Re: [PATCH bpf v2 2/4] bpf: map_poke_descriptor is being called with an unstable poke_tab[]

Alexei Starovoitov wrote:

On Wed, Jun 16, 2021 at 03:55:19PM -0700, John Fastabend wrote:

quoted

When populating poke_tab[] of a subprog we call map_poke_track() after
doing bpf_jit_add_poke_descriptor(). But, bpf_jit_add_poke_descriptor()
may, likely will, realloc the poke_tab[] structure and free the old
one. So that prog->aux->poke_tab is not stable. However, the aux pointer
is referenced from bpf_array_aux and poke_tab[] is used to 'track'
prog<->map link. This way when progs are released the entry in the
map is dropped and vice versa when the map is released we don't drop
it too soon if a prog is in the process of calling it.

I wasn't able to trigger any errors here, for example having map_poke_run
run with a poke_tab[] pointer that was free'd from
bpf_jit_add_poke_descriptor(), but it looks possible and at very least
is very fragile.

This patch moves poke_track call out of loop that is calling add_poke
so that we only ever add stable aux->poke_tab pointers to the map's
bpf_array_aux struct. Further, we need this in the next patch to fix
a real bug where progs are not 'untracked'.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 kernel/bpf/verifier.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6e2ebcb0d66f..066fac9b5460 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -12126,8 +12126,12 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 			}
 
 			func[i]->insnsi[insn_idx - subprog_start].imm = ret + 1;
+		}
 
-			map_ptr = func[i]->aux->poke_tab[ret].tail_call.map;
+		for (j = 0; j < func[i]->aux->size_poke_tab; j++) {
+			int ret;
+
+			map_ptr = func[i]->aux->poke_tab[j].tail_call.map;

I don't see why it's necessary.

Agree its not necessary. Nothing else can get at poke_tab while we do
the realloc so I agree its fine as is. It still seems odd to me to do the
poke_track when we know we are about to do multiple reallocs in the
next round of the loop. Either way I'll reply on the feedback in 3/4 and
we can avoid this patch altogether I think.

poke_tab pointer will be re-read after bpf_jit_add_poke_descriptor().
The compiler is not allowed to cache it.

I've applied the patch 1.