Subject: [PATCH] efi: fdt: avoid FDT manipulation after ExitBootServices()

From: [email-masked] (Ingo Molnar)
Date: Wed, 1 Feb 2017 21:16:44 +0100

* Ard Biesheuvel <[email-masked]> wrote:

> On 1 February 2017 at 19:03, Ingo Molnar <[email-masked]> wrote:
> >
> > * Ard Biesheuvel <[email-masked]> wrote:
> >
> >> Some AArch64 UEFI implementations disable the MMU in ExitBootServices(),
> >> after which unaligned accesses to RAM are no longer supported.
> >>
> >> Commit abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the
> >> kernel") fixed an issue in the memory map handling of the stub FDT code,
> >> but inadvertently created an issue with such firmwares, by moving some
> >> of the FDT manipulation to after the invocation of ExitBootServices().
> >> Given that the stub's libfdt implementation uses the ordinary, accelerated
> >> string functions, which rely on hardware handling of unaligned accesses,
> >> manipulating the FDT with the MMU off may result in alignment faults.
> >>
> >> So fix the situation by moving the update_fdt_memmap() call into the
> >> callback function invoked by efi_exit_boot_services() right before it
> >> calls the ExitBootServices() UEFI service (which is arguably a better
> >> place for it anyway)
> >>
> >> Note that disabling the MMU in ExitBootServices() is not compliant with
> >> the UEFI spec, and carries great risk due to the fact that switching from
> >> cached to uncached memory accesses halfway through compiler generated code
> >> (i.e., involving a stack) can never be done in a way that is architecturally
> >> safe.
> >>
> >> Cc: <[email-masked]>
> >> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> >> Tested-by: Riku Voipio <[email-masked]>
> >> Signed-off-by: Ard Biesheuvel <[email-masked]>
> >> ---
> >>  drivers/firmware/efi/libstub/fdt.c | 14 +++-----------
> >>  1 file changed, 3 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/drivers/firmware/efi/libstub/fdt.c b/drivers/firmware/efi/libstub/fdt.c
> >> index 921dfa047202..260c4b4b492e 100644
> >> --- a/drivers/firmware/efi/libstub/fdt.c
> >> +++ b/drivers/firmware/efi/libstub/fdt.c
> >> @@ -187,6 +187,7 @@ static efi_status_t update_fdt_memmap(void *fdt, struct efi_boot_memmap *map)
> >>  struct exit_boot_struct {
> >>       efi_memory_desc_t *runtime_map;
> >>       int *runtime_entry_count;
> >> +     void *new_fdt_addr;
> >>  };
> >>
> >>  static efi_status_t exit_boot_func(efi_system_table_t *sys_table_arg,
> >> @@ -202,7 +203,7 @@ static efi_status_t exit_boot_func(efi_system_table_t *sys_table_arg,
> >>       efi_get_virtmap(*map->map, *map->map_size, *map->desc_size,
> >>                       p->runtime_map, p->runtime_entry_count);
> >>
> >> -     return EFI_SUCCESS;
> >> +     return update_fdt_memmap(p->new_fdt_addr, map);
> >>  }
> >>
> >>  /*
> >> @@ -300,22 +301,13 @@ efi_status_t allocate_new_fdt_and_exit_boot(efi_system_table_t *sys_table,
> >>
> >>       priv.runtime_map = runtime_map;
> >>       priv.runtime_entry_count = &runtime_entry_count;
> >> +     priv.new_fdt_addr = (void *)*new_fdt_addr;
> >
> > Nit: could the type cast by dropped?
> 
> new_fdt_addr is a pointer to unsigned long, so we can't drop the cast
> here without introducing it somewhere else.

Fair enough, I missed the dereference of new_fdt_addr.

Thanks,

	Ingo