[PATCH bpf v3 3/5] bpf: Fix NULL-ptr-deref in btf_var_show()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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 <jiayuan.chen@linux.dev>
---
 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 6e267c4c4e1f..3cf35ee49c10 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2926,7 +2926,15 @@ 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