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[efi] Distribute available entropy within stack cookie

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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>
This commit is contained in:
Michael Brown
2020-07-09 13:56:50 +01:00
parent f7ddda435c
commit fcdd9c0982
1 changed files with 17 additions and 6 deletions
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23
src/interface/efi/efi_init.c
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@@ -22,6 +22,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
#include <string.h> #include <string.h>
#include <errno.h> #include <errno.h>
#include <ipxe/init.h> #include <ipxe/init.h>
#include <ipxe/rotate.h>
#include <ipxe/efi/efi.h> #include <ipxe/efi/efi.h>
#include <ipxe/efi/efi_driver.h> #include <ipxe/efi/efi_driver.h>
#include <ipxe/efi/Protocol/LoadedImage.h> #include <ipxe/efi/Protocol/LoadedImage.h>
@@ -105,19 +106,29 @@ static void * efi_find_table ( EFI_GUID *guid ) {
*/ */
__attribute__ (( noinline )) unsigned long __attribute__ (( noinline )) unsigned long
efi_stack_cookie ( EFI_HANDLE handle ) { 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 /* There is no viable source of entropy available at this
* point. Construct a value that is at least likely to vary * point. Construct a value that is at least likely to vary
* between platforms and invocations. * 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 * runaway string terminator. Construct the NUL using a shift
* rather than a mask, to avoid losing valuable entropy in the * rather than a mask, to avoid losing valuable entropy in the
* low-order bits. * lower-order bits.
*/ */
return ( ( ( ( unsigned long ) handle ) ^ cookie <<= 8;
( ( unsigned long ) &handle ) ^
profile_timestamp() ^ build_id ) << 8 ); return cookie;
} }
/** /**