From: Kees Cook <email@example.com> To: Greg Kroah-Hartman <firstname.lastname@example.org> Cc: Michal Hocko <email@example.com>, Andrew Morton <firstname.lastname@example.org>, Alexey Dobriyan <email@example.com>, Lee Duncan <firstname.lastname@example.org>, Chris Leech <email@example.com>, Adam Nichols <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, Uladzislau Rezki <firstname.lastname@example.org> Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer Date: Wed, 17 Mar 2021 14:30:47 -0700 [thread overview] Message-ID: <202103171425.CB0F4619A8@keescook> (raw) In-Reply-To: <YFIikaNixD57o3pk@kroah.com> On Wed, Mar 17, 2021 at 04:38:57PM +0100, Greg Kroah-Hartman wrote: > On Wed, Mar 17, 2021 at 04:20:52PM +0100, Michal Hocko wrote: > > On Wed 17-03-21 15:56:44, Greg KH wrote: > > > On Wed, Mar 17, 2021 at 03:44:16PM +0100, Michal Hocko wrote: > > > > On Wed 17-03-21 14:34:27, Greg KH wrote: > > > > > On Wed, Mar 17, 2021 at 01:08:21PM +0100, Michal Hocko wrote: > > > > > > Btw. I still have problems with the approach. seq_file is intended to > > > > > > provide safe way to dump values to the userspace. Sacrificing > > > > > > performance just because of some abuser seems like a wrong way to go as > > > > > > Al pointed out earlier. Can we simply stop the abuse and disallow to > > > > > > manipulate the buffer directly? I do realize this might be more tricky > > > > > > for reasons mentioned in other emails but this is definitely worth > > > > > > doing. > > > > > > > > > > We have to provide a buffer to "write into" somehow, so what is the best > > > > > way to stop "abuse" like this? > > > > > > > > What is wrong about using seq_* interface directly? > > > > > > Right now every show() callback of sysfs would have to be changed :( > > > > Is this really the case? Would it be too ugly to have an intermediate > > buffer and then seq_puts it into the seq file inside sysfs_kf_seq_show. > > Oh, good idea. > > > Sure one copy more than necessary but it this shouldn't be a hot path or > > even visible on small strings. So that might be worth destroying an > > inherently dangerous seq API (seq_get_buf). > > I'm all for that, let me see if I can carve out some time tomorrow to > try this out. The trouble has been that C string APIs are just so impossibly fragile. We just get too many bugs with it, so we really do need to rewrite the callbacks to use seq_file, since it has a safe API. I've been trying to write coccinelle scripts to do some of this refactoring, but I have not found a silver bullet. (This is why I've suggested adding the temporary "seq_show" and "seq_store" functions, so we can transition all the callbacks without a flag day.) > But, you don't get rid of the "ability" to have a driver write more than > a PAGE_SIZE into the buffer passed to it. I guess I could be paranoid > and do some internal checks (allocate a bunch of memory and check for > overflow by hand), if this is something to really be concerned about... Besides the CFI prototype enforcement changes (which I can build into the new seq_show/seq_store callbacks), the buffer management is the primary issue: we just can't hand drivers a string (even with a length) because the C functions are terrible. e.g. just look at the snprintf vs scnprintf -- we constantly have to just build completely new API when what we need is a safe way (i.e. obfuscated away from the caller) to build a string. Luckily seq_file does this already, so leaning into that is good here. -- Kees Cook
next prev parent reply other threads:[~2021-03-17 21:31 UTC|newest] Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-15 17:48 Kees Cook 2021-03-15 18:33 ` Al Viro 2021-03-15 20:43 ` Kees Cook 2021-03-16 7:24 ` Greg Kroah-Hartman 2021-03-16 12:43 ` Al Viro 2021-03-16 12:55 ` Greg Kroah-Hartman 2021-03-16 13:01 ` Michal Hocko 2021-03-16 19:18 ` Kees Cook 2021-03-17 10:44 ` Greg Kroah-Hartman 2021-03-16 8:31 ` Michal Hocko 2021-03-16 19:08 ` Kees Cook 2021-03-17 12:08 ` Michal Hocko 2021-03-17 13:34 ` Greg Kroah-Hartman 2021-03-17 14:44 ` Michal Hocko 2021-03-17 14:56 ` Greg Kroah-Hartman 2021-03-17 15:20 ` Michal Hocko 2021-03-17 15:38 ` Greg Kroah-Hartman 2021-03-17 15:48 ` Michal Hocko 2021-03-17 21:30 ` Kees Cook [this message] 2021-03-18 8:07 ` Greg Kroah-Hartman 2021-03-18 15:51 ` Kees Cook 2021-03-18 17:56 ` Greg Kroah-Hartman 2021-03-19 14:07 ` [seq_file] 5fd6060e50: stress-ng.eventfd.ops_per_sec -49.1% regression kernel test robot 2021-03-19 19:31 ` Kees Cook
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