[PATCH bpf-next v3 01/15] bpf: Read a kfunc's __sz argument only when it is in a register

[Yonghong Song <yonghong.song@linux.dev>]

Commit e0b7b91c72db ("bpf: Support stack arguments for kfunc calls")
supported stack arguments for kfunc's. In bpf_kfunc_stack_access_bytes(),
the size of a ptr + __sz pair is read from const_reg_vals[] at index
'BPF_REG_1 + arg + 1'. Past the fifth argument that index leaves the
argument registers and reaches 6 through 9, which are the callee saved
registers R6 through R9. The verifier does record constants for those,
so a __sz argument passed on the stack can take the value of an
unrelated register as its size.

Fix it by guard size_reg which has to be less than or equal to
MAX_BPF_FUNC_REG_ARGS.

Fixes: e0b7b91c72db ("bpf: Support stack arguments for kfunc calls")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/verifier.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9e79750e2480..fed576b8f7fe 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -13771,13 +13771,14 @@ s64 bpf_kfunc_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn *
 		goto out;
 	}
 
-	/* ptr + __sz/__szk pair: size is in the next register */
+	/* ptr + __sz/__szk pair: the size follows the pointer */
 	if (arg + 1 < nargs &&
 	    (btf_param_match_suffix(btf, &args[arg + 1], "__sz") ||
 	     btf_param_match_suffix(btf, &args[arg + 1], "__szk"))) {
 		int size_reg = BPF_REG_1 + arg + 1;
 
-		if (aux->const_reg_mask & BIT(size_reg)) {
+		if (size_reg <= MAX_BPF_FUNC_REG_ARGS &&
+		    (aux->const_reg_mask & BIT(size_reg))) {
 			size = (s64)aux->const_reg_vals[size_reg];
 			goto out;
 		}
-- 
2.52.0