Re: [PATCH v2] Avoid crash calling PrintErrMesg from efi_multiboot2

[Jan Beulich <jbeulich@suse.com>]

On 19.08.2024 14:35, 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: 00d5d5ce23e6 ("work around Clang generating .data.rel.ro section for init-only files")

I don't think this is right. While this is where the array was introduced,
it was correct at that time afaict. It went wrong when MB2 support was added
about a year later. 9180f5365524 ("x86: add multiboot2 protocol support for
EFI platforms") may be reasonable to blame, albeit I'm not sure that was the
final one, after which MB2 support was considered complete.

> @@ -308,7 +325,7 @@ static void __init PrintErrMesg(const CHAR16 *mesg, EFI_STATUS ErrCode)
>      PrintErr(L": ");
>  
>      if( (ErrIdx < ARRAY_SIZE(ErrCodeToStr)) && ErrCodeToStr[ErrIdx] )
> -        mesg = ErrCodeToStr[ErrIdx];
> +        mesg = (const CHAR16 *)((const void *)&ErrorStrings + ErrCodeToStr[ErrIdx]);

As said when suggesting the alternative form in reply to v1: This is now too
long and hence needs wrapping.

Jan