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

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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>

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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