Re: [PATCH] binfmt_misc: fix shift-out-of-bounds in check_special_flags

[Kees Cook <keescook@chromium.org>]

On Sat, Nov 19, 2022 at 05:29:22AM +0000, Al Viro wrote:
> On Wed, Nov 02, 2022 at 10:51:23AM +0800, Liu Shixin wrote:
> > UBSAN reported a shift-out-of-bounds warning:
> > 
> >  left shift of 1 by 31 places cannot be represented in type 'int'
> >  Call Trace:
> >   <TASK>
> >   __dump_stack lib/dump_stack.c:88 [inline]
> >   dump_stack_lvl+0x8d/0xcf lib/dump_stack.c:106
> >   ubsan_epilogue+0xa/0x44 lib/ubsan.c:151
> >   __ubsan_handle_shift_out_of_bounds+0x1e7/0x208 lib/ubsan.c:322
> >   check_special_flags fs/binfmt_misc.c:241 [inline]
> >   create_entry fs/binfmt_misc.c:456 [inline]
> >   bm_register_write+0x9d3/0xa20 fs/binfmt_misc.c:654
> >   vfs_write+0x11e/0x580 fs/read_write.c:582
> >   ksys_write+0xcf/0x120 fs/read_write.c:637
> >   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >   do_syscall_64+0x34/0x80 arch/x86/entry/common.c:80
> >   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >  RIP: 0033:0x4194e1
> > 
> > Since the type of Node's flags is unsigned long, we should define these
> > macros with same type too.
> 
> We are limited to 32 bits anyway.  More interesting question here is what's
> the point of having those bits that high anyway?

Hm, looks like it was designed to avoid the enum just before the
defines:

enum {Enabled, Magic};
#define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
#define MISC_FMT_OPEN_BINARY (1 << 30)
#define MISC_FMT_CREDENTIALS (1 << 29)
#define MISC_FMT_OPEN_FILE (1 << 28)

But both appear to be entirely internally defined bits. I think these
can all just be part of the enum, and we can quit mixing "&" tests and
test_bit() calls:

...
        if (e->flags & MISC_FMT_OPEN_FILE)
                *dp++ = 'F';
        *dp++ = '\n';

        if (!test_bit(Magic, &e->flags)) {
                sprintf(dp, "extension .%s\n", e->magic);
...


-- 
Kees Cook