When a kfunc was trying to access data from context in a syscall eBPF
program, the verifier was rejecting the call.
This is because the syscall context is not known at compile time, and
so we need to check this when actually accessing it.

Check for the valid memory access and allow such situation to happen.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Benjamin Tissoires <redacted>

---

changes in v8:
- fixup comment
- return -EACCESS instead of -EINVAL for consistency

changes in v7:
- renamed access_t into atype
- allow zero-byte read
- check_mem_access() to the correct offset/size

new in v6
---
 kernel/bpf/verifier.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7c1e056624f9..c807c5d7085a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -248,6 +248,7 @@ struct bpf_call_arg_meta {
        struct bpf_map *map_ptr;
        bool raw_mode;
        bool pkt_access;
+       bool is_kfunc;
        u8 release_regno;
        int regno;
        int access_size;

@@ -5170,6 +5171,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
                                   struct bpf_call_arg_meta *meta)
 {
        struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
+       enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
        u32 *max_access;

        switch (base_type(reg->type)) {

@@ -5223,6 +5225,24 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
                                env,
                                regno, reg->off, access_size,
                                zero_size_allowed, ACCESS_HELPER, meta);
+       case PTR_TO_CTX:
+               /* in case of a kfunc called in a program of type SYSCALL, the context is
+                * user supplied, so not computed statically.
+                * Dynamically check it now
+                */
+               if (prog_type == BPF_PROG_TYPE_SYSCALL && meta && meta->is_kfunc) {