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Message-ID: <Y6XcJ+tcf1Cg1jkZ@momentum.pseudorandom.co.uk>
Date: Fri, 23 Dec 2022 16:49:43 +0000
From: Simon McVittie <smcv@...ian.org>
To: oss-security@...ts.openwall.com
Subject: Re: [Linux] /proc/pid/stat parsing bugs
On Thu, 22 Dec 2022 at 10:04:48 -0500, Shawn Webb wrote:
> We knew way back then the dangers of VFS-based wizardry. Did we lose
> that knowledge somehow?
To me this seems like a parsing problem, not a VFS problem. Some
pseudo-files in Linux /proc are one file per item (/proc/self/oom_adj,
/proc/self/sessionid, most of /proc/sys) and those are fine[1]: the
structure is implicit in the filesystem layout, and the file contents
are trivial to "parse". Others have a simple and well-defined format
(like /proc/self/environ and /proc/self/cmdline, which are sequences of
\0-terminated bytestrings), and those also seem fine.
It's the pseudo-files that contain more than one item, particularly
those with a semi-consistent format that aims for human-readability, that
can easily get into escaping and parsing issues. If those pseudo-files
made *more* use of the VFS (one new file in /proc/self for each field
in the current /proc/self/stat?) then they would suffer from different
issues instead, like inability to read all fields atomically and maybe
performance issues for heavy users, but parsing would become a non-issue.
smcv
[1] or when they're not fine, the issues are around things like how to
separate an AppArmor enforcement mode from the label, which again is
a matter of parsing a human-readable format with structure
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