Re: [PATCH] arm64: efi: Disable only the misbehaving runtime service on sync exceptions

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Wed, Nov 09, 2022 at 04:57:14PM +0100, Ard Biesheuvel wrote:
> On Wed, 9 Nov 2022 at 15:42, Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> >
> > Hi,
> >
> > On Tue, Nov 08, 2022 at 04:15:09PM +0100, Ard Biesheuvel wrote:
> > > Alexandru reports that his Ampere Altra machine, whose buggy firmware
> > > triggers a synchronous exception in its implementation of SetTime() when
> > > called without SetVirtualAddressMap() having been called first, doesn't
> > > quite recover from this, and starts spewing error messages into the log
> > > that are unrelated to the buggy runtime service.
> > >
> > > The driver in question is the EFI RTC driver, which should be fixed in
> > > any case, as flooding the log like that (or doing any logging to the
> > > kernel log at all on something whuch is not a severe issue) is not ok.
> > >
> > > However, in this particular case, it would be beneficial for both
> > > ordinary use as well as diagnostics regarding broken firmware if we only
> > > prevent the broken runtime service from being called again, and permit
> > > others (such as GetTime() which triggers the logging or the variable
> > > services) from being used as normal.
> > >
> > > So wire up the existing efi.runtime_supported_mask, and clear the
> > > service's bit in the mask if the generic runtime service wrapper
> > > observes a return value of EFI_ABORTED, which only happens if a service
> > > call is aborted due to an exception. (EFI_ABORTED is not documented as a
> > > valid error code for any of the EFI runtime services).
> >
> > With a kernel built from v6.1-rc4, when doing efibootmgr after the EFI panic
> > happens (so with runtime services disabled), this is what I get:
> >
> > # efibootmgr
> > Skipping unreadable variable "Boot0001": Interrupted system call
> > Skipping unreadable variable "Boot0002": Interrupted system call
> > show_order(): Interrupted system call
> >
> > And dmesg shows:
> >
> > [   55.941312] efi: EFI Runtime Services are disabled!
> >
> > With this patch on top of v6.1-rc4:
> >
> > # efibootmgr
> > Skipping unreadable variable "Boot0001": Invalid argument
> > Skipping unreadable variable "Boot0002": Invalid argument
> > show_order(): Invalid argument
> >
> > Same thing happens if I cat the Boot001 efivars file. Nothing is printed
> > on dmesg.
> >
> 
> OK, this strongly suggests that the EFI runtime services end up in a
> funny state after the crash of SetTime(), and subsequent calls to any
> of them no longer work as expected.
> 
> > Changed efi_call_rts() to print the return value, status is
> > 0x8000_0000_0000_000f (or 15 in decimal if casted into an int). Tried to
> > debug further, but I'm not familiar with all the structs and what they
> > represent (for example, efi_call_virt(get_variable, args) calls
> > efi_call_virt_pointer(efi.runtime, get_variable, args), does it end up as
> > __efi_rt_asm_wrapper((efi.runtime)->get_variable, "get_variable", args?)
> 
> Indeed.
> 
> The value of the function pointer is used to make the indirect call,
> and the string is only used if an error occurs, so we can print it to
> the log. The remaining arguments are simply the arguments to the
> firmware call.

Got it, thanks for the explanation.

> 
> > As
> > an aside, it would be really helpful to document the arguments for
> > __efi_rt_asm_wrapper. Pointers here how to debug further would be very
> > welcome.
> >
> 
> If the log is completely silent, there is not a lot to debug, really.
> 
> The error value you are observing is EFI_ACCESS_DENIED, and looking at
> the open source version of the Mt.Jade firmware, this might be the
> value returned from the secure world helper.
> 
> One other thing I would like to try is disabling set_time specifically
> using a command line parameter.

Hm... had a look at kernel_parameters.txt and the rtc-efi driver and
couldn't figure out how to do that. I could just return 0 early from
efi_set_time(), I suppose. Is that what you had in mind?

> 
> Btw could you share the output of dmidecode as well?

Sure, you can find it at [1] (expires in 6 months, pastebin is unlisted).
If it makes a difference, dmidecode was run with the stock Ubuntu kernel
(v5.15).

[1] https://pastebin.com/eaL2LRCf

Thanks,
Alex

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