On Wed, Feb 21, 2018 at 12:04:30PM -0800, Cong Wang wrote: > On Thu, Feb 15, 2018 at 11:31 AM, Guillaume Nault <g.na...@alphalink.fr> > wrote: > > On Thu, Feb 15, 2018 at 06:01:16PM +0200, Denys Fedoryshchenko wrote: > >> On 2018-02-15 17:55, Guillaume Nault wrote: > >> > On Thu, Feb 15, 2018 at 12:19:52PM +0200, Denys Fedoryshchenko wrote: > >> > > Here we go: > >> > > > >> > > <srv> [24558.921549] > >> > > ================================================================== > >> > > <srv> [24558.922167] BUG: KASAN: use-after-free in > >> > > ppp_ioctl+0xa6a/0x1522 > >> > > [ppp_generic] > >> > > <srv> [24558.922776] Write of size 8 at addr ffff8803d35bf3f8 by task > >> > > accel-pppd/12622 > >> > > <srv> [24558.923113] > >> > > <srv> [24558.923451] CPU: 0 PID: 12622 Comm: accel-pppd Tainted: G > >> > > W > >> > > 4.15.3-build-0134 #1 > >> > > <srv> [24558.924058] Hardware name: HP ProLiant DL320e Gen8 v2, > >> > > BIOS P80 > >> > > 04/02/2015 > >> > > <srv> [24558.924406] Call Trace: > >> > > <srv> [24558.924753] dump_stack+0x46/0x59 > >> > > <srv> [24558.925103] print_address_description+0x6b/0x23b > >> > > <srv> [24558.925451] ? ppp_ioctl+0xa6a/0x1522 [ppp_generic] > >> > > <srv> [24558.925797] kasan_report+0x21b/0x241 > >> > > <srv> [24558.926136] ppp_ioctl+0xa6a/0x1522 [ppp_generic] > >> > > <srv> [24558.926479] ? ppp_nl_newlink+0x1da/0x1da [ppp_generic] > >> > > <srv> [24558.926829] ? sock_sendmsg+0x89/0x99 > >> > > <srv> [24558.927176] ? __vfs_write+0xd9/0x4ad > >> > > <srv> [24558.927523] ? kernel_read+0xed/0xed > >> > > <srv> [24558.927872] ? SyS_getpeername+0x18c/0x18c > >> > > <srv> [24558.928213] ? bit_waitqueue+0x2a/0x2a > >> > > <srv> [24558.928561] ? wake_atomic_t_function+0x115/0x115 > >> > > <srv> [24558.928898] vfs_ioctl+0x6e/0x81 > >> > > <srv> [24558.929228] do_vfs_ioctl+0xa00/0xb10 > >> > > <srv> [24558.929571] ? sigprocmask+0x1a6/0x1d0 > >> > > <srv> [24558.929907] ? sigsuspend+0x13e/0x13e > >> > > <srv> [24558.930239] ? ioctl_preallocate+0x14e/0x14e > >> > > <srv> [24558.930568] ? SyS_rt_sigprocmask+0xf1/0x142 > >> > > <srv> [24558.930904] ? sigprocmask+0x1d0/0x1d0 > >> > > <srv> [24558.931252] SyS_ioctl+0x39/0x55 > >> > > <srv> [24558.931595] ? do_vfs_ioctl+0xb10/0xb10 > >> > > <srv> [24558.931942] do_syscall_64+0x1b1/0x31f > >> > > <srv> [24558.932288] entry_SYSCALL_64_after_hwframe+0x21/0x86 > >> > > <srv> [24558.932627] RIP: 0033:0x7f302849d8a7 > >> > > <srv> [24558.932965] RSP: 002b:00007f3029a52af8 EFLAGS: 00000206 > >> > > ORIG_RAX: > >> > > 0000000000000010 > >> > > <srv> [24558.933578] RAX: ffffffffffffffda RBX: 00007f3027d861e3 RCX: > >> > > 00007f302849d8a7 > >> > > <srv> [24558.933927] RDX: 00007f3023f49468 RSI: 000000004004743a RDI: > >> > > 0000000000003a67 > >> > > <srv> [24558.934266] RBP: 00007f3029a52b20 R08: 0000000000000000 R09: > >> > > 000055c8308d8e40 > >> > > <srv> [24558.934607] R10: 0000000000000008 R11: 0000000000000206 R12: > >> > > 00007f3023f49358 > >> > > <srv> [24558.934947] R13: 00007ffe86e5723f R14: 0000000000000000 R15: > >> > > 00007f3029a53700 > >> > > <srv> [24558.935288] > >> > > <srv> [24558.935626] Allocated by task 12622: > >> > > <srv> [24558.935972] ppp_register_net_channel+0x5f/0x5c6 > >> > > [ppp_generic] > >> > > <srv> [24558.936306] pppoe_connect+0xab7/0xc71 [pppoe] > >> > > <srv> [24558.936640] SyS_connect+0x14b/0x1b7 > >> > > <srv> [24558.936975] do_syscall_64+0x1b1/0x31f > >> > > <srv> [24558.937319] entry_SYSCALL_64_after_hwframe+0x21/0x86 > >> > > <srv> [24558.937655] > >> > > <srv> [24558.937993] Freed by task 12622: > >> > > <srv> [24558.938321] kfree+0xb0/0x11d > >> > > <srv> [24558.938658] ppp_release+0x111/0x120 [ppp_generic] > >> > > <srv> [24558.938994] __fput+0x2ba/0x51a > >> > > <srv> [24558.939332] task_work_run+0x11c/0x13d > >> > > <srv> [24558.939676] exit_to_usermode_loop+0x7c/0xaf > >> > > <srv> [24558.940022] do_syscall_64+0x2ea/0x31f > >> > > <srv> [24558.940368] entry_SYSCALL_64_after_hwframe+0x21/0x86 > >> > > <srv> [24558.947099] > >> > > >> > Your first guess was right. It looks like we have an issue with > >> > reference counting on the channels. Can you send me your ppp_generic.o? > >> http://nuclearcat.com/ppp_generic.o > >> Compiled with gcc version 6.4.0 (Gentoo 6.4.0-r1 p1.3) > >> > > From what I can see, ppp_release() and ioctl(PPPIOCCONNECT) are called > > concurrently on the same ppp_file. Even if this ppp_file was pointed at > > by two different file descriptors, I can't see how this could defeat > > the reference counting mechanism. I'm going to think more about it. > > For me it looks like pch->clist is not removed from the list ppp->channels > when destroyed via ppp_release(). But I don't want to pretend I understand > ppp logic. > I've thought about that too, but couldn't find a scenario that could trigger the bug.
To get ->private_data pointing to a struct channel pointer, a file needs to ioctl(PPPIOCATTCHAN) first. For this call to succeed, the channel must have been registered with ppp_register_net_channel(). Both operations take a reference on the channel, which means that, before adding pch->clist to a ppp->channels list (with ppp_connect_channel()), the channel is already held by a /dev/ppp file and by the code that registered the channel in the first place. Therefore, closing the /dev/ppp file can't be enough to make ppp_release() free the channel. We need to unregister the channel for the refcount to drop to 0. And ppp_unregister_channel(), removes pch->clist from ppp->channels before decrementing the reference counter. I'm simplifying a bit, and there could always be a race somewhere. But I couldn't find anything so far.
