Commit fc152d22 authored May 24, 2017 by Mateusz Jurczyk Committed by Thomas Gleixner May 24, 2017

x86/alternatives: Prevent uninitialized stack byte read in apply_alternatives()

In the current form of the code, if a->replacementlen is 0, the reference
to *insnbuf for comparison touches potentially garbage memory. While it
doesn't affect the execution flow due to the subsequent a->replacementlen
comparison, it is (rightly) detected as use of uninitialized memory by a
runtime instrumentation currently under my development, and could be
detected as such by other tools in the future, too (e.g. KMSAN).

Fix the "false-positive" by reordering the conditions to first check the
replacement instruction length before referencing specific opcode bytes.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/20170524135500.27223-1-mjurczyk@google.com

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent cbed27cd

arch/x86/kernel/alternative.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -409,8 +409,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
		memcpy(insnbuf, replacement, a->replacementlen);
		insnbuf_sz = a->replacementlen;

		/* 0xe8 is a relative jump; fix the offset. */
		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
		/*
		* 0xe8 is a relative jump; fix the offset.
		*
		* Instruction length is checked before the opcode to avoid
		* accessing uninitialized bytes for zero-length replacements.
		*/
		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
		(s32 )(insnbuf + 1) += replacement - instr;
		DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
		(s32 )(insnbuf + 1),