Re: usb: f_fs: Fix CFI failure in ki_complete

[Alexander Lobakin]

From: Dan Carpenter <error27@xxxxxxxxx>
Date: Fri, 23 Dec 2022 12:04:18 +0300

> On Thu, Dec 22, 2022 at 06:21:03PM +0530, Prashanth K wrote:
> > 
> > 
> > On 14-12-22 11:05 pm, David Laight wrote:
> > > From: Greg Kroah-Hartman
> > > > Sent: 12 December 2022 13:35
> > > > 
> > > > On Mon, Dec 12, 2022 at 06:54:24PM +0530, Prashanth K wrote:
> > > > > Function pointer ki_complete() expects 'long' as its second
> > > > > argument, but we pass integer from ffs_user_copy_worker. This
> > > > > might cause a CFI failure, as ki_complete is an indirect call
> > > > > with mismatched prototype. Fix this by typecasting the second
> > > > > argument to long.
> > > > 
> > > > "might"?  Does it or not?  If it does, why hasn't this been reported
> > > > before?
> > > 
> > > Does the cast even help at all.
> > Actually I also have these same questions
> > - why we haven't seen any instances other than this one?
> > - why its not seen on other indirect function calls?
> > 
> > Here is the the call stack of the failure that we got.
> > 
> > [  323.288681][    T7] Kernel panic - not syncing: CFI failure (target:
> > 0xffffffe5fc811f98)
> > [  323.288710][    T7] CPU: 6 PID: 7 Comm: kworker/u16:0 Tainted: G S    W
> > OE     5.15.41-android13-8-g5ffc5644bd20 #1
> > [  323.288730][    T7] Workqueue: adb ffs_user_copy_worker.cfi_jt
> > [  323.288752][    T7] Call trace:
> > [  323.288755][    T7]  dump_backtrace.cfi_jt+0x0/0x8
> > [  323.288772][    T7]  dump_stack_lvl+0x80/0xb8
> > [  323.288785][    T7]  panic+0x180/0x444
> > [  323.288797][    T7]  find_check_fn+0x0/0x218
> > [  323.288810][    T7]  ffs_user_copy_worker+0x1dc/0x204
> > [  323.288822][    T7]  kretprobe_trampoline.cfi_jt+0x0/0x8
> > [  323.288837][    T7]  worker_thread+0x3ec/0x920
> > [  323.288850][    T7]  kthread+0x168/0x1dc
> > [  323.288859][    T7]  ret_from_fork+0x10/0x20
> > [  323.288866][    T7] SMP: stopping secondary CPUs
> > 
> > And from address to line translation, we got know the issue is from
> > ffs_user_copy_worker+0x1dc/0x204
> > 		||
> > io_data->kiocb->ki_complete(io_data->kiocb, ret);
> > 
> > And "find_check_fn" was getting invoked from ki_complete. Only thing that I
> > found suspicious about ki_complete() is its argument types. That's why I
> > pushed this patch here, so that we can discuss this out here.
> 
> I think the problem is more likely whatever ->ki_complete() points to
> but I have no idea what that is on your system.  You're using an Android
> kernel so it could be something out of tree as well...

Correct, CFI would *never* trigger a failure due to passing int as
long. It triggers only on prototype-implementation mismatches. The
author should go and check carefully whether there are any places
where some implementation differs and then ::ki_complete() gets
passed with a function typecast. Also, there can be places where a
proto has an argument as enum and an implementation has it as int.
Compilers don't warn on such mismatches, CFI does. The latest LLVM
Git snapshot with `-Wcast-function-type-strict` enabled could help
hunt such.

> 
> regards,
> dan carpenter

Thanks,
Olek