CVE-2025-39813

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.