Messages in this thread | | | Date | Thu, 22 Dec 2022 18:21:03 +0530 | Subject | Re: usb: f_fs: Fix CFI failure in ki_complete | From | Prashanth K <> |
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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.
Thanks in advance
> > ... >>> - io_data->kiocb->ki_complete(io_data->kiocb, ret); >>> + io_data->kiocb->ki_complete(io_data->kiocb, (long)ret); > ... > > If definition of the parameter in the structure member ki_complete() > definition is 'long' then the compiler has to promote 'ret' to long > anyway. CFI has nothing to do with it. > > OTOH if you've used a cast to assign a function with a > different prototype to ki_complete then 'all bets are off' > and you get all the run time errors you deserve. > CFI just converts some of them to compile time errors. > > For instance if you assign xx_complete(long) to (*ki_complete)(int) > then it is very likely that xx_complete() will an argument > with some of the high bits set. > But adding a cast to the call - ki_complete((long)int_var) > will make absolutely no difference. > The compiler wont zero/sign extend int_var to 64bits for you, > that will just get optimised away and the high bits will > be unchanged. > > You're description seems to be the other way around (which might > be safe, but CFI probably still barfs). > But you need to fix the indirect calls so the function types > match. So does that mean, we need to add casts in al indirect calls to match the function signature? > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
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