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| author | Mateusz Jurczyk <mjurczyk@google.com> | 2017-05-24 15:55:00 +0200 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2017-05-24 16:18:12 +0200 |
| commit | fc152d22d6e9fac95a9a990e6c29510bdf1b9425 (patch) | |
| tree | c784fbbc87c38aa14979ec3ff7d4a70741761ffa /arch/x86 | |
| parent | x86/PAT: Fix Xorg regression on CPUs that don't support PAT (diff) | |
| download | linux-fc152d22d6e9fac95a9a990e6c29510bdf1b9425.tar.xz 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.c | 9 |
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), |