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efi: Only register shim_lock verifier if shim_lock protocol is found and SB enabled

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The shim_lock module registers a verifier to call shim's verify, but the
handler is registered even when the shim_lock protocol was not installed.

This doesn't cause a NULL pointer dereference in shim_lock_write() because
the shim_lock_init() function just returns GRUB_ERR_NONE if sl isn't set.

But in that case there's no point to even register the shim_lock verifier
since won't do anything. Additionally, it is only useful when Secure Boot
is enabled.

Finally, don't assume that the shim_lock protocol will always be present
when the shim_lock_write() function is called, and check for it on every
call to this function.

Reported-by: Michael Chang <mchang@suse.com>
Reported-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Javier Martinez Canillas 2020-12-03 16:01:49 +01:00 committed by Daniel Kiper
parent d7e54b2e5f
commit 132ddc42c7
1 changed files with 10 additions and 7 deletions
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17
grub-core/commands/efi/shim_lock.c
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@ -20,6 +20,7 @@
#include <grub/dl.h>
#include <grub/efi/efi.h>
#include <grub/efi/sb.h>
#include <grub/err.h>
#include <grub/file.h>
#include <grub/misc.h>
@ -28,7 +29,6 @@
GRUB_MOD_LICENSE ("GPLv3+");
static grub_efi_guid_t shim_lock_guid = GRUB_EFI_SHIM_LOCK_GUID;
static grub_efi_shim_lock_protocol_t *sl;
/* List of modules which cannot be loaded if UEFI secure boot mode is enabled. */
static const char * const disabled_mods[] = {"iorw", "memrw", "wrmsr", NULL};
@ -43,9 +43,6 @@ shim_lock_init (grub_file_t io, enum grub_file_type type,
*flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION;
if (!sl)
return GRUB_ERR_NONE;
switch (type & GRUB_FILE_TYPE_MASK)
{
case GRUB_FILE_TYPE_GRUB_MODULE:
@ -100,6 +97,11 @@ shim_lock_init (grub_file_t io, enum grub_file_type type,
static grub_err_t
shim_lock_write (void *context __attribute__ ((unused)), void *buf, grub_size_t size)
{
grub_efi_shim_lock_protocol_t *sl = grub_efi_locate_protocol (&shim_lock_guid, 0);
if (sl == NULL)
return grub_error (GRUB_ERR_ACCESS_DENIED, N_("shim_lock protocol not found"));
if (sl->verify (buf, size) != GRUB_EFI_SUCCESS)
return grub_error (GRUB_ERR_BAD_SIGNATURE, N_("bad shim signature"));
@ -115,12 +117,13 @@ struct grub_file_verifier shim_lock =
GRUB_MOD_INIT(shim_lock)
{
sl = grub_efi_locate_protocol (&shim_lock_guid, 0);
grub_verifier_register (&shim_lock);
grub_efi_shim_lock_protocol_t *sl = grub_efi_locate_protocol (&shim_lock_guid, 0);
if (!sl)
if (sl == NULL || grub_efi_get_secureboot () != GRUB_EFI_SECUREBOOT_MODE_ENABLED)
return;
grub_verifier_register (&shim_lock);
grub_dl_set_persistent (mod);
}