linux-kernel - [PATCH] module: Fix use-after-free bug in read_file_mod

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Date:   Thu, 27 Apr 2023 22:59:33 -0700
From:   Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
To:     mcgrof@...nel.org, linux-modules@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, error27@...il.com,
        kernel-janitors@...r.kernel.org,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
Subject: [PATCH] module: Fix use-after-free bug in read_file_mod_stats()

Smatch warns:
	kernel/module/stats.c:394 read_file_mod_stats()
	warn: passing freed memory 'buf'

We are passing 'buf' to simple_read_from_buffer() after freeing it.

Fix this by changing the order of 'simple_read_from_buffer' and 'kfree'.

Fixes: df3e764d8e5c ("module: add debug stats to help identify memory pressure")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
---
Found with statis analysis, only compile tested.
---
 kernel/module/stats.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/module/stats.c b/kernel/module/stats.c
index ad7b6ada29f2..6ab2c94d6bc3 100644
--- a/kernel/module/stats.c
+++ b/kernel/module/stats.c
@@ -276,6 +276,7 @@ static ssize_t read_file_mod_stats(struct file *file, char __user *user_buf,
 	struct mod_fail_load *mod_fail;
 	unsigned int len, size, count_failed = 0;
 	char *buf;
+	int ret;
 	u32 live_mod_count, fkreads, fdecompress, fbecoming, floads;
 	unsigned long total_size, text_size, ikread_bytes, ibecoming_bytes,
 		idecompress_bytes, imod_bytes, total_virtual_lost;
@@ -390,8 +391,9 @@ static ssize_t read_file_mod_stats(struct file *file, char __user *user_buf,
 out_unlock:
 	mutex_unlock(&module_mutex);
 out:
+	ret = simple_read_from_buffer(user_buf, count, ppos, buf, len);
 	kfree(buf);
-        return simple_read_from_buffer(user_buf, count, ppos, buf, len);
+	return ret;
 }
 #undef MAX_PREAMBLE
 #undef MAX_FAILED_MOD_PRINT
-- 
2.40.0