ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

Overview

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

ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

When calling ftrace_dump_one() concurrently with reading trace_pipe, a WARN_ON_ONCE() in trace_printk_seq() can be triggered due to a race condition.

The issue occurs because:

CPU0 (ftrace_dump) CPU1 (reader) echo z > /proc/sysrq-trigger

!trace_empty(&iter) trace_iterator_reset(&iter) <- len = size = 0 cat /sys/kernel/tracing/trace_pipe trace_find_next_entry_inc(&iter) __find_next_entry ring_buffer_empty_cpu <- all empty return NULL

trace_printk_seq(&iter.seq) WARN_ON_ONCE(s->seq.len >= s->seq.size)

In the context between trace_empty() and trace_find_next_entry_inc() during ftrace_dump, the ring buffer data was consumed by other readers. This caused trace_find_next_entry_inc to return NULL, failing to populate iter.seq. At this point, due to the prior trace_iterator_reset, both iter.seq.len and iter.seq.size were set to 0. Since they are equal, the WARN_ON_ONCE condition is triggered.

Move the trace_printk_seq() into the if block that checks to make sure the return value of trace_find_next_entry_inc() is non-NULL in ftrace_dump_one(), ensuring the 'iter.seq' is properly populated before subsequent operations.

Affected

• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < f299353e7ccbcc5c2ed8993c48fbe7609cbe729a
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < 5ab0ec206deb99eb3baf8f1d7602aeaa91dbcc85
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < a6f0f8873cc30fd4543b09adf03f7f51d293f0e6
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < e80ff23ba8bdb0f41a1afe2657078e4097d13a9a
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < 28c8fb7ae2ad27d81c8de3c4fe608c509f6a18aa
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < ced94e137e6cd5e79c65564841d3b7695d0f5fa3
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < fbd4cf7ee4db65ef36796769fe978e9eba6f0de4
• Linux >= d769041f865330034131525ee6a7f72eb4af2a24 < 4013aef2ced9b756a410f50d12df9ebe6a883e4a
• Linux 2.6.28

Remediation

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