Hi Kees,
On Tue, 1 Dec 2020 16:48:55 -0800
Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Hi,
>
> There appears to be a problem with prefix counting for the instruction
> decoder. It looks like insn_get_prefixes() isn't keeping "nb" and "nbytes"
> in sync correctly:
>
> while (inat_is_legacy_prefix(attr)) {
> /* Skip if same prefix */
> for (i = 0; i < nb; i++)
> if (prefixes->bytes[i] == b)
> goto found;
> if (nb == 4)
> /* Invalid instruction */
> break;
> prefixes->bytes[nb++] = b;
> ...
> found:
> prefixes->nbytes++;
> insn->next_byte++;
> lb = b;
> b = peek_next(insn_byte_t, insn);
> attr = inat_get_opcode_attribute(b);
> }
>
> (nbytes is incremented on repeated prefixes, but "nb" isn't)
>
> However, it looks like nbytes is used as an offset:
>
> static inline int insn_offset_rex_prefix(struct insn *insn)
> {
> return insn->prefixes.nbytes;
> }
> static inline int insn_offset_vex_prefix(struct insn *insn)
> {
> return insn_offset_rex_prefix(insn) + insn->rex_prefix.nbytes;
> }
Yes, it is designed to do that. nbytes counts how many bytes the prefix is,
and nb is how many bytes of the prefix->bytes consumed.
Since the legacy prefix can be repeated and more than 4 (bytes), we can not
store all of those.
>
> Which means everything that iterates over prefixes.bytes[] is buggy,
> since they may be trying to read past the end of the array:
Good catch! All following usage are wrong...
>
> $ git grep -A3 -E '< .*prefixes(\.|->)nbytes'
> boot/compressed/sev-es.c: for (i = 0; i < insn->prefixes.nbytes; i++) {
> boot/compressed/sev-es.c- insn_byte_t p =
> insn->prefixes.bytes[i];
> boot/compressed/sev-es.c-
> boot/compressed/sev-es.c- if (p == 0xf2 || p == 0xf3)
> --
> kernel/uprobes.c: for (i = 0; i < insn->prefixes.nbytes; i++) {
> kernel/uprobes.c- insn_attr_t attr;
> kernel/uprobes.c-
> kernel/uprobes.c- attr = inat_get_opcode_attribute(insn->prefixes.bytes[i]);
> --
> kernel/uprobes.c: for (i = 0; i < insn->prefixes.nbytes; i++) {
> kernel/uprobes.c- if (insn->prefixes.bytes[i] == 0x66)
> kernel/uprobes.c- return -ENOTSUPP;
> kernel/uprobes.c- }
> --
> lib/insn-eval.c: for (i = 0; i < insn->prefixes.nbytes; i++) {
> lib/insn-eval.c- insn_byte_t p = insn->prefixes.bytes[i];
> lib/insn-eval.c-
> lib/insn-eval.c- if (p == 0xf2 || p == 0xf3)
> --
> lib/insn-eval.c: for (i = 0; i < insn->prefixes.nbytes; i++) {
> lib/insn-eval.c- insn_attr_t attr;
> lib/insn-eval.c-
> lib/insn-eval.c- attr = inat_get_opcode_attribute(insn->prefixes.bytes[i]);
>
> I don't see a clear way to fix this.
For the loop, we can check the insn.prefixes.bytes[i] == 0 since
it is initialized by 0 and 0x0 is not a prefix like this.
for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
...
}
Thank you,
>
> -Kees
>
> On Mon, Sep 21, 2020 at 09:20:07PM -0700, syzbot wrote:
> > syzbot has bisected this issue to:
> >
> > commit 4b2bd5fec007a4fd3fc82474b9199af25013de4c
> > Author: John Stultz <john.stultz@xxxxxxxxxx>
> > Date: Sat Oct 8 00:02:33 2016 +0000
> >
> > proc: fix timerslack_ns CAP_SYS_NICE check when adjusting self
> >
> > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1697348d900000
> > start commit: 325d0eab Merge branch 'akpm' (patches from Andrew)
> > git tree: upstream
> > final oops: https://syzkaller.appspot.com/x/report.txt?x=1597348d900000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1197348d900000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=b12e84189082991c
> > dashboard link: https://syzkaller.appspot.com/bug?extid=9b64b619f10f19d19a7c
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1573a8ad900000
> > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=164ee6c5900000
> >
> > Reported-by: syzbot+9b64b619f10f19d19a7c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 4b2bd5fec007 ("proc: fix timerslack_ns CAP_SYS_NICE check when adjusting self")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>
> --
> Kees Cook
--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>
