diff options
| author | Michael Brown <mcb30@ipxe.org> | 2020-07-09 13:56:50 +0100 |
|---|---|---|
| committer | Michael Brown <mcb30@ipxe.org> | 2020-07-09 13:56:50 +0100 |
| commit | fcdd9c0982e6c5a435fefdcc9444242fd4fbdd8b (patch) | |
| tree | bca63d1a2c2331c3bfffcf8c794ba218c7e8feb4 | |
| parent | f7ddda435cf0d116098351986f89517e3d88af13 (diff) | |
| download | ipxe-fcdd9c0982e6c5a435fefdcc9444242fd4fbdd8b.tar.gz | |
[efi] Distribute available entropy within stack cookie
Several of the values used to compute a stack cookie (in the absence
of a viable entropy source) will tend to have either all-zeroes or
all-ones in the higher order bits. Rotate the values in order to
distribute the (minimal) available entropy more evenly.
Suggested-by: Pete Beck <pete.beck@ioactive.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
| -rw-r--r-- | src/interface/efi/efi_init.c | 23 |
1 files changed, 17 insertions, 6 deletions
diff --git a/src/interface/efi/efi_init.c b/src/interface/efi/efi_init.c index df46bb17b..284c39b35 100644 --- a/src/interface/efi/efi_init.c +++ b/src/interface/efi/efi_init.c @@ -22,6 +22,7 @@ FILE_LICENCE ( GPL2_OR_LATER ); #include <string.h> #include <errno.h> #include <ipxe/init.h> +#include <ipxe/rotate.h> #include <ipxe/efi/efi.h> #include <ipxe/efi/efi_driver.h> #include <ipxe/efi/Protocol/LoadedImage.h> @@ -105,19 +106,29 @@ static void * efi_find_table ( EFI_GUID *guid ) { */ __attribute__ (( noinline )) unsigned long efi_stack_cookie ( EFI_HANDLE handle ) { + unsigned long cookie = 0; + unsigned int rotation = ( 8 * sizeof ( cookie ) / 4 ); /* There is no viable source of entropy available at this * point. Construct a value that is at least likely to vary * between platforms and invocations. - * - * Ensure that the value contains a NUL byte, to act as a + */ + cookie ^= ( ( unsigned long ) handle ); + cookie = roll ( cookie, rotation ); + cookie ^= ( ( unsigned long ) &handle ); + cookie = roll ( cookie, rotation ); + cookie ^= profile_timestamp(); + cookie = roll ( cookie, rotation ); + cookie ^= build_id; + + /* Ensure that the value contains a NUL byte, to act as a * runaway string terminator. Construct the NUL using a shift * rather than a mask, to avoid losing valuable entropy in the - * low-order bits. + * lower-order bits. */ - return ( ( ( ( unsigned long ) handle ) ^ - ( ( unsigned long ) &handle ) ^ - profile_timestamp() ^ build_id ) << 8 ); + cookie <<= 8; + + return cookie; } /** |