Subject: Re: UBSAN: array-index-out-of-bounds in arch_uprobe_analyze_insn

From: Kees Cook <[email-masked]>
Date: Tue, 1 Dec 2020 16:48:55 -0800

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;
}

Which means everything that iterates over prefixes.bytes[] is buggy,
since they may be trying to read past the end of the array:

$ 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.

-Kees

On Mon, Sep 21, 2020 at 09:20:07PM -0700, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit 4b2bd5fec007a4fd3fc82474b9199af25013de4c
> Author: John Stultz <[email-masked]>
> 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: [email-masked]
> 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