btf_var_show() calls btf_type_id_resolve() unconditionally, which
dereferences btf->resolved_ids. That is NULL for a base BTF - e.g. the
vmlinux BTF that bpf_snprintf_btf() renders against - since base BTF is
not resolved during parsing. btf_modifier_show() guards this with
'if (btf->resolved_ids)', but btf_var_show() does not.
A BPF program that passes the type_id of a BTF_KIND_VAR from the vmlinux
BTF to bpf_snprintf_btf() thus NULL-derefs:
KASAN: probably user-memory-access in range [0x46638-0x4663f]
RIP: 0010:btf_var_show (kernel/bpf/btf.c:2929)
Call Trace:
<TASK>
btf_type_show (kernel/bpf/btf.c:8259)
btf_type_snprintf_show (kernel/bpf/btf.c:8329)
bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047)
bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829)
__sys_bpf (kernel/bpf/syscall.c:4804)
do_syscall_64 (arch/x86/entry/syscall_64.c:84)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
</TASK>
Resolve the var's type directly with btf_type_skip_modifiers() when
resolved_ids is NULL, mirroring btf_modifier_show().
Fixes: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
Signed-off-by: Jiayuan Chen <[email protected]>
Acked-by: Ihor Solodrai <[email protected]>
---
kernel/bpf/btf.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 7835fe201384..7c634d639b7f 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2925,7 +2925,15 @@ static void btf_var_show(const struct btf *btf, const
struct btf_type *t,
u32 type_id, void *data, u8 bits_offset,
struct btf_show *show)
{
- t = btf_type_id_resolve(btf, &type_id);
+ /*
+ * btf_type_id_resolve() dereferences btf->resolved_ids, which is NULL
+ * for a base BTF (e.g. the vmlinux BTF that bpf_snprintf_btf() uses).
+ * Resolve the var's type directly in that case.
+ */
+ if (btf->resolved_ids)
+ t = btf_type_id_resolve(btf, &type_id);
+ else
+ t = btf_type_skip_modifiers(btf, t->type, &type_id);
btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
}
--
2.43.0
