bpf: Extend check_uarg_tail_zero() checks

The function check_uarg_tail_zero() was created from bpf(2) for
BPF_OBJ_GET_INFO_BY_FD without taking the access_ok() nor the PAGE_SIZE
checks. Make this checks more generally available while unlikely to be
triggered, extend the memory range check and add an explanation
including why the ToCToU should not be a security concern.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Link: https://lkml.kernel.org/r/CAGXu5j+vRGFvJZmjtAcT8Hi8B+Wz0e1b6VKYZHfQP_=DXzC4CQ@mail.gmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c653ee0..fbe09a0 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -48,6 +48,15 @@ static const struct bpf_map_ops * const bpf_map_types[] = {
 #undef BPF_MAP_TYPE
 };
 
+/*
+ * If we're handed a bigger struct than we know of, ensure all the unknown bits
+ * are 0 - i.e. new user-space does not rely on any kernel feature extensions
+ * we don't know about yet.
+ *
+ * There is a ToCToU between this function call and the following
+ * copy_from_user() call. However, this is not a concern since this function is
+ * meant to be a future-proofing of bits.
+ */
 static int check_uarg_tail_zero(void __user *uaddr,
                                size_t expected_size,
                                size_t actual_size)
@@ -57,6 +66,12 @@ static int check_uarg_tail_zero(void __user *uaddr,
        unsigned char val;
        int err;
 
+       if (unlikely(actual_size > PAGE_SIZE))  /* silly large */
+               return -E2BIG;
+
+       if (unlikely(!access_ok(VERIFY_READ, uaddr, actual_size)))
+               return -EFAULT;
+
        if (actual_size <= expected_size)
                return 0;
 
@@ -1393,17 +1408,6 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
        if (!capable(CAP_SYS_ADMIN) && sysctl_unprivileged_bpf_disabled)
                return -EPERM;
 
-       if (!access_ok(VERIFY_READ, uattr, 1))
-               return -EFAULT;
-
-       if (size > PAGE_SIZE)   /* silly large */
-               return -E2BIG;
-
-       /* If we're handed a bigger struct than we know of,
-        * ensure all the unknown bits are 0 - i.e. new
-        * user-space does not rely on any kernel feature
-        * extensions we dont know about yet.
-        */
        err = check_uarg_tail_zero(uattr, sizeof(attr), size);
        if (err)
                return err;