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

[Jan Beulich <jbeulich@xxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxxxxxxxxxxxxxx>

This had to be reverted, for breaking the build with old Clang. See the
respective Matrix conversation.

Jan