Re: [PATCH v4] Avoid crash calling PrintErrMesg from efi_multiboot2

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.09.2024 16:11, Andrew Cooper wrote:
> On 26/09/2024 2:26 pm, Marek Marczykowski-Górecki wrote:
>> On Mon, Aug 19, 2024 at 03:29:52PM +0100, 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  - 000000007DC29E46, CS  - 0000000000000038, RFLAGS - 0000000000210246
>>>   RAX  - 0000000000000000, RCX - 0000000000000050, RDX - 0000000000000000
>>>   RBX  - 000000007DAB4558, RSP - 000000007EFA1200, RBP - 0000000000000000
>>>   RSI  - FFFF82D040467A88, RDI - 0000000000000000
>>>   R8   - 000000007EFA1238, R9  - 000000007EFA1230, R10 - 0000000000000000
>>>   R11  - 000000007CF42665, R12 - FFFF82D040467A88, R13 - 000000007EFA1228
>>>   R14  - 000000007EFA1225, R15 - 000000007DAB45A8
>>>   DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
>>>   GS   - 0000000000000030, SS  - 0000000000000030
>>>   CR0  - 0000000080010033, CR2 - FFFF82D040467A88, CR3 - 000000007EC01000
>>>   CR4  - 0000000000000668, CR8 - 0000000000000000
>>>   DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
>>>   DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
>>>   GDTR - 000000007E9E2000 0000000000000047, LDTR - 0000000000000000
>>>   IDTR - 000000007E4E5018 0000000000000FFF,   TR - 0000000000000000
>>>   FXSAVE_STATE - 000000007EFA0E60
>>>   !!!! Find image based on IP(0x7DC29E46) (No PDB)  
>>> (ImageBase=000000007DC28000, EntryPoint=000000007DC2B917) !!!!
>>>
>>> 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)
>>>
>>> Fixes: 9180f5365524 ("x86: add multiboot2 protocol support for EFI 
>>> platforms")
>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>> I was hoping it would fix also an issue with xen.efi as the crash is
>> pretty similar
>> (https://github.com/QubesOS/qubes-issues/issues/8206#issuecomment-2366835136),
>> but it seems to be something different.
>>
>> Anyway, 
>>
>> Acked-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> 
> While I hate to drag this on further, there's a very relevant observation in
> 
> https://lore.kernel.org/xen-devel/20240925150059.3955569-31-ardb+git@xxxxxxxxxx/T/#u
> 
> which was posted yesterday.  Exactly the same is true of the early MB2
> code calling into regular EFI code, and I wonder if it's causing the
> other issue too.
> 
> I suspect following that pattern will be rather more robust.  Thoughts?

That builds upon there being a secondary mapping, which isn't the case here
I'm afraid.

Jan