pstore/ramoops: fixup driver removal

A basic rmmod ramoops segfaults. Let's see why.

Since commit 34f0ec82 ("pstore: Correct the max_dump_cnt clearing of
ramoops") sets ->max_dump_cnt to zero before looping over ->przs but we
didn't use it before that either.

And since commit ee1d2674 ("pstore: add pstore unregister") we free
that memory on rmmod.

But even then, we looped until a NULL pointer or ERR. I don't see where
it is ensured that the last member is NULL. Let's try this instead:
simply error recovery and free. Clean up in error case where resources
were allocated. And then, in the free path, rely on ->max_dump_cnt in
the free path.

Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org # 4.4.x-

fs/pstore/ram.c

@@ -377,13 +377,14 @@ static void ramoops_free_przs(struct ramoops_context *cxt)
 {
 	int i;
 
-	cxt->max_dump_cnt = 0;
 	if (!cxt->przs)
 		return;
 
-	for (i = 0; !IS_ERR_OR_NULL(cxt->przs[i]); i++)
+	for (i = 0; i < cxt->max_dump_cnt; i++)
 		persistent_ram_free(cxt->przs[i]);
+
 	kfree(cxt->przs);
+	cxt->max_dump_cnt = 0;
 }
 
 static int ramoops_init_przs(struct device *dev, struct ramoops_context *cxt,
@@ -408,7 +409,7 @@ static int ramoops_init_przs(struct device *dev, struct ramoops_context *cxt,
 			     GFP_KERNEL);
 	if (!cxt->przs) {
 		dev_err(dev, "failed to initialize a prz array for dumps\n");
-		goto fail_prz;
+		goto fail_mem;
 	}
 
 	for (i = 0; i < cxt->max_dump_cnt; i++) {
@@ -419,6 +420,11 @@ static int ramoops_init_przs(struct device *dev, struct ramoops_context *cxt,
 			err = PTR_ERR(cxt->przs[i]);
 			dev_err(dev, "failed to request mem region (0x%zx@0x%llx): %d\n",
 				cxt->record_size, (unsigned long long)*paddr, err);
+
+			while (i > 0) {
+				i--;
+				persistent_ram_free(cxt->przs[i]);
+			}
 			goto fail_prz;
 		}
 		*paddr += cxt->record_size;
@@ -426,7 +432,9 @@ static int ramoops_init_przs(struct device *dev, struct ramoops_context *cxt,
 
 	return 0;
 fail_prz:
-	ramoops_free_przs(cxt);
+	kfree(cxt->przs);
+fail_mem:
+	cxt->max_dump_cnt = 0;
 	return err;
 }
 
@@ -659,7 +667,6 @@ static int ramoops_remove(struct platform_device *pdev)
 	struct ramoops_context *cxt = &oops_cxt;
 
 	pstore_unregister(&cxt->pstore);
-	cxt->max_dump_cnt = 0;
 
 	kfree(cxt->pstore.buf);
 	cxt->pstore.bufsize = 0;