From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Sat, 19 Mar 2022 13:38:24 +0530
Subject: bpf: Check PTR_TO_MEM | MEM_RDONLY in check_helper_mem_access
Patch-mainline: v5.19-rc1
Git-commit: 97e6d7dab1ca4648821c790a2b7913d6d5d549db
References: jsc#PED-1377

The commit being fixed was aiming to disallow users from incorrectly
obtaining writable pointer to memory that is only meant to be read. This
is enforced now using a MEM_RDONLY flag.

For instance, in case of global percpu variables, when the BTF type is
not struct (e.g. bpf_prog_active), the verifier marks register type as
PTR_TO_MEM | MEM_RDONLY from bpf_this_cpu_ptr or bpf_per_cpu_ptr
helpers. However, when passing such pointer to kfunc, global funcs, or
BPF helpers, in check_helper_mem_access, there is no expectation
MEM_RDONLY flag will be set, hence it is checked as pointer to writable
memory. Later, verifier sets up argument type of global func as
PTR_TO_MEM | PTR_MAYBE_NULL, so user can use a global func to get around
the limitations imposed by this flag.

This check will also cover global non-percpu variables that may be
introduced in kernel BTF in future.

Also, we update the log message for PTR_TO_BUF case to be similar to
PTR_TO_MEM case, so that the reason for error is clear to user.

Fixes: 34d3a78c681e ("bpf: Make per_cpu_ptr return rdonly PTR_TO_MEM.")
Reviewed-by: Hao Luo <haoluo@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220319080827.73251-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 kernel/bpf/verifier.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4871,13 +4871,23 @@ static int check_helper_mem_access(struc
 		return check_map_access(env, regno, reg->off, access_size,
 					zero_size_allowed);
 	case PTR_TO_MEM:
+		if (type_is_rdonly_mem(reg->type)) {
+			if (meta && meta->raw_mode) {
+				verbose(env, "R%d cannot write into %s\n", regno,
+					reg_type_str(env, reg->type));
+				return -EACCES;
+			}
+		}
 		return check_mem_region_access(env, regno, reg->off,
 					       access_size, reg->mem_size,
 					       zero_size_allowed);
 	case PTR_TO_BUF:
 		if (type_is_rdonly_mem(reg->type)) {
-			if (meta && meta->raw_mode)
+			if (meta && meta->raw_mode) {
+				verbose(env, "R%d cannot write into %s\n", regno,
+					reg_type_str(env, reg->type));
 				return -EACCES;
+			}
 
 			max_access = &env->prog->aux->max_rdonly_access;
 		} else {