On Sun, 24 Mar 2024 at 22:48, Sasha Levin <sashal@xxxxxxxxxx> wrote: > > This is a note to let you know that I've just added the patch titled > > x86/efistub: Clear decompressor BSS in native EFI entrypoint > > to the 6.8-stable tree which can be found at: > http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary > > The filename of the patch is: > x86-efistub-clear-decompressor-bss-in-native-efi-ent.patch > and it can be found in the queue-6.8 subdirectory. > > If you, or anyone else, feels it should not be added to the stable tree, > please let <stable@xxxxxxxxxxxxxxx> know about it. > Thanks for taking this patch, but it breaks EFI mixed mode, so please wait for the fix for that to hit Linus' tree and take them both at the same time. (PR was sent today) https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git/commit/?id=df7ecce842b846a04d087ba85fdb79a90e26a1b0 > > > commit ea18347cb8519af39ec4d12963a1986a2c0ef1a8 > Author: Ard Biesheuvel <ardb@xxxxxxxxxx> > Date: Fri Mar 15 16:26:16 2024 +0100 > > x86/efistub: Clear decompressor BSS in native EFI entrypoint > > [ Upstream commit b3810c5a2cc4a6665f7a65bed5393c75ce3f3aa2 ] > > The EFI stub on x86 no longer invokes the decompressor as a subsequent > boot stage, but calls into the decompression code directly while running > in the context of the EFI boot services. > > This means that when using the native EFI entrypoint (as opposed to the > EFI handover protocol, which clears BSS explicitly), the firmware PE > image loader is being relied upon to ensure that BSS is zeroed before > the EFI stub is entered from the firmware. > > As Radek's report proves, this is a bad idea. Not all loaders do this > correctly, which means some global variables that should be statically > initialized to 0x0 may have junk in them. > > So clear BSS explicitly when entering via efi_pe_entry(). Note that > zeroing BSS from C code is not generally safe, but in this case, the > following assignment and dereference of a global pointer variable > ensures that the memset() cannot be deferred or reordered. > > Cc: <stable@xxxxxxxxxx> # v6.1+ > Reported-by: Radek Podgorny <radek@xxxxxxxxxxx> > Closes: https://lore.kernel.org/all/a99a831a-8ad5-4cb0-bff9-be637311f771@xxxxxxxxxxx > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> > > diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c > index 99429bc4b0c7e..681f576ec02a0 100644 > --- a/drivers/firmware/efi/libstub/x86-stub.c > +++ b/drivers/firmware/efi/libstub/x86-stub.c > @@ -21,6 +21,8 @@ > #include "efistub.h" > #include "x86-stub.h" > > +extern char _bss[], _ebss[]; > + > const efi_system_table_t *efi_system_table; > const efi_dxe_services_table_t *efi_dxe_table; > static efi_loaded_image_t *image = NULL; > @@ -465,6 +467,8 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle, > efi_status_t status; > char *cmdline_ptr; > > + memset(_bss, 0, _ebss - _bss); > + > efi_system_table = sys_table_arg; > > /* Check if we were booted by the EFI firmware */ > @@ -958,8 +962,6 @@ void __noreturn efi_stub_entry(efi_handle_t handle, > void efi_handover_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg, > struct boot_params *boot_params) > { > - extern char _bss[], _ebss[]; > - > memset(_bss, 0, _ebss - _bss); > efi_stub_entry(handle, sys_table_arg, boot_params); > }
