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 <m...@digikod.net> Cc: Alexei Starovoitov <a...@kernel.org> Cc: Daniel Borkmann <dan...@iogearbox.net> Cc: David S. Miller <da...@davemloft.net> Cc: Kees Cook <keesc...@chromium.org> Cc: Martin KaFai Lau <ka...@fb.com> Link: https://lkml.kernel.org/r/CAGXu5j+vRGFvJZmjtAcT8Hi8B+Wz0e1b6VKYZHfQP_=dxzc...@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..b884fdc371e0 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 dont know about yet. + * + * There is a ToCToU between this function call and the following + * copy_from_user() call. However, this should not be 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(!access_ok(VERIFY_READ, uaddr, actual_size))) + return -EFAULT; + + if (unlikely(actual_size > PAGE_SIZE)) /* silly large */ + return -E2BIG; + 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
