devices: work around kernel OOB write in BPF_PROG_QUERY - #69
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@opencontainers/cgroups-maintainers I want to release v0.0.9 once this is in. For analysis and repro, see #64 (comment) |
bpfAttrQuery described only the query member of union bpf_attr up to
prog_cnt, and that size (32 bytes) is what is passed to bpf(2).
Since Linux 6.17 (kernel commit 120933984460, "bpf: Implement mprog API
on top of existing cgroup progs"), kernel copies query.revision field
(offset 56) back to userspace unconditionally, without looking at the
size. So, the kernel writes past the end of the struct on every
BPF_PROG_QUERY.
Depending on how the compiler lays out the frame, those 8 bytes either
land on unused stack space, and nothing happens, or on live data. In
runc this showed up as corrupted defer records, crashing "runc restore"
in runtime.copystack, and as garbled strings.
The kernel bug was fixed by commit 21c4b99b27f3 ("bpf: fix
BPF_PROG_QUERY OOB write and cgroup backward compat", landed in v7.2).
Passing the larger size is safe on older kernels, as bpf(2) only
requires that the bytes beyond what it knows about are zero.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
kolyshkin
changed the title
AkihiroSuda
approved these changes
Aug 4, 2026
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bpfAttrQuery only described the query member of union bpf_attr up to prog_cnt, and that size (32 bytes) is what is passed to bpf(2). Since Linux 6.17 (kernel commit 120933984460, "bpf: Implement mprog API on top of existing cgroup progs"),
__cgroup_bpf_querycopiesquery.revisionback to userspace unconditionally, without looking at the size we passed. As revision sits at offset 56, the kernel writes 8 bytes past the end of the struct on everyBPF_PROG_QUERY.Depending on how the compiler lays out the frame, those 8 bytes either land on unused stack space, and nothing happens, or on live data. In runc this showed up as corrupted defer records, crashing "runc restore" in runtime.copystack, and as garbled strings. This can be seen in e.g. https://github.com/opencontainers/runc/actions/runs/30715630948. A small repro w/o runc is here: bpfcanary.go.
The kernel bug was fixed by commit 21c4b99b27f3 ("bpf: fix BPF_PROG_QUERY OOB write and cgroup backward compat", landed in v7.2).
Passing the larger size is safe on older kernels, as bpf(2) only
requires that the bytes beyond what it knows about are zero.