net/sched: fix potential stack infoleak in em_text_dump()

Overview

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

net/sched: fix potential stack infoleak in em_text_dump()

em_text_dump() allocates struct tcf_em_text on the stack without zeroing it. strscpy() writes the algorithm name and a NUL terminator into conf.algo[], leaving the remaining bytes uninitialised. nla_put_nohdr() then copies the full struct to the netlink response.

KMSAN on Linux 7.2-rc6 reports two kernel-infoleak splats from this path, one triggered via "tc filter show" and one via a raw RTM_GETTFILTER dump:

BUG: KMSAN: kernel-infoleak in _copy_to_iter+0x1c9/0x2620 nla_put_nohdr+0x83/0x130 em_text_dump+0x291/0x550 Local variable conf created at: em_text_dump+0x5d/0x550 Bytes 168-179 of 199 are uninitialized

I am not certain whether this constitutes a real security problem in practice: the test was conducted in a controlled KMSAN environment and the leaked stack bytes may or may not carry sensitive data on actual production kernels. I am reporting it because KMSAN flagged it as a kernel-infoleak and the fix is straightforward. I can provide a userspace reproducer on request.

The original code used strncpy() which zero-pads to the destination size. Commit b04202d6065c ("net/sched: replace strncpy with strscpy") replaced it with strscpy(), which does not pad, creating this condition. Zero-initialising the struct closes it.

Affected

• Linux >= b04202d6065cba5dfd51fb9866400b25573eac67 < 9923e1c537f91f3530b52d4efce85f3d9c131bf8
• Linux >= b04202d6065cba5dfd51fb9866400b25573eac67 < 0b160417826d22f24fc31a5b9cfae3b14aa70f76
• Linux >= b04202d6065cba5dfd51fb9866400b25573eac67 < 9c572a83037a7dcd653ba3a9cc468c16b857d0c9
• Linux 6.17

Remediation

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