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Subject[PATCH net-next v2 2/2] bpf: Extend check_uarg_tail_zero() checks
Date
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
---
kernel/bpf/syscall.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c653ee0bd162..fbe09a0cccf4 100644
--- 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;
--
2.13.3
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