Re: [PATCH] Revert "ACPI: custom_method: fix memory leaks"

[Rafael J. Wysocki]

On Mon, May 3, 2021 at 6:55 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, May 02, 2021 at 10:23:26AM -0700, Kees Cook wrote:
> > 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@xxxxxxxxxxx/
> > [2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
> > [3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/
> >
> > Cc: Wenwen Wang <wenwen@xxxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  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;
> >  }
> >
> > --
>
> Thanks for the revert, I'll queue it up on my larger "umn.edu reverts"
> branch that I'll be sending out for review in a day or so.

This will conflict with the material that I'm going to push on
Thursday that includes the two commits mentioned by Mark elsewhere in
this thread.