pstore: Set tfm to NULL on free_buf_for_compression

Set tfm to NULL on free_buf_for_compression() after crypto_free_comp().

This avoid a use-after-free when allocate_buf_for_compression()
and free_buf_for_compression() are called twice. Although
free_buf_for_compression() freed the tfm, allocate_buf_for_compression()
won't reinitialize the tfm since the tfm pointer is not NULL.

Fixes: 95047b05 ("pstore: Refactor compression initialization")
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -347,8 +347,10 @@ static void allocate_buf_for_compression(void)
 
 static void free_buf_for_compression(void)
 {
-	if (IS_ENABLED(CONFIG_PSTORE_COMPRESS) && tfm)
+	if (IS_ENABLED(CONFIG_PSTORE_COMPRESS) && tfm) {
 		crypto_free_comp(tfm);
+		tfm = NULL;
+	}
 	kfree(big_oops_buf);
 	big_oops_buf = NULL;
 	big_oops_buf_sz = 0;