net/sched: em_text: reject unterminated algo before autoload

Overview

In the Linux kernel, the following vulnerability has been resolved:

net/sched: em_text: reject unterminated algo before autoload

This is a follow-up to commit 9c572a83037a ("net/sched: fix potential stack infoleak in em_text_dump()"), which zero-initialised the dump-side struct. Sashiko pointed at a pre-existing bug that exposed the change-side input-validation gap open.

em_text_change() casts the raw netlink attribute payload to struct tcf_em_text and passes conf->algo, a 16-byte array with no enforced NUL terminator, to textsearch_prepare(). On the TS_AUTOLOAD retry textsearch_prepare() calls request_module("ts_%s", algo); vsnprintf() walks %s until it finds a NUL, so an algo[] with no NUL reads past the array into the adjacent struct fields and payload and feeds those bytes to the usermode helper command line. This is an out-of-bounds read and a kernel memory disclosure.

Reject the attribute when no NUL exists within TC_EM_TEXT_ALGOSIZ bytes, matching the check xt_string has had since 3ab720881b6e.

Conditions to recreate the bug:

• CAP_NET_ADMIN on the target netns (namespace-local via unshare -Urn is enough)
• install clsact on an interface, then add a basic filter with one text ematch whose algo[] is 16 bytes with no NUL (e.g. "A"*16) and pattern_len at least 1
• the add fails with -ENOENT and the kernel invokes modprobe with a module name made of "ts_" plus the 16 bytes and the adjacent payload

Affected

• Linux >= d675c989ed2d4ba23dff615330b04371aea83534 < 00bcc303000bac524d62eed054d77372b38ace0f
• Linux >= d675c989ed2d4ba23dff615330b04371aea83534 < 22182f91c6703af6e9832cffe6d22b381031cb5a
• Linux >= d675c989ed2d4ba23dff615330b04371aea83534 < 85e5512369d4fe6e5f2c43712d9f9e01d4ae8437
• Linux >= d675c989ed2d4ba23dff615330b04371aea83534 < b627c6ba20be20334338229b4820baacb9d96826
• Linux >= d675c989ed2d4ba23dff615330b04371aea83534 < c9728fcf2e7fb4a7a8d08ebd9524c3b02ae1a021
• Linux 2.6.13

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.