linux-kernel - [PATCH] Revert "ACPI: custom

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Date:   Sun,  2 May 2021 10:23:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Kees Cook <keescook@...omium.org>, Wenwen Wang <wenwen@...uga.edu>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: [PATCH] Revert "ACPI: custom_method: fix memory leaks"

This reverts commit 03d1571d9513369c17e6848476763ebbd10ec2cb.

While /sys/kernel/debug/acpi/custom_method is already a privileged-only
API providing proxied arbitrary write access to kernel memory[1][2],
with existing race conditions[3] in buffer allocation and use that could
lead to memory leaks and use-after-free conditions, the above commit
appears to accidentally make the use-after-free conditions even easier
to accomplish. ("buf" is a global variable and prior kfree()s would set
buf back to NULL.)

This entire interface needs to be reworked (if not entirely removed).

[1] https://lore.kernel.org/lkml/20110222193250.GA23913@outflux.net/
[2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
[3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/

Cc: Wenwen Wang <wenwen@...uga.edu>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/acpi/custom_method.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index 7b54dc95d36b..36d95a02cd30 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -53,10 +53,8 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 	if ((*ppos > max_size) ||
 	    (*ppos + count > max_size) ||
 	    (*ppos + count < count) ||
-	    (count > uncopied_bytes)) {
-		kfree(buf);
+	    (count > uncopied_bytes))
 		return -EINVAL;
-	}
 
 	if (copy_from_user(buf + (*ppos), user_buf, count)) {
 		kfree(buf);
@@ -76,7 +74,6 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE, LOCKDEP_NOW_UNRELIABLE);
 	}
 
-	kfree(buf);
 	return count;
 }
 
-- 
2.25.1