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@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