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SubjectRe: UBSAN: array-index-out-of-bounds in arch_uprobe_analyze_insn
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 <john.stultz@linaro.org>
> 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@syzkaller.appspotmail.com
> 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

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