Leaking kernel pointers to userspace is a bad idea, so let's try to do some analysis for it. The basic idea is that every pointer copied to userspace "should be" (but isn't necessarily) annotated with __user, and if it is not, then it's a potential infoleak. The motivation for this is stuff like [1], which is exactly a case of this. Based on a subsequent manual analysis of the uapi headers, I found [2]. Unfortunately in both of these cases, there is void * (for compat) trickery that masks them from actually turning up in this case. But hey, I wrote it and tried it out, so perhaps the code is useful for someone :) [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/block/floppy.c?id=65eea8edc315589d6c993cf12dbb5d0e9ef1fe4e [2]: https://lkml.org/lkml/2018/11/3/142 Signed-off-by: Tycho Andersen <tycho@xxxxxxxx> --- sparse.c | 93 ++++++++++++++++++++++++++++++++++++++- validation/copy_to_user.c | 31 +++++++++++++ 2 files changed, 123 insertions(+), 1 deletion(-) diff --git a/sparse.c b/sparse.c index 217a5bf..8bcbe16 100644 --- a/sparse.c +++ b/sparse.c @@ -171,13 +171,104 @@ static void check_memset(struct position pos, struct expression_list *args) check_byte_count(pos, size); } +static struct symbol *resolve_arg_type(struct position pos, struct expression *arg) +{ + struct expression *uncast; + + uncast = arg; + switch (arg->type) { + case EXPR_CAST: + case EXPR_FORCE_CAST: + case EXPR_IMPLIED_CAST: + /* + * Undo any casting done by sparse to the function's + * argument type. + */ + uncast = arg->cast_expression; + break; + case EXPR_SYMBOL: + break; + case EXPR_PREOP: + /* + * handle derefs; these are really just the type of the + * resulting expression. + */ + break; + case EXPR_BINOP: + /* TODO: resolve this pointer math if possible? */ + return NULL; + default: + warning(pos, "huh? arg not a cast or symbol? %d", arg->type); + return NULL; + } + + return uncast->ctype->ctype.base_type; +} + +static void check_ptr_in_other_as(struct position pos, struct symbol *sym, int this_as) +{ + struct ident *ident = sym->ident; + + if (sym->type == SYM_NODE) + sym = sym->ctype.base_type; + + switch (sym->type) { + case SYM_ARRAY: + case SYM_PTR: { + if (sym->ctype.as != this_as) + warning(pos, "member %s is a kernel pointer copied to userspace", show_ident(ident)); + check_ptr_in_other_as(pos, sym->ctype.base_type, this_as); + break; + } + case SYM_STRUCT: + case SYM_UNION: { + struct symbol *member; + + FOR_EACH_PTR(sym->symbol_list, member) { + check_ptr_in_other_as(pos, member, this_as); + } END_FOR_EACH_PTR(member); + break; + } + default: + /* + * scalar types are ok + * TODO: what about SYM_LABEL/PREPROCESSOR? + */ + break; + } +} + +static void check_no_kernel_pointers(struct position pos, struct expression_list *args) +{ + struct expression *src = ptr_list_nth_entry((struct ptr_list *)args, 1); + struct symbol *base = NULL; + + if (!Waddress_space) + return; + + /* get the type of src */ + base = resolve_arg_type(pos, src); + + /* + * And deref it to *src; src will *always* be a kernel pointer, and + * we're really after members of structures here, not the pointers + * themselves. So we do this deref at the top level. + */ + base = base->ctype.base_type; + + check_ptr_in_other_as(pos, base, 1); } #define check_memcpy check_memset -#define check_ctu check_memset #define check_cfu check_memset +void check_ctu(struct position pos, struct expression_list *args) +{ + check_memset(pos, args); + check_no_kernel_pointers(pos, args); +} + struct checkfn { struct ident *id; void (*check)(struct position pos, struct expression_list *args); diff --git a/validation/copy_to_user.c b/validation/copy_to_user.c new file mode 100644 index 0000000..d9cded6 --- /dev/null +++ b/validation/copy_to_user.c @@ -0,0 +1,31 @@ +#define __user __attribute__((address_space(1))) + +struct bar { + char *bar_kptr; +}; + +struct foo { + char *foo_kptr; + char __user *uptr; + struct bar bar; +}; + +static void copy_to_user(void __user *to, const void *from, unsigned long n) +{ +} + +static void bar(void) +{ + struct foo f; + void __user *p = (void __user *)0; + + copy_to_user(p, &f, sizeof(f)); +} +/* + * check-name: copy_to_user arguments + * + * check-error-start +copy_to_user.c:22:9: warning: member foo_kptr is a kernel pointer copied to userspace +copy_to_user.c:22:9: warning: member bar_kptr is a kernel pointer copied to userspace + * check-error-end + */ -- 2.19.1
