net: don't unconditionally copy_from_user a struct ifreq for socket ioctls

commit d0efb16294d145d157432feda83877ae9d7cdf37 upstream.

A common implementation of isatty(3) involves calling a ioctl passing
a dummy struct argument and checking whether the syscall failed --
bionic and glibc use TCGETS (passing a struct termios), and musl uses
TIOCGWINSZ (passing a struct winsize). If the FD is a socket, we will
copy sizeof(struct ifreq) bytes of data from the argument and return
-EFAULT if that fails. The result is that the isatty implementations
may return a non-POSIX-compliant value in errno in the case where part
of the dummy struct argument is inaccessible, as both struct termios
and struct winsize are smaller than struct ifreq (at least on arm64).

Although there is usually enough stack space following the argument
on the stack that this did not present a practical problem up to now,
with MTE stack instrumentation it's more likely for the copy to fail,
as the memory following the struct may have a different tag.

Fix the problem by adding an early check for whether the ioctl is a
valid socket ioctl, and return -ENOTTY if it isn't.

Fixes: 44c02a2c3dc5 ("dev_ioctl(): move copyin/copyout to callers")
Signed-off-by: Peter Collingbourne <>
Cc: <> # 4.19
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
2 files changed