On Fri, May 18, 2018 at 06:02:23PM +0200, Guillaume Nault wrote: > On Sun, May 13, 2018 at 11:11:55PM -0700, Eric Biggers wrote: > > [+ppp list and maintainer] > > > > This is a bug in ppp_generic.c; it still happens on Linus' tree and it's easily > > reproducible, see program below. The bug is that the PPPIOCDETACH ioctl doesn't > > consider that the file can still be attached to epoll instances even when > > ->f_count == 1. > > Right. What would it take to remove the file for the epoll instances? > Sorry for the naive question, but I'm not familiar with VFS and didn't > find a helper function we could call. > There is eventpoll_release_file(), but it's not exported to modules. It might work to call it, but it seems like a hack. > > Also, the reproducer doesn't test this but I think ppp_poll(), > > ppp_read(), and ppp_write() can all race with PPPIOCDETACH, causing > > use-after-frees as well. > > I also believe so. ppp_release() resets ->private_data, and even though > functions like ppp_read() test ->private_data before executing, there's > no synchronisation mechanism to ensure that the update is visible > before the unit or channel is destroyed. Is that the kind of race you > had in mind? Yes, though after looking into it more I *think* these additional races aren't actually possible, due to the 'f_count < 2' check. These races could only happen with a shared fd table, but in that case fdget() would increment f_count for the duration of each operation, resulting in 'f_count >= 2' if both ioctl() and something else is running on the same file concurrently. Note that this also means PPPIOCDETACH doesn't work at all if called from a multithreaded application... > > > Any chance that PPPIOCDETACH can simply be removed, > > given that it's apparently been "deprecated" for 16 years? > > Does anyone use it? > > The only users I'm aware of are pppd versions older than ppp-2.4.2 > (released in November 2003). But even at that time, there were issues > with PPPIOCDETACH as pppd didn't seem to react properly when this call > failed. An Internet search on the "PPPIOCDETACH file->f_count=" kernel > log string, or on the "Couldn't release PPP unit: Invalid argument" > error message of pppd, returns several related bug reports. > > Originally, PPPIOCDETACH never failed, but testing ->f_count was > later introduced to fix crashes when the file descriptor had been > duplicated. It seems that this was motivated by polling issues too. > > Long story short, it looks like PPPIOCDETACH never has worked well > and we have at least two more bugs to fix. Given how it has proven > fragile, I wouldn't be surprised if there were even more lurking > around. I'd say that it's probably safer to drop it than to add more > workarounds and playing wack-a-mole with those bugs. IMO, if we can get away with removing it without any users noticing, that would be much better than trying to fix it with a VFS-level hack, and probably missing some cases. I'll send a patch to get things started... - Eric -- To unsubscribe from this list: send the line "unsubscribe linux-ppp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
