get_perf_callchain() returns a per-CPU perf_callchain_entry buffer and releases its recursion slot via put_callchain_entry() before returning, so nothing keeps the entry reserved while __bpf_get_stack() consumes it below. A preemptible BPF program (e.g. a non-sleepable raw tracepoint program on a PREEMPT kernel, which runs under migrate_disable() but not preempt_disable()) can be scheduled out between obtaining the entry and the copy. Another task scheduled on the same CPU then reuses the same per-CPU buffer and overwrites trace->nr with a larger value. copy_len is then computed from the inflated trace->nr and can exceed the caller's buffer, causing an out-of-bounds write in the memcpy() and in the build_id path. The rcu_read_lock() taken here alone does not prevent this. It is only taken on the may_fault path, and under CONFIG_PREEMPT_RCU it does not disable preemption; it merely keeps perf's callchain buffer array alive (freed via call_rcu()) and does nothing to stop another task from reusing the entry. Disable preemption around obtaining the callchain entry and copying it into the caller's buffer, so the entry cannot be reused underneath us and trace->nr stays bounded by max_depth. Build ID resolution may fault and is therefore deferred until after preemption is re-enabled; by then the instruction pointers have already been copied into buf, so it operates only on that private copy. Note, preempt_disable() also subsumes the buffer-lifetime guarantee the rcu_read_lock() provided, since a preempt-disabled section is an RCU read-side critical section for the callchain buffers' call_rcu() reclaim. Fixes: c195651e565a ("bpf: add bpf_get_stack helper") Reported-by: Tao Chen <chen.dylane@linux.dev> Reported-by: STAR Labs SG <info@starlabs.sg> Signed-off-by: Daniel Borkmann <borkmann@iogearbox.net> Signed-off-by: Jiri Olsa <jolsa@kernel.org> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/bpf/20260803210149.296496-11-jolsa@kernel.org
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| @@ -815,6 +815,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, · 814 unmodified lines | |||
| 815 | 815 | ||
| 816 | max_depth = stack_map_calculate_max_depth(size, elem_size, flags); | 816 | max_depth = stack_map_calculate_max_depth(size, elem_size, flags); |
| 817 | 817 | ||
| 818 | preempt_disable(); | ||
| 818 | if (may_fault) | 819 | if (may_fault) |
| 819 | rcu_read_lock(); /* need RCU for perf's callchain below */ | 820 | rcu_read_lock(); /* need RCU for perf's callchain below */ |
| 820 | 821 | ||
| @@ -828,6 +829,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, · 7 unmodified lines | |||
| 828 | if (unlikely(!trace) || trace->nr < skip) { | 829 | if (unlikely(!trace) || trace->nr < skip) { |
| 829 | if (may_fault) | 830 | if (may_fault) |
| 830 | rcu_read_unlock(); | 831 | rcu_read_unlock(); |
| 832 | preempt_enable(); | ||
| 831 | goto err_fault; | 833 | goto err_fault; |
| 832 | } | 834 | } |
| 833 | 835 | ||
| @@ -836,6 +838,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, · 2 unmodified lines | |||
| 836 | /* trace should not be dereferenced after this point */ | 838 | /* trace should not be dereferenced after this point */ |
| 837 | if (may_fault) | 839 | if (may_fault) |
| 838 | rcu_read_unlock(); | 840 | rcu_read_unlock(); |
| 841 | preempt_enable(); | ||
| 839 | 842 | ||
| 840 | return callchain_finalize(buf, size, trace_nr, elem_size, flags, may_fault); | 843 | return callchain_finalize(buf, size, trace_nr, elem_size, flags, may_fault); |
| 841 | 844 | ||