On 4/24/2019 6:37 PM, Al Viro wrote:
On Wed, Apr 24, 2019 at 05:40:40PM +0530, Mukesh Ojha wrote:
Al,
i tried to put traceprintk inside ioctl after fdget and fdput on a simple
call of open => ioctl => close
in a loop, and multithreaded, presumably?
on /dev/uinput.
uinput-532 [002] .... 45.312044: SYSC_ioctl: 2 <= f_count
<After fdget()
uinput-532 [002] .... 45.312055: SYSC_ioctl: 2
<After fdput()
uinput-532 [004] .... 45.313766: uinput_open: uinput: 1
uinput-532 [004] .... 45.313783: SYSC_ioctl: 1
uinput-532 [004] .... 45.313788: uinput_ioctl_handler:
uinput: uinput_ioctl_handler, 1
uinput-532 [004] .... 45.313835: SYSC_ioctl: 1
uinput-532 [004] .... 45.313843: uinput_release: uinput: 0
So while a ioctl is running the f_count is 1, so a fput could be run and do
atomic_long_dec_and_test
this could call release right ?
Look at ksys_ioctl():
int ksys_ioctl(unsigned int fd, unsigned int cmd, unsigned long arg)
{
int error;
struct fd f = fdget(fd);
an error or refcount bumped
if (!f.file)
return -EBADF;
not an error, then. We know that ->release() won't be called
until we drop the reference we've just acquired.
error = security_file_ioctl(f.file, cmd, arg);
if (!error)
error = do_vfs_ioctl(f.file, fd, cmd, arg);
... and we are done with calling ->ioctl(), so
fdput(f);
... we drop the reference we'd acquired.
Seeing refcount 1 inside ->ioctl() is possible, all right:
CPU1: ioctl(2) resolves fd to struct file *, refcount 2
CPU2: close(2) rips struct file * from descriptor table and does fput() to drop
it;
refcount reaches 1 and fput() is done; no call of ->release() yet.
CPU1: we get arouund to ->ioctl(), where your trace sees refcount 1
CPU1: done with ->ioctl(), drop our reference. *NOW* refcount gets to 0, and
->release() is called.
Thanks for the detail reply, Al
This was my simple program no multithreading just to understand f_counting
int main()
{
int fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
ioctl(fd, UI_SET_EVBIT, EV_KEY);
close(fd);
return 0;
}
uinput-532 [002] .... 45.312044: SYSC_ioctl: 2 <=
f_count > <After fdget()
uinput-532 [002] .... 45.312055: SYSC_ioctl:
2 <After fdput()
uinput-532 [004] .... 45.313766: uinput_open: uinput:
1 /* This is from the uinput driver uinput_open()*/
=>>>> /* All the above calls happened for the
open() in userspace*/
uinput-532 [004] .... 45.313783: SYSC_ioctl: 1 /* This
print is for the trace, i put after fdget */
uinput-532 [004] .... 45.313788: uinput_ioctl_handler:
uinput: uinput_ioctl_handler, 1 /* This print is from the uinput_ioctl
driver */
uinput-532 [004] .... 45.313835: SYSC_ioctl: 1 /* This
print is for the trace, i put after fdput*/
uinput-532 [004] .... 45.313843: uinput_release:
uinput: 0 /* And this is from the close() */
Should fdget not suppose to increment the f_count here, as it is coming 1 ?
This f_count to one is done at the open, but i have no idea how this
below f_count 2 came before open() for
this simple program.
uinput-532 [002] .... 45.312044: SYSC_ioctl: 2 <= f_count
> <After fdget()
uinput-532 [002] .... 45.312055: SYSC_ioctl:
2 <After fdput()
-Mukesh
IOW, in your trace fput() has already been run by close(2); having somebody else
do that again while we are in ->ioctl() would be a bug (to start with, where
did they get that struct file * and why wasn't that reference contributing to
struct file refcount?)
In all cases we only call ->release() once all references gone - both
the one(s) in descriptor tables and any transient ones acquired by
fdget(), etc.
I would really like to see a reproducer for the original use-after-free
report...
