Hi Sorry for the delaying.
> On Sat, Oct 10, 2009 at 2:32 AM, KOSAKI Motohiro > <[email protected]> wrote: > > >> It does seem like a maximum spin count should be put in there - and > >> maybe a timeout as well (since with FUSE etc it's possible to engineer > >> page faults that take arbitrarily long). > >> Also, it occurs to me that: > > > > makes sense. > > I like maximum spin rather than timeout. > > I'm worried about the scenario where process A sets its cmdline buffer > to point to a page which will take a _VERY_ long time to pagein (maybe > forever), and then process B goes to try to read its cmdline. What > happens now? Honestly, I don't worry about so much. if attacker want DoS attack, fork bomb is efficient than this way. then, attacker never use this. > Process A can arrange for this to happen by using a FUSE filesystem > that sits on a read forever. And since the first thing the admin's > likely to do to track down the problem is 'ps awux', this is liable to > be a rather nasty DoS... Probably, I haven't understand this paragraph. Why is this FUSE related issue? > Of course, this is no worse than it is now - it's already possible to > replace the page in question. But we should think about ways this > could be fixed for good... Plus, please look my mesurement data as another post. seqlock implementation is very fast although contention occured. > >>> + do { > >>> + seq = read_seqbegin(&mm->arg_lock); > >>> + > >>> + len = mm->arg_end - mm->arg_start; > >>> + if (len > PAGE_SIZE) > >>> + len = PAGE_SIZE; > >> > >> If arg_end or arg_start are modified after this, is it truly safe to > >> assume that len will remain <= PAGE_SIZE without a memory barrier > >> before the conditional? > > > > 1) access_process_vm() doesn't return error value. > > 2) read_seqretry(&mm->arg_lock, seq)) check seq, not mm->arg_start or len. > > > > then, if arg_{start,end} is modified, access_process_vm() may return 0 > > and strnlen > > makes bad calculation, but read_seqretry() can detect its modify > > rightly. I think. > > No, I'm worried about what if the compiler decides to rewrite like so: > if (mm->arg_end - mm->arg_start > PAGE_SIZE) > len = PAGE_SIZE; > else /* here we reload arg_end/arg_start! */ > len = mm->arg_end - mm->arg_start; > > Now we might write into buffer more than PAGE_SIZE bytes, which is > probably a buffer overrun into kernel space... Rgiht. I'll fix this issue at next spin. Thank you. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
