On Tue, May 04, 2021 at 04:59:40PM +0200, Rafael J. Wysocki wrote: > 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. I'll drop this from my patch series now that you all have this covered. thanks! greg k-h