Messages in this thread | | | From | valdis.kletnieks@vt ... | Subject | Re: [PATCH] bpf: btf: Fix a missing check bug | Date | Mon, 08 Oct 2018 17:17:08 -0400 |
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On Mon, 08 Oct 2018 13:51:09 -0700, Song Liu said: > On Sun, Oct 7, 2018 at 1:26 PM Wenwen Wang <wang6495@umn.edu> wrote:
> > same value. Given that the btf data is in the user space, a malicious user > > can race to change the data between the two copies. By doing so, the user > > can provide malicious data to the kernel and cause undefined behavior.
> These two numbers are copied from same memory location, right? So I > think this check is not necessary?
Security researchers call this a TOCTOU bug - Time of Check - Time of Use.
What can happen:
1) We fetch the value (say we get 90) from userspace and stash it in hdr_len.
2) We do some other stuff like check the hdr_len isn't too big, etc..
meanwhile, on another CPU running another thread of the process... 3) malicious code stuffs a 117 into that field
4) We fetch the entire header, incliding a now-changed hdr_len (now 117) and stick it in btf->hdr->hdr_len.
5) Any code that assumes that hdr_len and btf->hdr->hdr_len are the same value explodes in interesting ways.
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