summaryrefslogtreecommitdiffstats
path: root/arch/x86
diff options
context:
space:
mode:
authorMateusz Jurczyk <mjurczyk@google.com>2017-05-24 15:55:00 +0200
committerThomas Gleixner <tglx@linutronix.de>2017-05-24 16:18:12 +0200
commitfc152d22d6e9fac95a9a990e6c29510bdf1b9425 (patch)
treec784fbbc87c38aa14979ec3ff7d4a70741761ffa /arch/x86
parentcbed27cdf0e3f7ea3b2259e86b9e34df02be3fe4 (diff)
downloadlinux-fc152d22d6e9fac95a9a990e6c29510bdf1b9425.tar.gz
linux-fc152d22d6e9fac95a9a990e6c29510bdf1b9425.tar.bz2
linux-fc152d22d6e9fac95a9a990e6c29510bdf1b9425.zip
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>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/kernel/alternative.c9
1 files changed, 7 insertions, 2 deletions
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f760473c..32e14d137416 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),