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Message-ID: <bfffe2d9-1d2a-4376-abb6-a8746a8a3a69@linux.dev>
Date: Thu, 7 Aug 2025 12:05:23 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Arnaud Lecomte <contact@...aud-lcm.com>
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
daniel@...earbox.net, eddyz87@...il.com, haoluo@...gle.com,
john.fastabend@...il.com, jolsa@...nel.org, kpsingh@...nel.org,
linux-kernel@...r.kernel.org, martin.lau@...ux.dev, sdf@...ichev.me,
song@...nel.org, syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH 2/2] bpf: fix stackmap overflow check in
__bpf_get_stackid()
On 8/7/25 10:52 AM, Arnaud Lecomte wrote:
> Syzkaller reported a KASAN slab-out-of-bounds write in __bpf_get_stackid()
> when copying stack trace data. The issue occurs when the perf trace
> contains more stack entries than the stack map bucket can hold,
> leading to an out-of-bounds write in the bucket's data array.
>
> Reported-by: syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
> ---
> kernel/bpf/stackmap.c | 26 +++++++++++++++-----------
> 1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 14e034045310..d7ef840971f0 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -250,7 +250,7 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
> }
>
> static long __bpf_get_stackid(struct bpf_map *map,
> - struct perf_callchain_entry *trace, u64 flags)
> + struct perf_callchain_entry *trace, u64 flags, u32 max_depth)
> {
> struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
> struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
> @@ -266,6 +266,8 @@ static long __bpf_get_stackid(struct bpf_map *map,
>
> trace_nr = trace->nr - skip;
> trace_len = trace_nr * sizeof(u64);
> + trace_nr = min(trace_nr, max_depth - skip);
> +
> ips = trace->ip + skip;
> hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
> id = hash & (smap->n_buckets - 1);
> @@ -325,19 +327,19 @@ static long __bpf_get_stackid(struct bpf_map *map,
> BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
> u64, flags)
> {
> - u32 max_depth = map->value_size / stack_map_data_size(map);
> - u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
> + u32 elem_size = stack_map_data_size(map);
> bool user = flags & BPF_F_USER_STACK;
> struct perf_callchain_entry *trace;
> bool kernel = !user;
> + u32 max_depth;
>
> if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
> BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
> return -EINVAL;
>
> - max_depth += skip;
> - if (max_depth > sysctl_perf_event_max_stack)
> - max_depth = sysctl_perf_event_max_stack;
> + max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> + if (max_depth < 0)
> + return -EFAULT;
the above condition is not needed.
>
> trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
> false, false);
> @@ -346,7 +348,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
> /* couldn't fetch the stack trace */
> return -EFAULT;
>
> - return __bpf_get_stackid(map, trace, flags);
> + return __bpf_get_stackid(map, trace, flags, max_depth);
> }
>
> const struct bpf_func_proto bpf_get_stackid_proto = {
> @@ -378,6 +380,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
> bool kernel, user;
> __u64 nr_kernel;
> int ret;
> + u32 elem_size, pe_max_depth;
pe_max_depth -> max_depth.
>
> /* perf_sample_data doesn't have callchain, use bpf_get_stackid */
> if (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN))
> @@ -396,24 +399,25 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
> return -EFAULT;
>
> nr_kernel = count_kernel_ip(trace);
> -
> + elem_size = stack_map_data_size(map);
> if (kernel) {
> __u64 nr = trace->nr;
>
> trace->nr = nr_kernel;
> - ret = __bpf_get_stackid(map, trace, flags);
> + pe_max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> + ret = __bpf_get_stackid(map, trace, flags, pe_max_depth);
>
> /* restore nr */
> trace->nr = nr;
> } else { /* user */
> u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
> -
please keep an empty line here.
> skip += nr_kernel;
> if (skip > BPF_F_SKIP_FIELD_MASK)
> return -EFAULT;
>
> flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> - ret = __bpf_get_stackid(map, trace, flags);
> + pe_max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> + ret = __bpf_get_stackid(map, trace, flags, pe_max_depth);
> }
> return ret;
> }
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