")
>>>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
>>>>>
>>>>> I tried testing this patch, but it seems I cannot reproduce the original
>>>>> failure...
>>>>>
>>>>> I did as the commit message suggests here:
>>>>> https://gitlab.com/xen-project/people/marmarek/xen/-/commit/ca3d6911c448eb886990f33d4380b5646617a982
>>>>>
>>>>> With blexit() in PrintErrMesg(), it went back to the bootloader, so I'm
>>>>> sure this code path was reached. But with blexit() commented out, Xen
>>>>> started correctly both with and without this patch... The branch I used
>>>>> is here:
>>>>> https://gitlab.com/xen-project/people/marmarek/xen/-/commits/automation-tests?ref_type=heads
>>>>>
>>>>> Are there some extra condition to reproduce the issue? Maybe it depends
>>>>> on the compiler version? I guess I can try also on QEMU, but based on
>>>>> the description, I would expect it to crash in any case.
>>>>>
>>>>
>>>> Did you see the correct message in both cases?
>>>> Did you use Grub or direct EFI?
>>>>
>>>> With Grub and without this patch you won't see the message, with grub
>>>> with the patch you see the correct message.
>>>
>>> I did use grub, and I didn't see the message indeed.
>>> But in the case it was supposed to crash (with added PrintErrMesg(),
>>> commented out blexit and without your patch) it did _not_ crashed and
>>> continued to normal boot. Is that #PF non-fatal here?
>>>
>>
>> Hi,
>>    I tried again with my test environment.
>> Added the PrintErrMesg line before efi_arch_edd call, I got a #PF, in
>> my case the system hangs. With the fix patch machine is rebooting and
>> I can see the message in the logs.
>> I'm trying with Xen starting inside Qemu, EFI firmware, xen.gz
>> compiled as ELF file. Host system is an Ubuntu 22.04.5 LTS. Gcc is
>> version 11.4.
> 
> My test was wrong, commenting out blexit made "mesg" variable unused.
> After fixing that, I can reproduce it on both QEMU and real hardware:
> without your patch it crashes and with your patch it works just fine.
> While there may be more places with similar issue, this patch clearly
> improves the situation, so:
> 
> Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

While I would have preferred my comment to be addressed, I'll accept the
maintainer ack to have the patch go in as-is. Andrew, what about your
comment? Can you accept "this is an improvement" as enough of a reason
for it to go in despite not fully addressing the underlying issue? (In
case of no reply until after my upcoming vacation, I'll take this as
silent agreement and put the patch in.)

Jan