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apparmor: avoid per-cpu hold underflow in aa_get_buffer

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When aa_get_buffer() pulls from the per-cpu list it unconditionally
decrements cache->hold. If hold reaches 0 while count is still non-zero,
the unsigned decrement wraps to UINT_MAX. This keeps hold non-zero for a
very long time, so aa_put_buffer() never returns buffers to the global
list, which can starve other CPUs and force repeated kmalloc(aa_g_path_max)
allocations.

Guard the decrement so hold never underflows.
Fixes: ea9bae12d0 ("apparmor: cache buffers on percpu list if there is lock contention")

Signed-off-by: Zhengmian Hu <huzhengmian@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
This commit is contained in:
Zhengmian Hu 2026-01-19 19:03:07 -05:00 • committed by John Johansen
parent 796c146fa6
commit 640cf2f095
1 changed files with 2 additions and 1 deletions
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3
security/apparmor/lsm.c
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@ -2137,7 +2137,8 @@ char *aa_get_buffer(bool in_atomic)
if (!list_empty(&cache->head)) {
aa_buf = list_first_entry(&cache->head, union aa_buffer, list);
list_del(&aa_buf->list);
cache->hold--;
if (cache->hold)
cache->hold--;
cache->count--;
put_cpu_ptr(&aa_local_buffers);
return &aa_buf->buffer[0];