Re: [PATCH 2/2] arm64: efi: Pass reboot cmd parameter to efi_reboot()

[Sumit Garg <sumit.garg@kernel.org>]

On Fri, Nov 14, 2025 at 04:47:18PM +0100, Ard Biesheuvel wrote:
> On Fri, 14 Nov 2025 at 13:16, Sumit Garg <sumit.garg@kernel.org> wrote:
> >
> > On Fri, Nov 14, 2025 at 10:35:33AM +0100, Ard Biesheuvel wrote:
> > > On Fri, 14 Nov 2025 at 10:33, Ard Biesheuvel <ardb@kernel.org> wrote:
> > > >
> > > > On Fri, 14 Nov 2025 at 10:31, Sumit Garg <sumit.garg@kernel.org> wrote:
> > > > >
> > > > > On Fri, Nov 14, 2025 at 10:26:03AM +0100, Ard Biesheuvel wrote:
> > > > > > On Fri, 14 Nov 2025 at 09:51, Sumit Garg <sumit.garg@kernel.org> wrote:
> > > > > > >
> > > > > > > From: Sumit Garg <sumit.garg@oss.qualcomm.com>
> > > > > > >
> > > > > > > EFI ResetSystem runtime service allows for platform specific reset type
> > > > > > > allowing the OS to pass reset data for the UEFI implementation to take
> > > > > > > corresponding action. So lets pass the reboot cmd parameter for the EFI
> > > > > > > driver to determine whether it's a platform specific reset requested or
> > > > > > > not.
> > > > > > >
> > > > > > > Signed-off-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
> > > > > > > ---
> > > > > > >  arch/arm64/kernel/process.c | 2 +-
> > > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > >
> > > > > > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > > > > > > index fba7ca102a8c..51784986c568 100644
> > > > > > > --- a/arch/arm64/kernel/process.c
> > > > > > > +++ b/arch/arm64/kernel/process.c
> > > > > > > @@ -136,7 +136,7 @@ void machine_restart(char *cmd)
> > > > > > >          * ResetSystem().
> > > > > > >          */
> > > > > > >         if (efi_enabled(EFI_RUNTIME_SERVICES))
> > > > > > > -               efi_reboot(reboot_mode, NULL);
> > > > > > > +               efi_reboot(reboot_mode, cmd);
> > > > > > >
> > > > > >
> > > > > > I agree with the general principle. However, there are already
> > > > > > existing callers of kernel_restart() that would end up passing a
> > > > > > random string to efi_reboot(), resulting in platform specific reset
> > > > > > with undefined result.
> > > > >
> > > > > Yeah true but the UEFI spec says:
> > > > >
> > > > > "If the platform does not recognize the EFI_GUID in ResetData the platform
> > > > > must pick a supported reset type to perform. The platform may optionally
> > > > > log the parameters from any non-normal reset that occurs."
> > > > >
> > > > > So, in these cases the UEFI implementation can fallback to normal reset
> > > > > optionally logging the reset data being passed. Does that sounds
> > > > > reasonable to you?
> > > > >
> > > >
> > > > What the UEFI spec says might deviate from how real platforms in the
> > > > field will behave when being passed a reset type that nobody ever
> > > > tried passing before.
> >
> > I suppose from OS point of view, we need to follow the UEFI
> > specification. However, there will be scope for quirks later if the real
> > world problems occur. Currently, in case of EFI reboot we are just
> > ignoring the reboot cmd parameter.
> >
> > If you have in mind any sanity checks we should do here then feel free
> > to propose and I can try to implement them.
> >
> > >
> > > Also, the GUID is expected to follow an unbounded NULL terminated
> > > UTF-16 string in memory, so we could easily cause a crash by doing
> > > this if \0\0 doesn't appear in the memory following the string.
> >
> > Okay I see, would following change on top of this patchset address this
> > concern?
> >
> > --- a/drivers/firmware/efi/reboot.c
> > +++ b/drivers/firmware/efi/reboot.c
> > @@ -5,6 +5,7 @@
> >   */
> >  #include <linux/efi.h>
> >  #include <linux/reboot.h>
> > +#include <linux/ucs2_string.h>
> >
> >  static struct sys_off_handler *efi_sys_off_handler;
> >
> > @@ -14,11 +15,18 @@ void efi_reboot(enum reboot_mode reboot_mode, const char *data)
> >  {
> >         const char *str[] = { "cold", "warm", "shutdown", "platform" };
> >         int efi_mode, cap_reset_mode;
> > +       unsigned long reset_data_sz = 0;
> > +       efi_char16_t *reset_data = NULL;
> >
> >         if (!efi_rt_services_supported(EFI_RT_SUPPORTED_RESET_SYSTEM))
> >                 return;
> >
> >         if (data) {
> > +               reset_data_sz = ucs2_strlen(data) * sizeof(efi_char16_t);
> 
> You can't just run ucs2_strlen() on an arbitrary buffer.
> 
> > +               reset_data = kzalloc(reset_data_sz + 2, GFP_KERNEL);
> > +               memcpy(reset_data, data, reset_data_sz);
> > +               reset_data_sz += 2;
> > +
> 
> What happened to the GUID? It comes after the UTF-16 string, no?

Ah, I missed putting the GUID here.

> 
> >                 efi_mode = EFI_RESET_PLATFORM_SPECIFIC;
> >         } else {
> >                 switch (reboot_mode) {
> > @@ -47,8 +55,7 @@ void efi_reboot(enum reboot_mode reboot_mode, const char *data)
> >                 efi_mode = cap_reset_mode;
> >         }
> >
> > -       efi.reset_system(efi_mode, EFI_SUCCESS, sizeof(data),
> > -                        (efi_char16_t *)data);
> > +       efi.reset_system(efi_mode, EFI_SUCCESS, reset_data_sz, reset_data);
> >  }
> >
> 
> I think the main issue here is tying machine_restart(), which takes a
> u8[] argument, to efi_reboot(), which takes a (u16[]) + L"\0" + GUID
> buffer. So the change to efi_reboot() looks fine to me, we just cannot
> call it directly from machine_restart() as you are suggesting.

It mostly looks like the concerns you are highlighing are related to
random commands being passed to UEFI platform specific reset API. I
suppose this can be addressed using following allow list (based on
analysis done in patch-set [1]) for platform specific reset types. Your
views?

static const efi_platform_reset_type_t platform_reset_types[] = {
        {EFI_RESET_BOOTLOADER_GUID,                     L"bootloader"           },
        {EFI_RESET_DM_VERITY_GUID,                      L"dm-verity-device-corrupted"   },
        {EFI_RESET_EDL_GUID,                            L"edl"                  },
        {EFI_RESET_FASTBOOT_GUID,                       L"fastboot"             },
        {EFI_RESET_LOADER_GUID,                         L"loader"               },
        {EFI_RESET_REBOOT_AB_UPDATE_GUID,               L"reboot-ab-update"     },
        {EFI_RESET_RECOVERY_GUID,                       L"recovery"             },
        {EFI_RESET_RESCUE_GUID,                         L"rescue"               },
        {EFI_RESET_SHUTDOWN_THERMAL_GUID,               L"shutdown-thermal"     },
        {EFI_RESET_SHUTDOWN_THERMAL_BATTERY_GUID,       L"shutdown-thermal-battery"     },
}

[1] https://lore.kernel.org/all/20251109-arm-psci-system_reset2-vendor-reboots-v17-0-46e085bca4cc@oss.qualcomm.com/

-Sumit