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Date: Thu, 22 Jul 2021 22:30:45 +0000
From: Qualys Security Advisory <>
To: "" <>
Subject: Re: CVE-2021-33909: size_t-to-int vulnerability in Linux's filesystem

Hi all,

A few people have asked us how we discovered CVE-2021-33909
(size_t-to-int conversion vulnerability in Linux's filesystem layer):

- We were reading (not auditing) the kernel code that generates some of
  the files in /proc/pid. We eventually reached seq_read_iter() and
  noticed the "while (1) { ... m->size <<= 1 ... }".

- We immediately wondered if there was a limit to this exponential
  growth; there was none. Next, we searched for /proc/pid seq_files
  whose records could become very large; and we knew from CVE-2020-28010
  that directory paths on Linux can be much longer than PATH_MAX.

- So we found show_mountinfo() (which generates /proc/pid/mountinfo) and
  the signed int buflen in the subsequent dentry_path(), which led to
  the out-of-bounds write of the "//deleted" string.

We are at your disposal for questions, comments, and further
discussions. Thank you very much! With best regards,

the Qualys Security Advisory team


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