[PATCH bpf 4/7] bpf/s390: prevent trampoline attachment when args location on stack is uncertain

[Alexis Lothoré (eBPF Foundation)]

When the target function receives more arguments than available
registers, the additional arguments are passed on stack, and so the
generated trampoline needs to read those to prepare the bpf context, but
also to prepare the target function stack when it is in charge of
calling it. This works well for scalar types, but if the value is a
struct, we can not know for sure the exact struct location, as it may
have been packed or manually aligned to a greater value.

Prevent wrong readings by refusing trampoline attachment if the target
function receives a struct on stack. While doing so, move the existing
check (ensuring that the number of args passed on stack is not higher
than MAX_NR_STACK_ARGS) into the newly created check function.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore at bootlin.com>
---
 arch/s390/net/bpf_jit_comp.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index c7f8313ba449716a8f18eafdeb6c77ed3b23f52e..b441feb20e993f54cc0e9a39c67a726f4b61d9f2 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -2566,6 +2566,27 @@ static int alloc_stack(struct bpf_tramp_jit *tjit, size_t size)
 /* -mfentry generates a 6-byte nop on s390x. */
 #define S390X_PATCH_SIZE 6
 
+static int validate_args(const struct btf_func_model *m)
+{
+	int i = 0, nr_reg_args, nr_stack_args;
+
+	nr_reg_args = min_t(int, m->nr_args, MAX_NR_REG_ARGS);
+	nr_stack_args = m->nr_args - nr_reg_args;
+
+	if (nr_stack_args == 0)
+		return 0;
+
+	/* Support as many stack arguments as "mvc" instruction can handle. */
+	if (nr_stack_args > MAX_NR_STACK_ARGS)
+		return -ENOTSUPP;
+
+	for (i = nr_reg_args; i < m->nr_args; i++)
+		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
+			return -ENOTSUPP;
+
+	return 0;
+}
+
 static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 					 struct bpf_tramp_jit *tjit,
 					 const struct btf_func_model *m,
@@ -2579,13 +2600,17 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	int nr_bpf_args, nr_reg_args, nr_stack_args;
 	struct bpf_jit *jit = &tjit->common;
 	int arg, bpf_arg_off;
-	int i, j;
+	int i, j, ret;
+
+	/* make sure that any argument can be located and processed by the
+	 * trampoline
+	 */
+	ret = validate_args(m);
+	if (ret)
+		return ret;
 
-	/* Support as many stack arguments as "mvc" instruction can handle. */
 	nr_reg_args = min_t(int, m->nr_args, MAX_NR_REG_ARGS);
 	nr_stack_args = m->nr_args - nr_reg_args;
-	if (nr_stack_args > MAX_NR_STACK_ARGS)
-		return -ENOTSUPP;
 
 	/* Return to %r14 in the struct_ops case. */
 	if (flags & BPF_TRAMP_F_INDIRECT)

-- 
2.49.0