From: Alexei Starovoitov stackid_init() gives a bucket of the stack map min(trace_nr - skip, max_depth - skip) trace entries. Here max_depth is the size of a bucket plus skip, but at most perf_event_max_stack. Context markers can make trace_nr larger than that, so the skip test can pass when max_depth < skip. Then 'max_depth - skip' wraps, and the kernel can write after the bucket. Clamp trace_nr to max_depth before the skip test, as __bpf_get_stack_pe() does. The result changes only when skip >= max_depth. Then it is -EFAULT. Until now skip == max_depth could give the id of an empty trace. get_perf_callchain() releases the trace, so an NMI can change trace->nr. Use READ_ONCE() in bpf_get_stackid(), so the compiler reads it one time. Fixes: 23f852daa4ba ("bpf: Fix stackmap overflow check in __bpf_get_stackid()") Signed-off-by: Alexei Starovoitov --- kernel/bpf/stackmap.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index 7d90891cce0a..a19126f5db02 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -522,12 +522,14 @@ static int stackid_init(struct stackid *stackid, struct bpf_map *map, u32 skip = flags & BPF_F_SKIP_FIELD_MASK; u32 max_depth; + max_depth = stack_map_calculate_max_depth(map->value_size, stack_map_data_size(map), flags); + /* trace_nr can be above max_depth and max_depth below skip: clamp first */ + trace_nr = min_t(u32, trace_nr, max_depth); if (trace_nr <= skip) /* skipping more than usable stack trace */ return -EFAULT; - max_depth = stack_map_calculate_max_depth(map->value_size, stack_map_data_size(map), flags); - stackid->nr = min_t(u32, trace_nr - skip, max_depth - skip); + stackid->nr = trace_nr - skip; stackid->len = stackid->nr * sizeof(u64); stackid->ips = trace->ip + skip; stackid->hash = jhash2((const u32 *)stackid->ips, stackid->len / sizeof(u32), 0); @@ -642,7 +644,8 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, /* couldn't fetch the stack trace */ return -EFAULT; - err = stackid_fastpath(&stackid, map, trace, trace->nr, flags); + /* the entry is released already, an NMI can change trace->nr */ + err = stackid_fastpath(&stackid, map, trace, READ_ONCE(trace->nr), flags); if (err != -ENOENT) return err; -- 2.55.0