x86, mce: Fix stack out-of-bounds write in mce-inject.c:flags_read()

x86, mce: Fix stack out-of-bounds write in mce-inject.c:flags_read()

[Luck, Tony]

Each of the strings that we want to put into the buf[MAX_FLAG_OPT_SIZE]
in flags_read() is two characters. But the sprintf() adds a trailing newline
and will add a terminating NUL byte. So MAX_FLAG_OPT_SIZE needs to be 4.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>
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diff --git a/arch/x86/kernel/cpu/mcheck/mce-inject.c b/arch/x86/kernel/cpu/mcheck/mce-inject.c
index 475cb4f5f14f..c805a06e14c3 100644
--- a/arch/x86/kernel/cpu/mcheck/mce-inject.c
+++ b/arch/x86/kernel/cpu/mcheck/mce-inject.c
@@ -48,7 +48,7 @@ static struct dentry *dfs_inj;
 
 static u8 n_banks;
 
-#define MAX_FLAG_OPT_SIZE	3
+#define MAX_FLAG_OPT_SIZE	4
 #define NBCFG			0x44
 
 enum injection_type {

x86, mce: Fix stack out-of-bounds write in mce-inject.c:flags_read()

[Borislav Petkov]

On Fri, Apr 27, 2018 at 09:37:08AM -0700, Luck, Tony wrote:
> Each of the strings that we want to put into the buf[MAX_FLAG_OPT_SIZE]
> in flags_read() is two characters. But the sprintf() adds a trailing newline
> and will add a terminating NUL byte. So MAX_FLAG_OPT_SIZE needs to be 4.

... and I dumped the n here:

	n = sprintf(buf, "%s\n", flags_options[inj_type])

and it was 3 but sprintf() calls vsnprintf() and *that* does return:

" * The return value is the number of characters which would
 * be generated for the given input, excluding the trailing
 * '\0', as per ISO C99."

So I'll extend the commit message with that and apply it.

Thanks.