x86/mtrr: Don't copy out-of-bounds data in mtrr_write

Don't access the provided buffer out of bounds - this can cause a kernel
out-of-bounds read when invoked through sys_splice() or other things that
use kernel_write()/__kernel_write().

Fixes: 7f8ec5a4f01a ("x86/mtrr: Convert to use strncpy_from_user() helper")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180706215003.156702-1-jannh@google.com

diff --git a/arch/x86/kernel/cpu/mtrr/if.c b/arch/x86/kernel/cpu/mtrr/if.c
index 4021d3859499c77c14eaa1c40864c752547df68c..40eee6cc412484470daba013f2a197439163707a 100644 (file)
--- a/arch/x86/kernel/cpu/mtrr/if.c
+++ b/arch/x86/kernel/cpu/mtrr/if.c
@@ -106,7 +106,8 @@ mtrr_write(struct file *file, const char __user *buf, size_t len, loff_t * ppos)
 
        memset(line, 0, LINE_SIZE);
 
-       length = strncpy_from_user(line, buf, LINE_SIZE - 1);
+       len = min_t(size_t, len, LINE_SIZE - 1);
+       length = strncpy_from_user(line, buf, len);
        if (length < 0)
                return length;