[PATCH 02/69] Revert "ACPI: custom_method: fix memory leaks"

Greg Kroah-Hartman
2021-05-03T12:00:59Z

From: Kees Cook <keescook@chromium.org>

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@cs.uga.edu>
Fixes: 03d1571d9513 ("ACPI: custom_method: fix memory leaks")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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 443fdf62dd22..72469a49837d 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;
}

--
2.31.1