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bpf: Tighten the requirements for preallocated hash maps

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The assumption that only programs attached to perf NMI events can deadlock
on memory allocators is wrong. Assume the following simplified callchain:

 kmalloc() from regular non BPF context
  cache empty
   freelist empty
    lock(zone->lock);
     tracepoint or kprobe
      BPF()
       update_elem()
        lock(bucket)
          kmalloc()
           cache empty
            freelist empty
             lock(zone->lock);  <- DEADLOCK

There are other ways which do not involve locking to create wreckage:

 kmalloc() from regular non BPF context
  local_irq_save();
   ...
    obj = slab_first();
     kprobe()
      BPF()
       update_elem()
        lock(bucket)
         kmalloc()
          local_irq_save();
           ...
            obj = slab_first(); <- Same object as above ...

So preallocation _must_ be enforced for all variants of intrusive
instrumentation.

Unfortunately immediate enforcement would break backwards compatibility, so
for now such programs still are allowed to run, but a one time warning is
emitted in dmesg and the verifier emits a warning in the verifier log as
well so developers are made aware about this and can fix their programs
before the enforcement becomes mandatory.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200224145642.540542802@linutronix.de
This commit is contained in:
Thomas Gleixner 2020-02-24 15:01:32 +01:00 committed by Alexei Starovoitov
parent 8eece07c01
commit 94dacdbd5d
1 changed files with 28 additions and 11 deletions
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39
kernel/bpf/verifier.c
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@ -8143,26 +8143,43 @@ static bool is_tracing_prog_type(enum bpf_prog_type type)
}
}
static bool is_preallocated_map(struct bpf_map *map)
{
if (!check_map_prealloc(map))
return false;
if (map->inner_map_meta && !check_map_prealloc(map->inner_map_meta))
return false;
return true;
}
static int check_map_prog_compatibility(struct bpf_verifier_env *env,
struct bpf_map *map,
struct bpf_prog *prog)
{
/* Make sure that BPF_PROG_TYPE_PERF_EVENT programs only use
* preallocated hash maps, since doing memory allocation
* in overflow_handler can crash depending on where nmi got
* triggered.
/*
* Validate that trace type programs use preallocated hash maps.
*
* For programs attached to PERF events this is mandatory as the
* perf NMI can hit any arbitrary code sequence.
*
* All other trace types using preallocated hash maps are unsafe as
* well because tracepoint or kprobes can be inside locked regions
* of the memory allocator or at a place where a recursion into the
* memory allocator would see inconsistent state.
*
* For now running such programs is allowed for backwards
* compatibility reasons, but warnings are emitted so developers are
* made aware of the unsafety and can fix their programs before this
* is enforced.
*/
if (prog->type == BPF_PROG_TYPE_PERF_EVENT) {
if (!check_map_prealloc(map)) {
if (is_tracing_prog_type(prog->type) && !is_preallocated_map(map)) {
if (prog->type == BPF_PROG_TYPE_PERF_EVENT) {
verbose(env, "perf_event programs can only use preallocated hash map\n");
return -EINVAL;
}
if (map->inner_map_meta &&
!check_map_prealloc(map->inner_map_meta)) {
verbose(env, "perf_event programs can only use preallocated inner hash map\n");
return -EINVAL;
}
WARN_ONCE(1, "trace type BPF program uses run-time allocation\n");
verbose(env, "trace type programs with run-time allocated hash maps are unsafe. Switch to preallocated hash maps.\n");
}
if ((is_tracing_prog_type(prog->type) ||