[PATCH] sysctl.conf: prevent unintentional writes into attacker-controlled files and FIFOs

["Peter Müller" <peter.mueller@ipfire.org>]

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Similar to hard- and symlink protection introduced a while ago, this
patch enables protections against unintentional writes into
attacker-controlled regular files or FIFOs, where a program expected to
create new ones. This makes exploiting TOCTOU flaws harder.

See also: https://www.kernel.org/doc/Documentation/sysctl/fs.txt

Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
---
 config/etc/sysctl.conf | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/config/etc/sysctl.conf b/config/etc/sysctl.conf
index d48c7734e..be7c07c85 100644
--- a/config/etc/sysctl.conf
+++ b/config/etc/sysctl.conf
@@ -49,6 +49,11 @@ kernel.dmesg_restrict = 1
 fs.protected_symlinks = 1
 fs.protected_hardlinks = 1
 
+# Don't allow writes to files and FIFOs that we don't own in world writable sticky
+# directories, unless they are owned by the owner of the directory.
+fs.protected_fifos = 2
+fs.protected_regular = 2
+
 # Minimal preemption granularity for CPU-bound tasks:
 # (default: 1 msec#  (1 + ilog(ncpus)), units: nanoseconds)
 kernel.sched_min_granularity_ns = 10000000
-- 
2.26.2