From: Tom Parkin <firstname.lastname@example.org> To: email@example.com Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: Re: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe Date: Mon, 15 Mar 2021 09:57:54 +0000 [thread overview] Message-ID: <20210315095754.GA4219@katalix.com> (raw) In-Reply-To: <2ad7aaa2.fcad.17826e87afb.Coremail.email@example.com> [-- Attachment #1: Type: text/plain, Size: 2631 bytes --] On Fri, Mar 12, 2021 at 22:47:53 +0800, firstname.lastname@example.org wrote: > > > > > -----原始邮件----- > > 发件人: "Tom Parkin" <email@example.com> > > 发送时间: 2021-03-12 18:12:58 (星期五) > > 收件人: firstname.lastname@example.org > > 抄送: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com > > 主题: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe > > > > Thanks for the report! > > > > On Thu, Mar 11, 2021 at 20:34:44 +0800, firstname.lastname@example.org wrote: > > > File: drivers/net/ppp/ppp_generic.c > > > > > > In ppp_unregister_channel, pch could be freed in ppp_unbridge_channels() > > > but after that pch is still in use. Inside the function ppp_unbridge_channels, > > > if "pchbb == pch" is true and then pch will be freed. > > > > Do you have a way to reproduce a use-after-free scenario? > > > > From static analysis I'm not sure how pch would be freed in > > ppp_unbridge_channels when called via. ppp_unregister_channel. > > > > In theory (at least!) the caller of ppp_register_net_channel holds > > a reference on struct channel which ppp_unregister_channel drops. > > > > Each channel in a bridged pair holds a reference on the other. > > > > Hence on return from ppp_unbridge_channels, the channel should not have > > been freed (in this code path) because the ppp_register_net_channel > > reference has not yet been dropped. > > > > Maybe there is an issue with the reference counting or a race of some > > sort? > > > > > I checked the commit history and found that this problem is introduced from > > > 4cf476ced45d7 ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls"). > > > > > > I have no idea about how to generate a suitable patch, sorry. > > This issue was reported by a path-sensitive static analyzer developed by our Lab, > thus i have not a crash or bug log. Does the analyzer report the sequence of events that lead to a possible use-after-free? Is it starting from the assumption that ppp_unbridge_channels is called with pch->file.refcnt == 1? > As the return type of ppp_unbridge_channels() is a int, can we return a value to > inform caller that the channel is freed? If ppp_unbridge_channels frees the channel in the ppp_unregister_channel call path, that implies that ppp_unregister_channel is called when only the bridge is keeping the channel alive. So the caller is attempting to drop a reference it doesn't in fact have. It might be preferable to defensively code against that possibility of course. [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 488 bytes --]
next prev parent reply other threads:[~2021-03-15 9:57 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-11 12:34 lyl2019 2021-03-12 10:12 ` Tom Parkin 2021-03-12 14:47 ` lyl2019 2021-03-15 9:57 ` Tom Parkin [this message] 2021-03-15 12:18 ` Guillaume Nault 2021-03-15 16:58 ` Tom Parkin 2021-03-15 12:04 ` Guillaume Nault 2021-03-15 11:50 ` Guillaume Nault
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20210315095754.GA4219@katalix.com \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --subject='Re: Re: [BUG] net/ppp: A use after free in ppp_unregister_channe' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).