s390: fix nospec table alignments

Add proper alignment for .nospec_call_table and .nospec_return_table in
vmlinux.

[[email protected]]: The problem with the missing alignment of the nospec
tables exist since a long time, however only since commit e6ed91f
("s390/alternatives: remove padding generation code") and with
CONFIG_RELOCATABLE=n the kernel may also crash at boot time.

The above named commit reduced the size of struct alt_instr by one byte,
so its new size is 11 bytes. Therefore depending on the number of cpu
alternatives the size of the __alt_instructions array maybe odd, which
again also causes that the addresses of the nospec tables will be odd.

If the address of __nospec_call_start is odd and the kernel is compiled
With CONFIG_RELOCATABLE=n the compiler may generate code that loads the
address of __nospec_call_start with a 'larl' instruction.

This will generate incorrect code since the 'larl' instruction only works
with even addresses. In result the members of the nospec tables will be
accessed with an off-by-one offset, which subsequently may lead to
addressing exceptions within __nospec_revert().

Fixes: f19fbd5 ("s390: introduce execute-trampolines for branches")
Signed-off-by: Josh Poimboeuf <[email protected]>
Link: https://lore.kernel.org/r/8719bf1ce4a72ebdeb575200290094e9ce047bcc.1661557333.git.jpoimboe@kernel.org
Cc: <[email protected]> # 4.16
Reviewed-by: Heiko Carstens <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>

arch/s390/kernel/vmlinux.lds.S

@@ -131,6 +131,7 @@ SECTIONS
 	/*
 	 * Table with the patch locations to undo expolines
 	*/
+	. = ALIGN(4);
 	.nospec_call_table : {
 		__nospec_call_start = . ;
 		*(.s390_indirect*)