Commit-ID: fc152d22d6e9fac95a9a990e6c29510bdf1b9425 Gitweb: http://git.kernel.org/tip/fc152d22d6e9fac95a9a990e6c29510bdf1b9425 Author: Mateusz Jurczyk <[email protected]> AuthorDate: Wed, 24 May 2017 15:55:00 +0200 Committer: Thomas Gleixner <[email protected]> CommitDate: Wed, 24 May 2017 16:18:12 +0200
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 <[email protected]> Reviewed-by: Borislav Petkov <[email protected]> Cc: Andy Lutomirski <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Thomas Gleixner <[email protected]> --- arch/x86/kernel/alternative.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index c5b8f76..32e14d1 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -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),
