[PATCH bpf-next v3 11/17] bpf: Detect IP == ksym.end as part of BPF program

Kumar Kartikeya Dwivedi
2023-09-12T23:32:27Z

Now that bpf_throw kfunc is the first such call instruction that has
noreturn semantics within the verifier, this also kicks in dead code
elimination in unprecedented ways. For one, any instruction following
a bpf_throw call will never be marked as seen. Moreover, if a callchain
ends up throwing, any instructions after the call instruction to the
eventually throwing subprog in callers will also never be marked as
seen.

The tempting way to fix this would be to emit extra 'int3' instructions
which bump the jited_len of a program, and ensure that during runtime
when a program throws, we can discover its boundaries even if the call
instruction to bpf_throw (or to subprogs that always throw) is emitted
as the final instruction in the program.

An example of such a program would be this:

do_something():
...
r0 = 0
exit

foo():
r1 = 0
call bpf_throw
r0 = 0
exit

bar(cond):
if r1 != 0 goto pc+2
call do_something
exit
call foo
r0 = 0 // Never seen by verifier
exit //

main(ctx):
r1 = ...
call bar
r0 = 0
exit

Here, if we do end up throwing, the stacktrace would be the following:

bpf_throw
foo
bar
main

In bar, the final instruction emitted will be the call to foo, as such,
the return address will be the subsequent instruction (which the JIT
emits as int3 on x86). This will end up lying outside the jited_len of
the program, thus, when unwinding, we will fail to discover the return
address as belonging to any program and end up in a panic due to the
unreliable stack unwinding of BPF programs that we never expect.

To remedy this case, make bpf_prog_ksym_find treat IP == ksym.end as
part of the BPF program, so that is_bpf_text_address returns true when
such a case occurs, and we are able to unwind reliably when the final
instruction ends up being a call instruction.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
kernel/bpf/core.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 7849b9cca749..8f921b6d6981 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -623,7 +623,11 @@ static __always_inline int bpf_tree_comp(void *key, struct latch_tree_node *n)

if (val < ksym->start)
return -1;
- if (val >= ksym->end)
+ /* Ensure that we detect return addresses as part of the program, when
+ * the final instruction is a call for a program part of the stack
+ * trace. Therefore, do val > ksym->end instead of val >= ksym->end.
+ */
+ if (val > ksym->end)
return 1;

return 0;
--
2.41.0