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efi: Generate stack protector canary at build time if urandom is available

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Generating the canary at build time allows the canary to be different for
every build which could limit the effectiveness of certain exploits.
Fallback to the statically generated random bytes if /dev/urandom is not
readable, e.g. Windows.

On 32-bit architectures, which use a 32-bit canary, reduce the canary to
4 bytes with one byte being NUL to filter out string buffer overflow attacks.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Glenn Washburn 2023-12-19 00:00:00 -06:00 • committed by Daniel Kiper
parent e424e945c9
commit dcc1af5d68
3 changed files with 23 additions and 1 deletions
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2
config.h.in
View file

@ -64,6 +64,8 @@
# define GRUB_TARGET_CPU "@GRUB_TARGET_CPU@"
# define GRUB_PLATFORM "@GRUB_PLATFORM@"
# define GRUB_STACK_PROTECTOR_INIT @GRUB_STACK_PROTECTOR_INIT@
# define RE_ENABLE_I18N 1
# define _GNU_SOURCE 1

20
configure.ac
View file

@ -1438,6 +1438,26 @@ else
AC_MSG_ERROR([invalid value $enable_stack_protector for --enable-stack-protector])
fi
TARGET_CPPFLAGS="$TARGET_CPPFLAGS -DGRUB_STACK_PROTECTOR=1"
if test -r /dev/urandom; then
# Generate the 8 byte stack protector canary at build time if /dev/urandom
# is able to be read. The first byte should be NUL to filter out string
# buffer overflow attacks.
GRUB_STACK_PROTECTOR_INIT="$($PYTHON -c 'import codecs; rf=open("/dev/urandom", "rb"); print("0x00"+codecs.encode(rf.read(7), "hex").decode("ascii"))')"
else
# Some hosts may not have a urandom, e.g. Windows, so use statically
# generated random bytes
GRUB_STACK_PROTECTOR_INIT="0x00f2b7e2f193b25c"
fi
if test x"$target_m32" = x1 ; then
# Make sure that the canary default value is 24-bits by only using the
# lower 3 bytes on 32 bit systems. This allows the upper byte to be NUL
# to filter out string buffer overflow attacks.
GRUB_STACK_PROTECTOR_INIT="0x00$(echo "$GRUB_STACK_PROTECTOR_INIT" | sed 's/.*\(......\)$/\1/')"
fi
AC_SUBST([GRUB_STACK_PROTECTOR_INIT])
fi
CFLAGS="$TARGET_CFLAGS"

2
grub-core/kern/efi/init.c
View file

@ -46,7 +46,7 @@ static grub_guid_t rng_protocol_guid = GRUB_EFI_RNG_PROTOCOL_GUID;
static grub_efi_uint8_t stack_chk_guard_buf[32];
/* Initialize canary in case there is no RNG protocol. */
grub_addr_t __stack_chk_guard = (grub_addr_t) 0x00f2b7e2f193b25c;
grub_addr_t __stack_chk_guard = (grub_addr_t) GRUB_STACK_PROTECTOR_INIT;
void __attribute__ ((noreturn))
__stack_chk_fail (void)